4.1.1. Basal cell tumor (basal cell epithelioma)

The basal cell carcinoma, also known as basalioma, is one of the most frequent skin tumors in dogs and cats, being also diagnosed in other animal species. Basal cells, in neoplastic disease, are similar to fetal germ cells, having a marked pluripotential character. This, as well as other considerations, under the conditions of neoplastic changes, requires concomitant or successive treatment of basal cell carcinomas and adnexal epidermal tumors (pilose follicles, sweat glands, sebaceous glands, ceruminous glands, perianal glands).

As a rule, basal cell carcinomas do not evolve concomitantly with adnexal epithelial tumors. The following risk factors are considered in the cytology of these tumors: genetic sensitivity as well as chemical and endocrine carcinogens. In humans, exposure to solar radiation is a risk factor, in animals this remains to be demonstrated.

The incidence of basaliomas is higher in dogs and cats. In dogs, statistics indicate a rate of 3–5–10% of all tumors, and in cats a percentage of up to 18%. In terms of age, in both dogs and cats, the adult age, 7 years in dogs and 9 years in cats, has a higher incidence. Males are more frequently affected by basaliomas, compared to females, and the dog breeds that seem to be more sensitive are Cocker Spaniel and Caniche [62]. Persian and older cats have a predisposition for basaliomas, presenting ulcerated plaques at the level of the head, extremities and neck. These malignant tumors have continuity with the epidermis, evolving with local invasion, but without metastases. In dogs, basal cell carcinomas histologically show cornification, being termed basosquamous carcinomas that are found in old Saint Bernard, Scottish Terrier and Norwegian Elkhound dog breeds. They may be located in any body area, under the form of endo-exophytic nodules or plaques. They have local invasive growth, without metastases.

Locations and the macroscopic aspect are characterized by single or multiple formations, in both dogs and cats. In 1986, FEHRER and LIN reported a multicentric basal cell tumor in a cat. The most frequent location is in the head, neck and shoulder. In general, lesions are not extensive, being well circumscribed, up to 2.5 cm in diameter; a 10 cm tumor has been exceptionally reported in a dog.

Histologically, basalioma cells are characterized by oval prominent nuclei and a small amount of cytoplasm, they generally have small sizes and are uniform. Basalioma cells are very similar to epidermal basal cells. The mitotic index, in the case of this tumor, is high. Basal cells frequently contain abundant melanin amounts, especially in cats, which is why they should not be confused with pigmented tumors.

The literature includes multiple classifications of epidermal tumors, all being based on the histological components of this skin segment [62, 97].

Basal cell tumor, a benign neoplastic proliferation of cells that recapitulate the basal cell layer of the normal epidermis. The neoplastic cells are either small and round or polyhedral, with little cytoplasm and ovoid nuclei. Most tumors show little nuclear pleomorphism and variable mitotic activity (Fig. 4.1). Central cystic degeneration of the tumor lobules may be found. Melanophages may be present in the stroma and within the center of the cyst. Melanin pigment can also be present within the cytoplasm of tumor cells. Basal cell tumor is differentiated from basal cell carcinoma by the lack of invasion and associated fibroplasias at the periphery of the benign tumors [103].

Fig. 4.1

Basal cell tumor

Basal cell carcinoma, a low-grade malignant proliferation of cells that recapitulate the basal cell layer of the normal epidermis or the adnexa.

Infiltrative basal cell carcinoma (Fig. 4.2.) may have an association with the overlying epidermis. Cords of primitive basaloid cells extend into the dermis and the subcutis. The tumor cells have small, hyperchromatic nuclei with scant cytoplasm, and mitotic figures may be numerous. There is often an extensive proliferation of stromal fibroblasts [103].

Fig. 4.2

Basal cell carcinoma, infiltrative type.

Clear-cell basal cell carcinoma is an uncommon variant, the cells are large and polygonal and have clear or finely granular cytoplasm; the nuclei are ovoid and uniform, nucleoli are small, and the number of mitoses found may be quite variable [103].

The following 5 types are microscopically differentiated, depending on the cell arrangement:

1. tumors disposed as ribbons or cords, having the appearance of a palisade with hyperchromatic ovoid cells;
2. tumors formed by compact, trabecular cell groups - solid tumors;
3. tumors with medusoid disposition;
4. adenoid tumors;
5. cystic tumors or basosquamous cell arrangement, frequent in cats, with a keratinized nucleus [61, 62]. In some cases, the same tumor can present a mixed arrangement.

The investigations performed by DITERS and WALSH (1984), on 124 cats with basal cell tumors, indicate a higher frequency of cystic forms compared to compact forms.

Basal cell tumors had a lower incidence in horses, the following tumors being histologically identified: adenoid tumors, solid tumors and medullary-like tumors, and a lower proportion with a cystic cord and space structure. Solid forms were intensely pigmented. In all the cases monitored, no postoperative recurrences were found, after 3 up to 8 years [86].

Cord-like cell tumors have the appearance of cell fascicles disposed as a palisade, with frequent mucinous degeneration of the stroma between the cords.

Solid tumors are formed by cells with variable sizes, the row of cells being frequently disposed at the periphery, nuclei are perpendicular to the stromal tissue delimiting the neoplasm.

Medusoid tumors appear as small nests formed by basal cells, which project into the surrounding stroma, under the form of fascicles, infiltrative growth aspect.

Adenoid tumors display cells arranged as multiple islands, with round or ovoid spaces with homogeneous cells in the center, while at the periphery cells are perpendicular to the connective stroma, rich in fundamental mass. The general appearance is that of adenomatous proliferations (Fig. 4.4.).

Fig. 4.4

Squamous cell carcinoma, differentiated.

Cystic tumors, more common in cats, may appear as a large single cyst or multiple smaller cysts. Cysts are formed by undifferentiated basal cells, without a limiting basal membrane, and squamous differentiations may frequently occur. Cysts have an eosinophilic content, probably as a result of the degeneration of basal cells from the solid mass [49],

These histological forms of basaliomas seem to suggest developmental stages of neoplasms. Basal cell tumors have a slow growth, they are usually encapsulated, they do not develop metastases and have no postoperative recurrences.

4.1.2. Squamous cell carcinoma

Squamous cell carcinoma is a highly malignant neoplasm, frequently associated with solar dermatosis, being the tumor of the cells of the malpighian layer from the epidermis.

It has a high incidence, being reported in all species of domestic animals, with a higher frequency in horses, dogs and cats, especially in adult and old animals. Although breed does not seem to be a risk factor, some authors [21] report a higher sensitivity for Labrador and black Caniche dogs, with location in the digits, and for Dalmatian, Beagle, Whippets and white English Bull Terrier breeds, with location in the flank and the abdomen.

In horses, SCHWINK (1987) noted the presence of squamous carcinomas, with ocular location, in heavy breeds; BERRY et al. (1991) reported an exceptional location in a horse, at the level of the hoof, a squamous carcinoma, in the laminar corium, cells with a high mitotic index.

Squamous cell carcinoma in hens is extremely uncommon, consequently the report of the Japanese authors SUGIYAMA et al. (1991) deserves to be mentioned, according to which a 204 day old laying hen, with a weight of 1.55 kg, had a tumor in the scapular region. The authors described a microscopic image with the proliferation of epidermal cells, corium and subcutaneous connective tissue, with the presence of heterophils, lymphocytes and plasmocytes. Tumor cells had eosinophilic cytoplasm, the cell shapes were round and irregular, pale. The nuclei, round or oval, with 1–2 nucleoli. Mitoses were numerous, keratotic pearls eosinophilic and lamellar. Intercellular desmosomes were identified by electron microscopy.

A very peculiar location was reported by PRAJAPATI et al. in cattle, at the level of the horn core epithelium, under the form of squamous cell carcinoma. Keratotic pearls were microscopically present, in the center of which current alkaline phosphatase activity was found, as well as an increased succinate dehydrogenase activity, in the peripheral cells of keratotic pearls.

Actinic keratosis (solar keratosis), a localized proliferation of epidermal keratinocytes caused by chronic exposure to ultraviolet light. The affected cells have larger, more hyperchromatic nuclei and may exhibit dyskeratosis. There is epidermal hyperplasia and parakeratosis of the stratum corneum. Dermal elastin fiber degeneration may be found. These lesions may progress to squamous cell carcinoma.

Multicentric squamous cell carcinoma in situ (Bowen-like disease) is a malignant tumor of epidermal cells without invasion through the basal lamina zone. This type of squamous cell carcinoma is not associated with ultraviolet light exposure, but an association with papillomavirus infection has been noted. The epidermis and external root sheath in affected areas are thickened and are often sharply demarcated from normal epidermis. Atypical neoplastic squamous cells, often with large, hyperchromatic nuclei, are found primarily in the basal and spinous layer and extend down the external root sheath of hair follicles. Lesions frequently contain abundant melanin pigment. Hyperkeratosis, parakeratosis, and hyperpigmentation of the stratum corneum may be present [103].

Squamous cell carcinoma, a malignant tumor of epidermal cells with varying degrees of keratinocyte (squamous cell) differentiation. Most cases of squamous cell carcinoma consist of islands, cords, and trabeculae of invasive epithelial cells that almost always have an association with the overlying epidermis, in which there has been a breaching of the basal lamina zone. There is often the formation of keratin pearls (concentric lamellae of keratin within the tumor) by invasive neoplastic cells. The cells and nuclei are large, nuclei are hyperchromatic, and chromatin often appears clumped. Nucleoli vary in size and may be prominent. Whereas those tumors that are well differentiated from keratin pearls, poorly differentiated tumors only show keratinization of individual cells.

Two uncommon histologic variants of squamous cell carcinoma occur:

• - acantholytic squamous cell carcinoma, in which there has been dyshesion and degeneration of neoplastic cells resulting in cyst formation with a single peripheral layer of neoplastic cells, producing a pseudoglandular pattern;
• - spindle cell squamous cell carcinoma, in which neoplastic cells are fusiform; cytokeratin stains are often positive and help identify these rare tumors as squamous cell carcinoma [103].

Basosquamous carcinoma, a low-grade malignancy composed of basal cells with foci of squamous or follicular infundibular differentiation, may have an association with the epidermis. Dyskeratotic cells may be seen among the basaloid cells. The keratinized cells display atypia, and occasionally numerous mitotic figures are observed. Melanin may be present in the peripheral basaloid cells [103].

MADEWELL and THEILEN (1987) synthesized the different locations by body regions and species, with moderate/high incidence of squamous cell carcinoma:

Ocular squamous cell carcinoma in cattle has a variable incidence in different geographical areas, but it has been diagnosed in all countries where cattle are raised. The risk factors incriminated are: genetic predisposition, UV radiation, nonpigmented skin in the orbital area, also including irritations produced by insects, chemical substances and even some viruses. The Hereford and Holstein breeds have a higher sensitivity to squamous cell carcinoma. In India, neoplasms have been diagnosed in buffaloes and sheep. Bovine herpesvirus-5 antigens have been identified in the cytoplasm of neoplastic cells. Bovine papillomavirus has been detected in bovine ocular lesions such as: conjunctival plaques, conjunctival papilloma, eyelid papilloma and cutaneous horn [61].

Ocular squamous cell carcinomas are preceded by precancerous lesions of the cornea and sclera and/or eyelids.

Ocular lesions progress gradually, starting with precancerous plaques located on the cornea and the sclerocorneal limbus. Conjunctival papillomas develop subsequently. The carcinoma in situ develops directly from the plaque or the papilloma. In a more advanced stage, the carcinoma invades the cornea and sclera [61].

According to the cited authors, histological aspects, starting with benign lesions, towards malignant lesions, have the following succession: acanthosis with ulcerative foci; keratosis, located at the limits of the mucocutaneous junction; papillomas located in the periocular skin, with keratotic lesion appearance; carcinoma in situ, difficult to diagnose due to the resemblance to inflammatory ulcers; infiltrative squamous cell carcinoma with diffuse proliferative, frequently hemorrhagic character, and with necrotic foci, superficial bacterial infections.

Metastases frequently occur in regional lymph nodes, but it can occasionally appear in the lungs and other visceral organs. A special case was that of a cow with a suspicion of tuberculous mastitis, with retromammary lymph node hypertrophy. Microscopy showed squamous cell carcinoma metastasis in the retromammary lymph nodes, with tumor lesions on the third eyelid, without affecting ocular structures and without metastases in other organs or lymph nodes [9].

Ocular squamous cell carcinoma in horses has a lower incidence compared to cattle. Advanced age, the absence of periocular pigmentation and UV exposure are risk factors.

In a study performed on 41 horses with ocular squamous cell carcinoma, SCHWINK (1987) found a higher incidence in heavy breeds (46.3%), which had recurrent postoperative neoplasms in a proportion of 42.4%, and 6% developed metastases. Similar incidences have been reported in USA, South Africa, as well as in other countries. In 15 up to 20% of cases, carcinomas have a bilateral evolution; the age group with the highest incidence is over 9 years [61].

Ocular squamous cell carcinomas in horses require differential diagnosis from other lesions with similar locations (eyelid sarcoid, ocular lymphoma, hemangiosarcoma, mastocytoma, bacterial or parasitic inflammation).

Squamous cell carcinoma in other species has been diagnosed in sheep, dogs and cats. Incidence is much lower compared to other species. In sheep, location is ocular and periocular, risk factors such as UV exposure and probably papillomavirus infection being incriminated.

In dogs, this neoplasm is uncommon, of a total number of 202 ocular neoplasms only 2% were squamous cell carcinomas; ocular papillomatous proliferations had a benign evolution, without being progressive [49]. Squamous cell carcinoma induced by exposure to solar radiation is most frequently reported in dogs. The cited authors used with promising results etretinate treatment, in early squamous cell carcinomas or in preneoplastic lesion carcinomas. In the case of preneoplastic lesions, evolution was stationary or progressive. In the case of multifocal lesions, treatment did not influence the disease evolution. The authors administered 1 mg/kg etretinate, two times a day, for at least 90 days.

In cats, this carcinoma is located in the skin and conjunctival mucosa of eyelids. A higher sensitivity is found in white cats, in which squamous cell carcinoma can develop simultaneously in eyelids, ears, nostrils and lips. Ocular neoplasms in the early stages are circumscribed, precisely located, facilitating surgical intervention. Metastases occur in regional lymph nodes. LIU et al. (1974) diagnosed 8 squamous cell carcinomas in cats, of which 6 were located in the head area, in the gums, causing osteolytic or osteoblastic periosteal lesions in the mandible. In two other subjects, location was in the interdigital skin, inducing osteolytic lesions in digital bones, with metastases in the regional lymph nodes, lungs and eyelids. The malignancy of squamous cell carcinomas in the cat manifests in a similar way to those of the dog, including the invasion of adjacent bones.

Squamous cell carcinoma located in the lips, ears and nostrils is more frequently found in dogs, cats and horses, and less frequently in other species. Squamous cell carcinoma especially occurs in old animals, with non-pigmented skin and UV exposure.

Cutaneous squamous cell carcinoma in the cat is usually located on the external side of the auricle or/and on the margin of the ear tips, lips and nostrils. In California, an incidence of 26.9 cases/100 000 cats is estimated, at the age of over 5 years, in white subjects [61]. The risk of squamous cell carcinoma in white cats is 13.4-fold higher compared to colored cats.

BAER and HELTON (1993) studied 12 cats with the mean age of 12 years, with multicentric squamous cell carcinomas. The affected skin was pigmented, and the neoplasms were located in the trunk, limbs, feet, head and neck, without having been exposed to solar radiation. Multiple skin lesions were 0.5–3.0 cm in diameter, irregular, slightly bulging, having the appearance of plaques or papillae, partially with alopecia. Following microscopic examination, the authors established differences between the multicentric squamous cell carcinoma in situ of the cat and precancerous Bowen dermatosis in humans. ASHLEY (1978) [11] showed that human Bowen dermatosis is an intradermal type of cancer rather than a precancerous condition. The result of the study on multicentric squamous carcinoma in cats demonstrates that this is a neoplasm morphologically similar to the bowenoid squamous cell carcinoma in situ present in humans. In spite of this, the cat disease is not identical to human Bowen disease and should be referred to as multicentric squamous cell carcinoma in situ. The neoplasm is limited to the epidermal epithelium and the follicular infundibulum.

Multicentric squamous cell carcinoma in situ in cats differs from Bowen disease in humans, by the multicentric aspect in 75% of the examined cats. In contrast with the predominantly solitary lesions found in human Bowen disease, multiple development centers are typical in humans with solar keratosis; however, the lesions of the cats included in the study did not appear in poorly pigmented or non-pigmented skin areas, and the majority of cats were minimally exposed to solar radiation.

In dogs, the locations in the lips, ears and nostrils are similar to those mentioned in cats, being associated with the lack of pigmentation of these skin areas. Malignant evolution with deep infiltrations and ulcerations is associated with metastases in regional lymph nodes.

Subungual keratoacanthoma (nailbed keratoacanthoma), a benign neoplasm, is symmetrical and unencapsulated with irregular scalloped borders and a central core of keratin; it is composed of sheets of keratinocytes that have abundant pale eosinophilic cytoplasm, distinct cell borders, and large vesicular nuclei. A variable number of apoptotic keratinocytes may be found; this tumor may cause lysis of the phalangeal bone [103].

Squamous cell carcinoma located in the epithelium of the horns and nails is found in cattle, a particularly high incidence being reported in zebu cattle, in India [82]. The authors report an extremely high frequency of squamous cell carcinomas at the base of the horns, mentioning 10% of the animals killed in slaughter houses, exclusively castrated males, around the age of 5 years. The possibility of actinocancer and/or repeated traumas associated with castration is mentioned. Epidemiological studies show the implication of the hormonal factor (castrated or cryptorchid males), repeated traumas, genetic predisposition and UV rays. The neoplasm in its progressive evolution causes osteolysis in the bone plug, invades the frontal sinus, and can extend to the nasal cavity, frontal bones, pituitary fossa and optical fossa (PACHAURI and PATHAC, 1969) [61].

The location in the nail epithelium has been relatively frequently reported in dogs, with chronic inflammations, repeated traumas and non-pigmentation of the area [61]. The cited authors mention the Poodel, Schnauzer and Labrador breeds with a higher incidence of squamous cell carcinoma in the nail epithelium. Following a study in 213 dogs with cutaneous squamous cell carcinoma, the same authors found 128 cases (60%) located in the nail epithelium, at a mean age of 9 years, without sex-dependent differences.

The location in the nail epithelium manifests with local invasion, the invasion of phalanges, of the metacarpal or metatarsal bone, and metastases occur in lymph nodes, lungs and other visceral organs.

Tumors of the hoof wall in horses are rare; BERRY et al. [17] describe a squamous cell carcinoma of laminar cells, with a high mitotic index and local invasion.

Surgical treatment, in the case of dogs, with the excision of the digit in the early stages, has good results.

Squamous cell carcinoma of the prepuce, penis, vulva, anus and perineal region. Squamous cell carcinoma of the penis and prepuce is more common in old horses and stallions, as well as in other animal species. Tumors are present in unpigmented skin areas and develop progressively as premalignant inflammatory processes that evolve into infiltrative and ulcerative squamous cell carcinoma. The accumulation of smegma in the prepuce is associated with the initiation of carcinogenesis.

In cows and mares, squamous cell carcinoma is frequently located in the vulva. In sheep, LAGADIC (1980) reports an enzootic of vulvar squamous cell carcinoma. The author mentions a perineal carcinoma enzootic in Angora goats from South Africa, studied by THOMAS (1929), while the same type of perineal neoplasm is reported in India by DAMODORAN and PARTHASARTH (1972). In most cases, in sheep and goats, lesions are located at the mucocutaneous junction or in the perineum, with depigmented skin. Lesions are of precancerous type such as keratosis, cutaneous horn or papillomas, and subsequent ulcerations occur, as well as secondary infections or other lesions. YERUHAM et al. (1993) described neoplastic lesions with the same characteristics in two goats, with perineal location, emphasizing that skin was pigmented and the possibility of UV effects more reduced. BABA (1995) reported a case of squamous cell carcinoma in sheep, located in the left thoracic side, caused by the application of hexachlorocyclohexane on a wound and exposure to solar radiation. The neoplasm developed slowly, but became generalized after several months by metastases in the majority of lymph nodes and in the lungs.

Vulvar cancers, in Ayrshire cattle, from Kenya, under the form of epizootics, diagnosed as squamous cell carcinomas, were reported by BURDIN [54]. About 3% of the livestock were affected, reaching 30%, and the neoplasm started with precancerous lesions, such as papilloma, hyperacanthosis, and hyperkeratosis [61].

Squamous cell carcinoma in the ventral abdominal region in the dog has epidemiologically proved to be an actinotumor. Tumors develop in skin areas with little or no pigmentation, following exposure to sunlight. The carcinoma develops as a multicentric neoplasm that is ulcerated, forms crusts and eschars. Microscopically, precancerous lesions appear, as well as epithelial hyperplasia, keratosis, and finally invasive carcinoma in situ and metastatic carcinoma, sometimes with scirrhoid reaction.

The morphology of squamous cell carcinoma is characterized by papillary growths of variable sizes, usually with a cauliflower-like appea-rance, firm and rough on palpation, of white-gray color, unclearly delimited, ulcerated and with red striae. At other times, the tumor develops as an ulceration covered by crusts, gradually becoming crateriform. In the case of the tip of the ear, in cats, the lesion starts as an erythema of the ear tip and margins, with progressive hair loss, desquamation, formation of crusts and ulcerations, with necroses of the ear structures (skin and cartilage).

Histologically, the neoplasm has a characteristic aspect, which facilitates a positive diagnosis. Hyperkeratotic, parakeratotic, ulcerative and acanthotic lesions are predominant. Neoplastic cells are easily recognized, having the characteristics of spinous cells, a polyhedral shape, vesicular nuclei and extremely prominent nucleoli. Cells are arranged irregularly or under the form of cords. Carcinomas with numerous large keratotic pearls and obvious cell differentiation with low mitotic index are considered to have a lower malignancy grade and slow evolution. Keratotic pearls are concentric arrangements of parakeratin layers and/or hyperkeratosis (Fig. 4.3–4.4). The high malignancy forms are composed of undifferentiated cells, large, round, atypical cells with eosinophilic cytoplasm, sometimes with multinucleate cells and a high mitotic index (Fig. 4.5–4.6).

Fig. 4.3

Squamous cell carcinoma. Islands and cords of malignant squamous cells surrounded by desmoplasia.

Fig. 4.5

Squamous cell carcinoma, poorly differentiated.

Fig. 4.6

Squamous cell carcinoma, poorly differentiated.

The periphery of neoplastic cell islands is always delimited by basal membrane and the presence of basal cells.

Laboratory studies have attempted to establish some criteria that could assess the malignancy grade, prognosis and optimal treatment for different evolution stages, forms and locations of squamous cell carcinoma.

BARRY and SHARKEY (1986) studied the possibility to establish a histological scale for the evaluation of squamous cell carcinoma malignancy. The authors used morphometric methods, assessing the keratinization degree and architectural aspects of the tumor. In this sense, the volume of neoplastic cells (basal cells, spinous cells and keratinized cells) was estimated, data being statistically processed. It should be mentioned that investigations were performed in 6 squamous cell carcinomas from affected individuals and from transplantations of these tumors in nude mice. The conclusion is that histological differentiation can be highly reliable and that the method is sensitive enough to be applied in the solution of practical problems of histological importance.

BABA and PRICĂ (1988) performed morphometric studies on squamous cell carcinomas in cattle. In all cases the nucleus/cytoplasm ratio was in favor of the nucleus; the mitotic index was two-fold higher in poorly keratinized or non-keratinized carcinomas, compared to intensely keratinized carcinomas; in the case of intensely keratinized carcinomas, mitoses were predominantly in the basal layer; nucleoli had larger sizes and were less numerous in carcinomas with a higher mitotic index, while nucleoli from carcinomas with a low mitotic index were smaller and more numerous. The conclusion is that one single parameter (nucleoli/nucleus, etc.) in the estimation of squamous cell carcinoma malignancy is not sufficient, and the morphometric assessment of several parameters is required in order to appreciate the malignancy grade.

Studies of a completely different nature, regarding the reaction of organisms with carcinomas, were performed by JUN and JOHNSON (1979 a, b). The authors found that in sheep with carcinomas, the blastogenic response of peripheral lymphocytes to phytomitogens and squamous cell carcinoma extracts decreases significantly with the increase in tumor maturity. The tumor avoids its own rejection by the direct or indirect formation of serum blastogenic inhibitor that causes the immunosuppression of T cell activity.

4.1.3. Papilloma and fibropapilloma

Papilloma and fibropapilloma are benign tumors, frequently found in cattle, horses and dogs, and exceptionally in cats; their etiology is recognized as being of viral nature, the papillomavirus, Papovaviridae family [1].

Papillomaviruses produce two types of lesions: squamous cell carcinoma and fibropapilloma, while intermediate lesions can also be found.

In the etiology of these tumors, in addition to papillomavirus infection, some co-carcinogenic factors are also incriminated, such as: genetic factors; immune depression; hormonal factors, sunlight (UV) exposure of pigmented skin; chemical synthesis substances, etc.

The incidence of these benign tumors is not generally influenced by sex and race; in cattle they may appear under the age of 2 years, and in horses more frequently between 1 and 2 years of age. Angora goats seem to be more sensitive, and in the Saanen breed papillomatosis is more frequent during the lactation period.

Papilloma and fibropapilloma in cattle are usually located in the head, neck, shoulders, neck folds, limbs and udder [89]. As it has been mentioned [46], viral etiology has been demonstrated, to which risk factors have been added. Tumor formations are multiple, they can be generalized in almost all body areas, having a typical papilloma appearance, of variable sizes, from 1–2 cm to large structures, which are rough, dense, cauliflower-like. Tumors are exophytic, sometimes with a large attachment base or pediculate.

The typical papilloma has a size of 1–2 cm, a filiform wart aspect, with exophytic growth, epidermal hyperplasia through the expansion of the spinous layer against a dermal structure. The cells of the spinous layer undergo hydropic ballooning of the cytoplasm that contains large eosinophilic keratohyaline granules (Fig. 4.8); nuclei are vesicular, containing viral particles, easily detectable by electron microscopy, and viral antigens detectable by immunofluorescence [35].

Fig. 4.8

Papilloma.

Fibropapilloma: characteristic lesions consist of hyperkeratosis, acanthosis and growth in depth under the form of epithelial growth in a massive dermal proliferation. Fibroblast cell proliferation is massive under the form of perpendicular fascicles. Frequently, epidermal proliferation is minimal, with slight acanthosis (Fig. 4.7.).

Fig. 4.7

Fibropapilloma.

Clinical and experimental studies have demonstrated that the development of one or the other tumor form, papilloma or fibropapilloma, depends on the anatomical location and histological structure, on the virus type, as well as on virus infection or inoculation, which can be superficial or deeper. In this way, the proliferative tissue reaction can be superficial (epidermal) or deep (dermal) [101].

Genital fibropapilloma is found in adult cattle, in females on the vulvar mucosa and in males on the penis. The virus is transmitted by breeding. Macroscopically, it starts with small formations that gradually reach 2–5 cm in diameter; since they are well vascularized, tumors bleed easily, and superinfections occur. Microscopic structure is characteristic of fibropapillomas with the deep penetration of the epithelium into the conjunctival vascular stroma [2].

Mammary gland papilloma appears with a multiple aspect, in adult cows, with two microscopic forms: fibropapilloma or papilloma and squamous cell papilloma [46].

Digestive tract papillomatosis is of viral nature, although feeding on fern is also incriminated [47], sometimes in association, or the transformation of benign forms into malignant ones, of carcinoma type. In an extensive study, from an area with enzootic hematuria, the disease was undoubtedly caused by the consumption of fern (Pteridium acquilinum), and digestive papillomatosis was sporadically present.

Digestive papillomas are located in the oral cavity, tongue, esophagus and rumen, in both young and adult cattle.

Fibropapilloma in the horse is of viral nature, and macro- and microscopic aspects are characteristic, having a pediculate or sessile appearance. It is commonly located in the lips, nose and legs, but it can also be found in the oral cavity, ocular area and genital system, at over 3 years of age. Proliferations usually are 2 to 10 cm in diameter, especially in solitary forms with ocular and genital location [93].

The literature reports skin papilloma cases in 1–5 day old foals, which suggests the possibility of papillomaviruses infection during intrauterine life [61].

Macroscopically, lesions are exophytic, circumscribed, horny masses and a cauliflower-like surface. Microscopically, the tissue reaction is that of squamous epithelium.

Canine papillomatosis, in the form of multiple skin proliferations, is most probably non-infectious. In multiple skin papillomatosis in dogs, canine papillomas can have the following locations: oral location, skin location, usually on the limbs, the posterior abdominal area and occasionally on the body; ocular locations, in the cornea and conjunctiva.

The oral location with typical lesions has been reported in dogs, coyotes and wolves. In internal genital organs, multiple papillomas have been described in males, after the administration of synthetic estrogens. Multiple oral papillomas are more common in young subjects, being located on the mucosa of the lips, gums, tongue and epiglottis. Papillomas develop relatively rapidly, as pediculate forms or in the depth of the mucosa, but sometimes they regress without completely disappearing. Eyelid papillomas may appear as juvenile papillomatosis or as a solitary lesion in old dogs.

Histologically, the surface of the oral papilloma is covered with stratified squamous epithelium, with acanthosis and hyperkeratosis. Spinous cells show obvious tonofibrils and cytoplasmic inclusion material in the cells of the granular layer. In the vesicular nucleus of spinous cells, intranuclear inclusions characteristic of papillomaviruses are identified by electron microscopy and immunofluorescence.

The skin location usually appears as a solitary tumor in old dogs, the cases of multiple cutaneous papillomatosis being more rare. Early lesions have a smooth surface, becoming rough in time, with deep crevasses. The literature does not mention the possibility of experimental transmission, although there is evidence that cutaneous papilloma in the dog is of viral nature.

The ocular location has been reported in both puppies and adult dogs. In tumor cells, viral particles have been evidenced, the lesion has been experimentally reproduced.

Papillomatosis in sheep and goats develops with locations in different body areas. In sheep, primary tumors occur in the head and ears, as papillomas and fibropapillomas, squamous cells become hypertrophied, forming horny skin plaques [1]. In goats, neoplasms appear both in pigmented and non-pigmented skin areas. They can be located in the head, neck, trunk and mammary gland, usually a multicentric form. In this species, viral etiology could not be demonstrated, but the action of solar rays seems to favor and/or induce proliferation. Clinical and epidemiological data demonstrate the evolution of papillomas and fibropapillomas into squamous cell carcinoma [61].

DIAGNOSIS in the case of papilloma or fibropapilloma is relatively easy to make, considering the somewhat specific locations for the different animals; in dogs, oral papillomas require differential diagnosis from melanotic nodules, by histological examination.

TREATMENT, for all species, will be surgical or electrosurgical, usually with good results. The possibility of the spontaneous disappearance of papillomas is considered. In dogs, the oral form involves the risk of the extension of the proliferation to the pharynx and larynx; consequently, electrosurgical excision is required. Vaccinations have very good results and autovaccination is recommended especially for cattle, dogs and horses. The vaccine is prepared from one part triturated tumor tissue in nine parts physiological serum, which is filtered and stored at 4°C. Inoculations are performed intradermally, in dogs 0.5–1 ml once a week, three treatments; in horses and cattle, intradermally as well, 1–5 ml once a week, three treatments being sufficient.

Autohemotherapy, in cattle with cutaneous papillomas, has promising results, the success rate being 70% in cows treated with 30 ml blood harvested and immediately inoculated subcutaneously, 3–4 injections at 7–10 day intervals [12].

Prognosis is favorable for skin infections in cattle, horses and dogs. In the case of goats, sheep and rabbits, papillomas or fibropapillomas may evolve into squamous cell carcinomas.

4.1.4. Infundibular keratinizing acanthoma (keratoacanthoma, intracutaneous cornifying epithelioma

This tumor is also known as scaly papilloma or intracutaneous cornifying epithelioma and has only been observed in dogs.

Incidence is higher around the age of 5 years, males being more frequently affected than females, and the breeds selected for hunting seem to be more sensitive, developing a generalized form.

The tumor consists in epidermal invagination, the keratoacanthoma has the appearance of a nodule, cyst, 1–3 cm in diameter, being located in the back, neck, thorax, more rarely in the limbs, head and tail. It usually occurs as a single tumor, but in some breeds it can also be multiple or in succession.

Microscopy shows a dermal cavity filled with keratin, which has an orifice towards the skin surface. The wall is formed by a thick layer of stratified and well differentiated scaly epithelium. Keratin appears as a concentric lamellar mass, with a keratotic pearl aspect. Some spinous cells are arranged as epithelial cords or islands in the keratin mass. The tumor formation exerts a compression force on the adjacent connective tissue, conferring an encapsulated appearance.

Microscopic diagnosis can be difficult due to certain similarities to squamous cell carcinoma (keratotic pearls).

Morphological characteristics

It should be mentioned that histological examination is the only one that can establish the malignant or benign nature of a melanic tumor, and that a certainty diagnosis is difficult to make, being subject to errors. Morphological data should be corroborated with history data regarding the time of appearance of the first melanic lesions, the evolution in time, the lymph node aspect, multiple tumor formations, the age and color of the animal, etc.

Primary melanotic tumors always originate in the skin. PET (pigmented extraepidermal tissue) mice have all body tissues pigmented, the embryonic musculature containing melanocytes. Although neoplastic extradermal tissue melanocytes have not been found in PET mice, it could be supposed that under certain conditions, extradermal melanocytes might become cancerous.

The presence of melanic tumor cells is histologically found in the epidermis, dermis and hypodermis. Tumor cells are grouped into clusters or columns, and sometimes they can invade tissues up to the underlying muscles. Tumor melanocytes multiplied in the dermis cause the disorganization of the basal layer, they compress the granular layer and induce the thickening of the horny layer, with the appearance of necrotic foci surrounded by inflammatory reaction. In advanced forms, the epidermis is disintegrated and ulcers occur.

Tumor melanocytes in the dermis are frequently grouped around vessels or nerves, into a collagen mass with fibroblasts and histiocytes. The intravascular presence of melanocytes is a sign of high malignancy. The melanin amount from melanocytes varies widely and does not represent a malignancy criterion.

Melanic tumor cells have either a fusiform appearance, similar to that of fibrosarcomas, or an epithelioid shape, mimicking undifferentiated carcinomas. Even in highly malignant melanomas, mitoses are less numerous, but giant cells can be noted.

Junctional melanic tumors are histologically characterized by an alteration of melanocytes or naevus cells, which are solitary or under the form of nests, with their obvious passage from the epidermis or pilose follicles to the dermis. Tumor cells have an extremely varied aspect and they are frequently recognized by their pigment content and their cluster or nest arrangement.

In the epidermis, melanocytes show a dark colored, dense nucleus and a clear cytoplasm. In the superficial area of the dermis they are cuboid or oval, similar to epithelioid cells. In the lower area of the dermis, cells are elongated, with a nucleus similar to fibroblasts. In general, the pigment content tends to diminish from the surface towards deeper areas. Junctional melanocytes are both a dermal and epidermal component, in an extremely variable proportion.

In some melanic tumors, almost all cells are situated at the dermoepidermal junction, while in others, their presence at the level of the junction is extremely scarce, almost all tumor cells being located in the dermis.

Dermal melanocytes, unlike Junctional ones, do not show changes and seem to be derived from the dermis. The majority of melanic dermal tumors can be classified as fibrous and cellular tumors, but some or them may also have a mixed structure. In the fibrous type, melanocytes are flat or dendritic, being grouped in irregular bundles mixed with fibroblasts, with an interlaced aspect. Cells are usually disposed parallel to the epidermis.

Compared to tumor cells, melanophages are present in a varied number, they are larger and contain rough melanin granules.

In dermal melanocytomas of cellular type, dense cell islands are found, with round or fusiform shapes. Larger islands are frequently subdivided into alveolar cell groups.

Melanomas, known by the old nomenclature as malignant melanomas, may develop from normal melanocytes from the epidermis and the buccal epithelium, as well as from dermal or junctional elements of melanocytes. In dermoepidermal junctional melanomas, a considerable junctional activity is noted, with a descending flow of anaplastic melanocytes form the epidermis to the dermis. Tumor cells frequently invade the upper epidermis, so that the epidermis disintegrates, inducing ulcerations. Reaching the dermis, tumor cells can manifest significant pleomorphism, having cuboid or fusiform shapes. Cells tend to form alveolar structures or irregularly ramified cords.

Melanomas, especially amelanotic ones, are formed by fusiform cells, resembling fibrosarcomas, while tumors formed by predominantly epithelial cells are similar to undifferentiated carcinomas. Mitoses in giant cell tumors, with a bizarre aspect, are not associated with a high malignancy grade (Fig. 4.26).

Fig. 4.26

Malignant melanoma, pleomorphic and giant cell type.

Malignant melanocytes contain variable melanin amounts, which can be evidenced by ammonium silver nitrate or, in fresh tumor tissue, the DOPA reaction may be performed. The melanin amount is not associated with the malignancy grade.

The diagnosis of a malignant tumor, a melanoma, compared to the benign form, the melanocytoma, is difficult, all the more so when prediction is involved. The difficulty consists in the differentiation of a junctional melanoma and the early recognition of the malignant transformation of the junctional melanocytoma. Another difficult problem is represented by diagnosis and the recognition of the malignant transformation of the dermal melanocytoma. The presence of mitoses and necrotic foci is indicative of malignancy. At least in the dog, fibrous dermal melanocytoma, blue naevus, has a malignant potential.

Melanomas can metastasize by blood and lymphatic route, regional lymph nodes usually being the first affected. Metastases frequently occur in the lungs.

The intraepidermal lentiginous melanocytoma is characterized by the presence of melanocytes that are located strictly intraepidermally, having the aspect of a pigment spot, without malignancy characteristics. However, malignant developments have been noted in humans.

The junctional naevus melanocytoma histologically presents melanocytes grouped in 3–20 cell nests, having a polygonal or round shape. Melanocytes are located in the epidermis, without exceeding the basal membrane.

The dermal and/or hypodermal melanocytoma of cellular or dermal naevus type has no junctional activity, a variable pigment content, and cells have a disorderly arrangement, being both melanocytes and melanophages. Mitoses are absent or completely sporadic. Tumor formations located in the chorion do not exceed the size of a nut (Fig. 4.17–4.20).

Fig. 4.17

Dermal melanocytoma, spindle cell type.

Fig. 4.18

Dermal melanocytoma, spindle cell type.

Fig. 4.19

Dermal melanocytoma, dendritic cell type.

Fig. 4.20

Dermal melanocytoma, round cell type.

The mixed, junctional and dermal melanoma, of mixed naevus type has the histological characteristics of the previous types, being considered as a benign tumor predisposed to malignization.

The epithelioid junctional melanoma is formed by cells similar to those of the carcinoma. Cells are grouped densely, resembling the medulla of the carcinoma or they can be arranged as lobules or trabeculae. The cytoplasm of melanocytes contains less pigment compared to other melanoma types, and nuclei are larger.

The dermal and/or hypodermal melanoma with fusiform cells is formed by typically bipolar fusiform cells, with variable sizes. Nuclei are elongated, with scarce chromatin, and nucleoli are obvious. The number of mitoses is greater than in other melanomas. Cells give the tumor a sarcomatous or fibromatous aspect, against a background of reticulin fibers (Fig. 4.21).

Fig. 4.21

Melanoma, intraepithelial nests with atypia and pleomorphism.

The dermal and/or hypodermal melanoma with dendritic and spiral cells is formed by cells with a dense, disorderly or band-like arrangement, with an extremely high melanin content. Cellular details can only be seen after discoloring. Cells have polyhedral or round shapes, being present in numerous small dendritic cells. Mitoses are not numerous, but necrotic foci are frequently found (Fig. 4.24).

Fig. 4.24

Melanoma, dendritic cell type.

The metastasizing mixed, junctional and dermal melanoma, with epithelioid and fusiform cells is frequently found on the buccal mucosa or skin. Microscopically, the presence of both cellular types is found, with the predominance of one type (Fig. 4.25).

Fig. 4.25

Malignant melanoma, epithelioid cell type.

The classification and histological description presented facilitates the identification of the morphological melanoma types, but the clinical aspect remains deficient, in particular prediction and prognosis. We consider valuable the use for diagnostic purposes and in veterinary oncology of the invasion level proposed by CLARK et al. (1969) and of the BRESLOW scale (1970), for the assessment of the malignancy grade according to the maximal thickness of the tumor invasion. For this purpose, we propose a scheme for the evaluation of the malignancy grade, adapted from CLARK (1969) and BRESLOW (1970):

Classification and assessment of the malignancy grade of melanic tumors, adapted from CLARK (1969) and BRESLOW (1970)

Usually, histological diagnosis does not pose special problems; rapid growth, metastases will be considered for malignant forms, especially in the lung and lymph nodes. Melanomas are frequently bordered by inflammatory reaction.

Amelanotic melanomas, as well as malignant forms with benign cells, pose diagnostic difficulties.

The following histological criteria are considered:

• – presence of dispersed and ungrouped melanic cells in the deep epidermal area and melanocyte pleomorphism;
• – sporadic presence of tumor cells in the superficial epidermis;
• – presence of inflammatory infiltrate, in the absence of traumas or infection and the presence of melanocytes in the lumen of lymphatic or blood vessels;
• – presence of abnormal mitoses and cytonuclear atypias;

Differential diagnosis is made with other more or less pigmented malignant skin tumors, such as epidermoid carcinomas and fibrosarcomas, but also with skin melanosis, brown or black tissue pigmentation.

ETIOLOGY

In animals, as well as in humans, the problem of the etiology of melanoma still remains at the stage of theories and suppositions, with controversies and certainties that are subsequently contradicted partially or totally. A multifactorial etiology may be admitted, in which there are favoring determining factors or risk factors.

Regardless of whether the neoplasm is initiated by chemical carcinogens or ultraviolet (UV) light, melanomas in all species share a similar biology in that they frequently recur and are predisposed to metastases to regional lymph nodes. Animal melanomas are common and provide a useful model for a deadly human disease. Initiation of as many as 65% of cutaneous melanomas in humans and in most of the melanomas diagnosed in Angora goats is thought to occur secondary to mutations generated by both UVA and UVB solar radiation.

In humans initiation within benign, the most important category of precursor lesion is the dysplastic nevus; malignant transformation of benign lesions is very uncommon in animals. Promoters can include chronic trauma, chemical exposure, burns, hormones, infections, drugs, and other causes for reactive hyperplasia [107].

The hereditary factor

The hereditary factor seems to be at least for some animal species (horses and swine) scientifically documented.

A phenomenon occurs in Lipizzaner horses that seems to be genetically programmed. At birth, foals of this breed have black hair and skin. With age, depigmentation of hair occurs, which becomes white-silver, and the skin remains totally or partially depigmented. When we refer to the color of the horse, we consider the color of hair, not of skin. Skin is usually black, but it can also have depigmented, pink or white spots, that are visible on the glabrous skin (mouth, perineum, etc.). The preferential areas for the development of melanomas will always be pigmented.

GEBHART and NIEBAUER (1977) and DESSER et al. (1980) maintain that depigmentation as well as the development of tumors in Lipizzaner horses are genetically determined. The color of hair is inherited according to Mendel's laws, the order of dominance being: white, bay, bluish-gray, brown, black and chestnut, but colors do not form an allelomorphic series. It has been noted that some lines of bluish-gray horses are more susceptible to develop the melanotic disease, with a more severe form, at a younger age (under 10 years), especially when consanguineous breeding has been practiced [58]. SEARLE [40] states that the "G" factor is responsible for the discoloring of animals, and the pleomorphic effect of this factor could also be responsible for the form of melanic tumors. The execution of this genetic program must be related to an activity or block of the hormonal, enzymatic, immunological or other regulating system. JIMBOW et al. [40] establish in Leghorn hens the genetic programming of death and the disappearance of pigment cells, which results in the completely white color of their feathers. This makes GEBHART and NIEBAUER (1977) affirm that something similar occurs in the case of Lipizzaner horses, in which melanocytes can no longer be identified histologically (DOPA reaction) or electron microscopically in depigmented spots. The place of these cells is taken over by cells whose significance is not yet known. The authors do not exclude the possibility that melanocyte migration deficiencies might occur in unpigmented spots.

The intervention of the genetic factor in the development and evolution of melanic tumors could be observed in Sinclair pigs. By evaluating the results obtained following an extensive study of a population of 349 pigs, HOOK et al. found in 1979, an increased incidence (54%) of melanomas in the newborns whose parents had both melanomas. By breeding a female without melanomas with a male with such tumors or a female with melanomas and a male without melanomas, the authors reported a decrease of up to 22% and 21%, respectively, in the incidence of melanomas in newborns. The pigs of parents without melanomas only had the lesion in 2% of the cases. These results suggest the existence of a transmissible factor from parents to offspring, in the appearance of melanic neoplasms. The incidence of melanomas in Sinclair pigs can be influenced by selection, which brings new information on the etiopathogenesis of melanic tumors in this species. New work hypotheses can be advanced that may contribute to the elucidation of the mystery that still surrounds the black neoplasm.

PET (pigmented extraepidermal tissue) mice manifest the pigmentation of all body tissues, melanocytes being found in the embryonic musculature. Although extradermal tissue melanocytes of PET mice have not been found under a cancerous form, it may be supposed that under certain circumstances, extradermal melanocytes can become cancerous.

Studies of the molecular base of carcinogenesis in melanomas have been oriented in three directions: activated oncogenes; cytogenetic analysis of chromosomal abnormalities; analysis of familial forms, with the research of the gene that can favor the appearance of uncontrolled melanocyte proliferation [18].

Oncogenes are capable of inducing malignant transformation features, after they are introduced in a melanocyte culture. Results suggest that in melanomas, the frequency of the activation of ras genes, detected by transformation experiments, is around 10%. Experiments performed with murine sarcoma viruses have shown that viral oncogenes induce malignant transformations in melanocytes.

Chromosomal alterations, such as translocations, deletions or variations in the number of chromosomes, have been noted in malignant melanocytes. Chromosomal abnormalities are found during the "vertical" growth of melanomas and especially in metastatic lesions. Researches have demonstrated that the "melanoma sensitivity" gene is related to the Rhesus blood group, situated on the short arm of chromosome 1. If an autosomal dominant gene is involved in familial melanoma and dysplastic naevus syndrome, its nature and location remain to be determined.

Genetically or environmentally initiated DNA instability facilitates subsequent neoplastic transformation. Human melanomas with p53 expression are common, 20–40% among all cases, and may have a different pathogenesis compared to those without p53. A canine case of multicentric melanocytoma was demonstrated to lack p53; p53 is not involved in equine melanoma tumorigenesis; most melanoma cases in humans lack detectable p53 mutations [107].

Expression of the apoptosis suppressor gene bcl-2 was detected in 16 of 18 human cutaneous melanomas examined by MORALES-DUCRET et al. 1995 (cite [107]) but the bcl-2 gene product was also found in resting melanocytes in normal skin. Atypical bcl-2 overexpression has been detected in human uveal melanoma.

Proto-oncogene mutation to oncogenes also favors proliferation and tumor development. Oncogenes detected in in vitro and in vivo melanoma studies include c-myc, c-erb-B-2, c-yes, c-kit and ras [107].

In the case of malignant melanomas, 5 to 10% can be considered familial. Familial melanoma frequently develops from a preexisting lesion, the dysplastic naevus, also present in other animal species. This lesion may correspond to an early stage of cancerization, a proliferation caused by chromosomal instability [22] or a genetic predisposition of melanocytes to proliferation or mutations under the effect of physical stimuli, such as ultraviolet light.

The hereditary component, as it has been demonstrated in some pathological cases, needs the intervention of an ecological factor for the clinical expression of the disease. The existence of the hereditary pathological potential allows the understanding of particular individual reactions to the aggressive action of ecological factors.

The hormonal factor

In the first place, sex hormones in animals, contrary to observations in humans, do not seem to have a determining or triggering role in the onset of melanomas. The authors who have described melanomas in horses, swine and dogs do not remark differences regarding frequency, evolution and prognosis between males and females.

Hypophyseal hormones, as well as pineal gland melatonin, have not been proved to play a role in the triggering and development of melanomas.

Irritating factors, of chemical nature, have been successfully used in the genesis of some neoplasms. In hamsters, skin melanomas have been induced by repeated paintings with carcinogenic substances.

Low intensity, long duration traumas, such as harness sores, have been incriminated in the etiology of skin melanomas, but the involvement of this factor has not been proved with certainty. It rather seems that skin melanomas become, as a result of their development, easily irritable sites, which subsequently ulcerate. The possibility that traumas subsequent to the development of tumors might favor the dissemination and onset of secondary neoplasms is not excluded.

Congenital neural crest abnormalities have been made responsible for the production of polymorphic dysplasias. LEVENE (1980) points out that melanomas in horses are located in certain body areas, such as the area of the head, neck, parotid glands and rhomboid muscles. The neural crest is the origin of pigment cells, autonomous and sensory ganglia of the peripheral nervous system, with supporting cells of the APUD cell system, the head and neck mesenchyma and the skull. The author maintains that following neural crest abnormalities, melanocytic ectopias would be initiated, which would subsequently become proliferation sites of melanic neoplasms. The intervention of the diffuse endocrine system in the development of melanocytes, sometimes with uncontrolled proliferations, may be supported.

The melanin production imbalance has been incriminated in the genesis of large melanin deposits by macrophagy or histiocytic infiltration [58]. There is a similarity to lipid storage diseases or Gaucher's disease, some forms of cellular myeloblastoma, xanthoma and hypomelanic reticulosis. Practical observation and the great number of cases made JAGER affirm as early as 1909 [58] that melanosarcomatosis in bluish-gray horses could be the manifestation of a metabolic abnormality. Current knowledge allows us, if this supposition is accepted, to consider the possibility of an inborn error of melanocytic cell metabolism. The role of the hereditary factor in the induction of melanotic neoplasms is rediscussed.

The blue naevus in animals is nothing else than a deposit of melanic pigment seen through the transparency of the dermis and epidermis. Comparative studies with what is known of humans have been performed by LEVENE (1980), who mentions the larger sizes and more intense pigmentation of the blue naevus in animals. The blue naevus has been observed more frequently in Terrier dogs and especially in fibrotic forms. Fibrotic pigmentary tumors are more frequently found in dogs and cats.

Horses that change their hair color, becoming white or gray, have a tendency to develop melanotic spots as blue naevi, which can subsequently become cancerous.

The brief review of some possible etiological factors of melanomas in animals includes no reference to the existing theories that are accepted in the etiology of cancer disease in general and that can also be adopted for melanomas.

Melanic neoplasms, like other diseases, seem to be generated by a complex of factors, on a favorable terrain. In this case, risk factors may be involved, which include both ecological conditions and the terrain, the organism with its genetic patrimony, more or less favorable to the onset and evolution of a neoplasm.

Epidemiological data on melanic tumors in animals

Epidemiological data reveal the presence of melanic tumors in all vertebrate species (mammals, birds, fish). Melanic tumors are commonly found in dogs, horses and some breeds of pigs. Incidence is lower in cattle and goats and extremely rare in cats.

In dogs, according to various bibliographic sources, melanic tumors represent between 4 and 7% of all tumors and between 9 and 20% of all skin tumors. The highest incidence is in dogs aged between 7–14 years and more frequently in Airdale Terrier, Cocker Spaniel, Scottish Terrier, Boxer and Boston Terrier breeds. Incidence is in general increased in dogs with pigmented skin. It seems that in males, incidence is higher, compared to females. Tumors are usually solitary, located on the skin or the buccal mucosa. Approximately 90% of oral tumors are malignant, while skin tumors are benign.

In a study over 40 years (1953–1974), by analyzing 3388 tumors from dogs, BASTIANELLO (1983) established the following locations: mesenchymal tissue, 33.7%; skin and skin adnexae 10.8%; female genital tract 10.2%; lymphohematopoietic tissue 8.9% and male genital tract 5.8%. The mastocytoma was the most common tumor type, with a frequency of 12.7% of all tumors, followed in decreasing order by lymphosarcoma, melanoma, squamous cell carcinoma, basalioma, hemangiosarcoma and histiocytoma. The main skin tumors were: basaliomas, squamous cell carcinomas, perianal gland tumors and melanomas. Over 80% of the tumors of the female genital tract were breast neoplasms, while the main tumors of the male genital tract were located in the testis, of which Sertoli cell neoplasms were the most frequent. The majority of digestive tract neoplasms were found in the oral cavity, where most of them were inflammatory epulis and melanomas. Osteosarcomas, neurofibromas and thyroid carcinomas were the most frequent neoplasms of the skeleton, nervous system and endocrine system, respectively, and pulmonary adenocarcinomas, melanomas and cholangiocarcinomas were the tumors of the lung, eye and liver.

The same author [14] found in a total number of 243 neoplasms from cats the following frequency of locations: skin, lymphoid tissue, digestive tract and genital tract, representing 76.6% of all tumors. Squamous cell carcinoma represented 26.7% of the 243 neoplasms analyzed, being the most frequent type of skin tumor, followed by basalioma, mastocytoma and melanoma. The digestive tract was involved with 13.5% of all tumors, with the following types: squamous cell carcinoma, lymphosarcoma and intestinal adenocarcinoma. Mammary gland tumors represented 9.5% of all tumors, and of these 61% were carcinomas, especially skin carcinomas, followed by cutaneous and ocular melanoma, and osteoma, hepatocellular carcinoma and hemangiocarcinoma.

In cats, melanic tumors are extremely rare, representing less than 1 % of all tumors and approximately 2% of all skin tumors. In this species, the following order of tumor frequency is noted: squamous cell carcinoma, lymphosarcoma and breast carcinoma.

In horses, these tumors represent 6–15% of skin tumors. A relation between the incidence of these tumors and the gray color is found, although horses of other colors are occasionally affected. There is an increased tendency to the development of melanic tumors in gray horses with age; approximately 80% of gray horses over 15 years old develop skin melanic growths that are clinically detectable.

In pigs, melanic tumors have been found in 3–5% of the killed pigs, and in some breeds incidence exceeds 20%. Melanocytic tumors occur congenitally or at a very young age and almost exclusively in the Duro-Jersey breed, in dwarf Sinclair and Harmel pigs.

In cattle, melanic tumors represent less than 2% of all tumors, with the mention that the literature reports a high incidence, between 17 and 24% of all skin neoplasms.

4.8.1. Horse melanoma (equine melanotic disease, EMD)

The first report of a horse melanoma belongs to GOHIER, in an extensive veterinary surgery study, published in Lyon in 1813 [58]. The author described in detail a fact of the natural history of EMD. A white stallion, of a rare beauty, was bred with a great number of mares, the majority of foals inheriting the color of their father. Over the next years, black perianal formations appeared in this stallion. In time, nodular formations extended, including the scrotum and prepuce. Nodules gradually increased their volume, becoming confluent, their development accelerated and the stallion soon died. The author underscored the fact that the disease was only transmitted to white offspring, while dark color horses and their offspring were not affected. GOHIER gave an extensive and detailed description of clinical signs and lesions found on necropsy. The attempt to induce the disease experimentally, by subcutaneous inoculations of tumor triturates in bay and light bay horses, in mules and dogs, was not successful. The author showed that the disease was not contagious, since it was not transmitted by copulation when some of the partners had melanomas on their genital organs. The idea of hereditary transmission was advanced, and the castration of white or gray stallions was proposed.

4.8.2. Dog melanoma (canine melanotic disease, CMD)

The first part of this chapter has reviewed the results obtained by WEISS and FRESE (1974) following an extensive study performed on 700 melanomas from dogs, as well as the classification proposed by these authors.

The incidence of melanomas seems to be higher in dogs with intensely pigmented skin. Tumor formations in CMD are usually cutaneous, being located on the oral mucosa and intraocularly. Cutaneous locations at the level of distal limb extremities and on the scrotum usually have a malignant evolution. According to BASTIANELLO (1983), the melanoma occupies the third place in terms of incidence, after the mastocytoma and lymphosarcoma.

COTCHIN, 1955, and GOLDSCHMIDT and SHOFER, 1992 (cite [107]) established: melanoma is relatively common in dogs, accounting for 3% of all neoplasms and up to 7%; the most frequently affected sites are the oral cavity (56%), lip (23%), skin (11%), and digit (8%), with other sites, including the eye, accounting for only 2%.

Cutaneous melanoma occurs more commonly in dogs with heavily pigmented skin, with the Miniature Schnauzer, Standard, Schnauzer, and Scottish Terrier at increased risk of developing neoplasms. Most dogs with melanoma are more than 10 years old with a range of 1–7 years; an increased frequency has been described in male dogs [107].

By examining 63 dogs with oral melanomas (32 males, 28 females and 3 castrated females), HARVEY et al. (1981) established the following locations: 38 gingival, 12 labial, 8 palatal and 5 lingual. Following clinical, radiological and necroptic examination, the authors mentioned that in 45 of the 63 dogs (71 %), the cause of death was melanoma. All melanotic tumors of the oral cavity are considered as malignant, whether they present metastases or not. This is so because they have the histological characteristics of malignant tumors, and evolution is also malignant, with metastases, generalization and death. Melanomas in dogs, in any location, have an unfavorable prognosis. Even in early surgical interventions, this neoplasm produces recurrences, lymph node, pulmonary metastases, and death. Surgery is the main treatment method of dogs with melanoma [62]. In conclusion, no correlation can be established between the prognosis of dogs with melanoma located in the oral cavity and the following parameters: the microscopic aspect of the tumor, the mitotic index and the degree of pigmentation, on the one hand, and early surgery, sex of the dogs, intraoral location, volume and type of surgery (removal by scalpel using the cryosurgical method), on the other hand. Survival in the case of 7 untreated dogs was 65 days compared to 242 days for dogs submitted to surgery.

Regardless of location, the following histological types can be diagnosed in CMD: junctional melanoma; cellular and fibromatous dermal melanocytoma; epithelioid melanoma; type A and B fusocellular melanoma; mixed melanoma and dendritic melanoma. In humans, osteogenic melanomas have been described, in dogs one case has been reported, with gingival location, with a microscopic nest or lobule arrangement of obviously anaplastic cells, containing melanin, and in the stroma, abundant osteoid [23].

Eyelid and conjunctival melanocytomas do not differ from skin melanocytomas.

Intraocular melanoma in dogs originates either in the iris or in the ciliary body, unlike in humans, in whom tumor growth starts in the choroid. At the level of the ocular globe, melanomas can have intraocular or epibulbar locations. Intraocular melanocytomas in dogs are exceptional. Intraocular melanoma is histologically characterized by the proliferation of fusiform, epithelioid, dendritic and mixed cells.

Mixed melanoma [51] seems to be the most frequent. RYAN and DITERS (1984) found the following incidence of ocular melanomas in dogs: melanomas at the level of the sclerocorneal limbus 22.5%; fusocellular intraocular melanomas, type A 30%; fusocellular intraocular melanomas, type B 22.5%; mixed intraocular melanomas 17.5%.

Epidemiological researches have revealed a higher incidence of ocular melanomas with distant metastases in the German Shepherd breed, especially at an advanced age [32, 38, 81]. Anterior uveal melanoma is the most frequent primary intraocular tumor in dogs. These primary tumors induce metastases in the kidney and the lung. Amelanotic melanomas with fusiform cells have been diagnosed [71].

A simplified classification of uveal melanomas in dogs is proposed - benign forms called melanocytes, and malignant forms called melanomas [99]. In general, uveal neoplasms are white-gray or black. They have a diffuse growth pattern, occupying most of the inside of the ocular globe. Polygonal cells are large, with abundant and pale cytoplasm, and fusiform cells are arranged in spirals or are interlaced, being usually stratified. Hematogenic metastases are frequent.

The iridociliary location in dogs appears as a primary malignant tumor. Macroscopically, cells have various shapes, from small nodules to masses that fill the ocular globe. Melanocytomas are well differentiated, with cuboid or columnar cells, with rare mitoses. Melanomas are formed by pleomorphic polygonal or fusiform, columnar cells, with frequent mitoses. Hematogenic metastases are rare and are usually pulmonary.

The clinical and histopathological study of 14 dogs with intraocular melanomas, performed by BUSSANICH et al. (1987) revealed intense pigmentation and extensive necrosis in the tumor mass. The authors established a higher incidence in dogs aged between 6 and 15 years, with a mean age of 10 years, mentioning that there were 9 females and 5 males, the left eye being involved in 8 cases, and the right eye in 6 cases.

Clinically, the inflammation of the anterior segment of the ocular globe was found in 10 dogs. In one dog, necropsy revealed metastases in the lung, liver and kidney.

Tumors were located in the ciliary body, in the iris, frequently filling and distorting the anterior segment of the ocular globe.

Histologically, 3 types of tumor cells were identified: round, fusiform and oval.

Round cells. The authors identified, in all 14 cases, round cells, which sometimes were dominant. Round cells presented numerous melanin granules, uniformly dispersed in the cytoplasm. Nuclei were pyknotic and slightly eccentric, and mitoses were practically absent. Electron microscopy showed small, irregular nuclei, with rough chromatin, and premelanosomes and mature melanosomes were present in the cytoplasm.

Fusiform cells formed a small tumor mass, nuclei were fusiform, nucleoli were difficult to detect, and the cytoplasm was abundant and less pigmented. Electron microscopy showed small, elongated nuclei, with rough chromatin, but with few melanosomes.

Ovoid cells presented the widest variation in number. Thus, in some tumors they were dominant, in others, they were virtually absent. Ovoid cells were well circumscribed in an obvious cytoplasmic membrane. Nuclei were centrally arranged, with a marginal chromatic mass, vesicular nuclei, and abundant cytoplasm with few melanin granules. Electron microscopically, ovoid cells had the largest, prominent nuclei, with fine, uniformly dispersed chromatin. In the cytoplasm, the rough endoplasmic reticulum and free ribosomes were abundant, but melanin granules were scarce.

Based on the analyzed material, the cited authors propose a classification of melanic tumors depending on the identification of the three types of cells: tumors with predominantly round cells, tumors with predominantly fusiform cells, and tumors with predominantly oval cells.

By analyzing the clinical evolution and morphological peculiarities, a similarity is found between melanic tumors with predominantly round cells in dogs and human melanocytomas. Regardless of the histological aspect, canine melanotic tumors are less aggressive than human tumors.

Limboscleral melanomas appear as slightly bulging, highly pigmented structures, situated between the sclera and the limbus. The duplication of the conjunctiva and corneal erosions are evident. Two cellular types have been histologically differentiated: small fusiform cells, with margins that are difficult to differentiate, and large, round, pigmented cells. Neoplastic cells have been constantly identified at the level of the sclerocorneal junction.

Type A fusocellular melanomas were clinically expressed by hydrophthalmia, pigmented tumor mass, identified following enucleation as anterior uveal melanoma. Histologically, the dominant population was formed by large, round cells, with fine pigment granules arranged uniformly in the cytoplasm, nuclei were round and vesicular, chromatin and nucleoli prominent. These pigmentary cells were numerous at the periphery of the tumor mass. Fusiform cells had oval, fusiform nuclei, cytoplasmic borders were poorly delimited. Mitotic images were rare.

Type B fusocellular melanomas clinically manifested by massive tumor growths, corneal edemas and hydrophthalmia. In all studied cases, the anterior tract was affected, and in some cases the tumor extended through the sclera towards the musculature of the ocular globe. Histologically, type B tumors were similar to type A tumors, except for larger and more pleomorphic type B fusiform cells, with round to oval nuclei and obvious nucleoli. Cytoplasmic borders were poorly delimited. The mitotic index was low. Round, large, pigmented cells were less numerous than fusiform cells. Type B melanomas were less pigmented than type A melanomas.

Epithelioid melanomas had no melanic pigment. The neoplasm included the iris, the ciliary body and the choroid. It sometimes extended to the muscles of the ocular globe, and in one case, hepatic, adrenal, pulmonary, myocardial and ganglionic metastases were found. The tumor extended along the optical nerve, up to the optic chiasma. Microscopically, the tumor was formed by large, round cells, the cytoplasm was abundant and eosinophilic. Nuclei were large and oval, nucleoli were evident and in numerous cells, they were multiple. Multinucleate cells were present, and the mitotic index was high [84].

Mixed cell melanomas were characterized by the constant involvement of the choroid. In one case, the anterior eye chamber was filled with a pigmented tumor mass. This case showed hepatic, splenic, pulmonary, renal and ganglionic metastases. Mixed melanomas were formed by round, large pigmented cells, epithelioid cells and fusiform cells. The most numerous were fusiform cells, and the less numerous large round cells. Epithelioid cells were grouped or dispersed in the tumor mass. The mitotic index was situated between that of pure epithelioid melanomas and fusiform melanomas.

Following the study performed, RYAN and DITERS (1984) estimated that no strict correlation could be established between the development and frequency of ocular melanomas in dogs, on the one hand, and sex, breed and age, on the other hand. However, the authors found the following: females were affected in 61% of cases; the German Shepherd breed had a three-fold incidence, and the Boxer breed a two-fold incidence compared to a control group; the German Shepherd breed was more frequently affected by type A fusocellular melanoma, located at the level of the sclerocorneal limbus; in the Boxer breed, melanoma was of mixed type.

The metastatic dissemination of ocular melanomas in dogs takes place by hematogenic route, and locally, dissemination occurs along vascular and nerve branches. The risk of metastasizing is highest in mixed and epithelioid melanomas. Type A and B fusocellular melanomas are less aggressive, so that prognosis will be more favorable compared to mixed and epithelioid melanomas.

The treatment applied in the case of intraocular melanomas is the enucleation of the ocular globe.

Epibulbar melanomas in dogs are generally located at the level of the sclerocorneal limbus. The cases presented by MARTIN (1981) showed that epibulbar melanomas in dogs originated exclusively in the sclerocorneal limbus; intraocular extension, located on the surface of the ciliary body, was noted in one case. In man, corneal invasion develops between the epithelium and the Bowman's membrane, and it tends to remain superficial, rarely penetrating the ocular globe. The invasion of the cornea in the dog was deeper or approximately up to the mid-stroma, probably involving a deeper origin of the tumor.

Epibulbar melanomas have a favorable prognosis if surgery is performed early. Lamellar keratectomy is preferred to the enucleation of the ocular globe, in melanomas of the sclerocorneal limbus. The author noted that in younger animals, aged between 2 and 4 years, the development of the tumor was more rapid, compared to 11 year-old subjects, in which slow evolution, with stationary states manifested. It is recommended that the graft should be used through all the thickness of the cornea and sclera, in order to avoid the dissection and dilaceration of the neoplasm. Based on the reported cases, the cited author considers prognosis favorable not only for survival, but also for the maintenance of eye function in younger animals, while old animals will only be monitored for signs of neoplastic development.

Feline melanoma is uncommon, accounting for less than 1 % of all feline oral neoplasms, and approximately 0.5% of feline skin tumors. The ocular and cutaneous forms are generally more common than the melanoma of the oral cavity. The most common cutaneous sites are the head, tail, distal extremities, and lumbar area. The prognosis is generally poor because of recurrence and regional metastasis in up to half the cases [107].

4.8.3. Pig melanoma (swine melanotic disease, SMD)

Melanic pigmentations have been described in the cerebellum of Hampshire pigs. The presence of melanocytes has been mentioned in the pia mater, the arachnoid, in and around vessel walls, in the presented cases melanosis being of genetic nature.

The literature before 1964–1966 considered SMD as completely sporadic and studies were limited to the presentation of isolated cases. The appearance of new pig breeds, their cross-breeding and the creation by selection of the laboratory dwarf pig has led to a sudden increase in the SMD incidence and has implicitly broadened the range of this field.

In 1965, STRAFUSS et al. [70] described the first cases of cutaneous melanomas in dwarf Sinclair swine. Subsequently, the dwarf Sinclair pig became the object of extensive researches, macro- and microscopic studies [63, 70], selective and genetic reproduction studies [45], intended to clarify SMD. In Romania, BABA et al. (1981) reported a case of malignant cutaneous melanoma in a Bazna pig. In this case, melanocytes were numerous in the papillary dermis, with the dislocation of the basal epidermal layer; in the peritumoral hypodermis there was a lymphohistiocytic reaction. Dendritic melanocytes were arranged as nests, surrounded by fusiform cells and with numerous melanic granules in the cytoplasm.

Potentially malignant melanomas were described, by NOMURA et al. (1974), in Duroc x Hampshire crossbred pigs.

The studies performed on Sinclair swine aimed to macroscopically examine a great number of subjects, to perform optical microscopic and electron microscopic examinations [69]. The authors mentioned that the benign exophytic tumor was formed by dendritic melanocytes, similar to those of human naevus. Melanophages were easy to detect ultrastructurally. In pulmonary melanomas, melanocytes that contained atypical forms of melanosomes and mitochondria were identified. There were lysosomes without melanosomes, while some phagolysosomes showed different stages of melanosome degradation. The authors mentioned that benign exophytic forms were similar to type B KLUG melanoma. Malignant tumors were similar to type A melanoma, with numerous nuclear changes, incomplete and atypical melanization.

MILLIKAN et al. (1973; 1974), as well as MANNING et al. (1974), identified several developmental stages of cutaneous lesions: smooth spots, fine nodules, red tumor and regressing tumor, which could coexist in the same individual.

SMD is more frequent in pig breeds with pigmented skin (Duroc, Jersey, Bazna, etc.) and their crossbreeds. The presence of exophytic melanomas at birth is surprising, which demonstrates their histogenesis during intrauterine life. The involution and complete regression of pigmentary cutaneous lesions has been noted even in intrauterine life [69].

HOOK et al. (1979) demonstrated the influence of selective reproduction on melanomas in Sinclair pigs. The incidence of melanomas was highest in offspring whose both parents had tumors. In offspring with only one parent affected, incidence was higher compared to descendants of normal parents and lower compared to the first category. The cited authors remarked the absence of the influence of a strictly maternal factor or of a sexual component. Results demonstrated that only 24.5% of cutaneous melanomas were present at birth and that 75.5% developed later. This observation does not exclude the possibility that melanomas might have derived from very small, histologically undetected melanocytic lesions, especially that the pigs included in the study were black.

Controlled breeding and the early observation of a high number of Sinclair pigs allowed the cited authors to reach some conclusions regarding the SMD incidence and evolution. The high incidence of melanomas in Sinclair swine is also influenced by a hereditary component, which is also found in the case of human malignant familial melanoma, where the hereditary factor would act as either a polygenic factor, or with the involvement of an autosomal gene.

One of the most interesting observations was that with the advancement of age, regressions occur, which can result in the disappearance of cutaneous melanomas. It is known that human melanoma is one of the few tumors that can regress and, although the regression mechanism is not known, certain serum or plasma factors are associated with melanoma regression in vivo and in vitro. In both human and swine tumors, regression has been associated with a high number of tumor-infiltrative lymphocytes. These lymphocytes have been identified as CD8+ cytotoxic T cells and display granulated lymphocyte morphology. These are the lymphocytes that react to Melan-A/MART (melanoma antigen recognized by T cells), a cytoplasmic marker that is weaker or not evident in the population of melanoma cells remaining after partial remission of human melanomas [107]. The observations on SMD in dwarf Sinclair pigs suggest the possibility of controlling and fighting the development of melanoma, a phenomenon acquired with age. The findings and suppositions in SMD increase the attraction of the study of this type of tumor, regarding host-tumor interactions during the development, growth and regression of melanoma.

SMD in Sinclair swine and the elucidation of the genetic mechanisms involved in the transmission and regression of neoplasms need to be further studied, making this unique model extremely attractive, through the possibilities offered by the study of melanoma.

4.8.4. Ruminant melanoma (Ruminant melanotic disease, RMD)

Melanoma in cattle, sheep and goats has been less studied, probably due to the low incidence of this neoplasm in ruminants. Cattle melanomas represent about 5% of the cases of tumors found in this species, and of all cases, 92% are benign. The melanoma cases found in young cattle made COTCHIN (1972) assume that this is a congenital disease. The author noted a higher incidence of RMD in Aberden Angus cattle. Congenital melanomas in calves are more rarely reported in the literature. These tumors have no preferential location, they can be found as naevi or pediculate tumors (trunk, limbs, jaws, abdomen) [68].

MILLER et al. (1995) reported two cases of neuroectodermal melanoma, which, like in children, is characterized by a preferential maxillomandibular location and which contained a high number of melanic cells and "like" cells surrounded by connective tissue. LAMBOTTE et al. (1989) described the first case of congenital melanoma in a Blanc-Blue-Beige calf, mentioning that congenital melanoma in calves has no preferential location. It can be found as a naevus or pediculate tumor. In a newborn calf, the authors described a pediculate, black, glabrous tumor formation of 4.5 kg in the thorax, in the upper posterior region of the shoulder. Six months after resection, the tumor did not relapse. Histological findings included: hyperkeratosis, the spinous layer presented vacuolized cells, with large intracellular spaces, acanthosis and spongiosis. In the proximity of the epidermis, there was an intensely colored area, with cells overloaded with melanin. In the depth of the tumor, cells were polyhedral, with variable melanin amounts, arranged as cords or disorderly. The authors established the diagnosis of benign cell melanoma.

BOURGOIN (1972) noted a predisposition to malignant melanocytoma in the African Angora goat. KIRCHER et al. (1974) reported a case of malignant melanoma in the mandibular sinus of a sheep.

In a synthesis study, NOBEL et al. (1979) found of 709 cases of neoplasms in domestic animals, 5 melanomas: 2 in horses, 2 in dogs and 1 in cattle.

DRIEUX and PRAT (1968) described a melanoma case in a cow killed in a slaughter house. History showed the absence of particular clinical signs, just a slight hesitation to rise from the decubitus position. At the post mortem examination, sternebrae and the body of dorsal and lumbar vertebrae were black; small gray-reddish areas appeared in the cross section. The hypophysis had the size of a nut, with intense black pigmentation, and its consistency was higher than normal.

Traces of the glomerular area and some cords from the fasciculate area of the adrenal gland were histologically identified. The rest of the tissue was neoplastic. Two types of cells were identified: some fusiform cells with small nuclei containing scarce chromatin, arranged as a palisade, and large cells, with irregular arrangements, clearly delimited cytoplasm and large nuclei. Both cellular types contained intracytoplasmatic melanin granules. The neuronal and glandular portions were impossible to identify in the hypophysis. The development of fasciculated tissue with numerous necrotic foci predominated in the hypophysis. Fusiform bundle cells dissociated and dislocated the hypophyseal cords and acini. Fusiform cells were loaded with melanin. Among acidophilic glandular cells, there were voluminous cell elements, isolated or grouped into small clusters, with the cytoplasm loaded with melanin granules. Based on the histological description, the presence of a mixed melanoma may be assumed, in which fusiform cells were dominant, but epithelioid cells were also present.

In another cow killed in a slaughter house, BABA et al. (1983) reported a malignant melanoma located at the level of the cerebellum. The neoplasm was formed by tumor masses with the size of an orange, intense black color and dense elastic consistence. The subject also presented black tumor formations in the subcutaneous connective tissue and myocardial tissue. Histologically, the cerebellar melanoma was formed by fusiform cells and round cells in the cytoplasm of which there were numerous melanic pigment granules. Thorough microscopic examination could only be performed after the bleaching of the sections. Malignant melanomas in domestic ruminants are, as it has been mentioned, neoplasms that are more rarely reported in these species. The short life and killing for economic reasons could explain the lack of data in this field.

4.8.5. Experimental, diagnostic and therapeutic considerations