A benign tumor arising from melanocytes. These cells may be in the epidermis, dermis, or adnexa, primarily the hair follicles

Gross Appearance:
•Intradermal mass 
•Usually covered by hyperpigmented epidermis 
•May extend through panniculus adiposus into the subcutis 
•Black, but degree of pigmentation is variable.

•Morphology of the cells which makeup these tumors is very variable 
•Many cell types may be found within a single mass 
•Amount of pigment within the cell cytoplasm is variable 
•Some cells may totally lack melanin granules 
•Cell morphology varies:
    small spindle cell
    large spindle cells 
    epithelioid cells
    polygonal cells 
    round cells
•Large amount of melanin within the cytoplasm that obscure the nucleus 
•Intraepithelial component:
    consists of nests of tumor cells in the lower layers
    nests of tumor cells in the follicle wall
•Dermal component is quite variable: 
    tumors may display a neuroidal differentiation
    spindle cells within an extensive collagenous matrix
    sheets of cells that are round to epithelioid in morphology and are highly melanized
•Dermal melanocytomas have no association with the overlying epidermis
•Others are dermal in the late stages of tumor growth in which epidermal component is no longer present
•Requires step-sectioning to find the epidermal component
•Uncommon variant is the balloon-cell melanocytoma:
    composed of large round and epithelioid cells
    abundant pale finely granular to foamy cytoplasm
    melanin granules are difficult to find
•Differentiation of melanocytoma from malignant melanoma can be very difficult
    presence of nests of neoplastic melanocytes in the upper epidermis
    infiltrative dermal or subcutaneous component
    in the dog but not in the cat finding more than 3 mitoses per 10 high-power fields is usually indicative of malignancy
    presence of lymphatic or vascular invasion is the best indicator of malignancy

Good following surgical excision

               Sex Distribution (N=2822)

Sex Status














(normal sex distribution)

Anatomic Location (N=2596)


Head  39.6
Forelimb  17.3
Hindlimb  10.4
Thorax  8.0
Abdomen  6.6
Back  6.0
Multiple  4.3
Neck  3.6
Perineum  1.9
Scrotum 1.3
Tail  1.0

Copyright © 2000 - University of Pennsylvania School of Veterinary Medicine
Faculty: Michael H. Goldschmidt, BVMS & Patricia Mcmanus,VMD, PhD
Student: Kyle Goldschmidt