Michael H. Goldschmidt, MSc, BVMS, MRCVS, Diplomate ACVP Professor and Head, Laboratory of Pathology and Toxicology Chief, Surgical Pathology Department of Pathobiology
Frances S. Shofer, PhD, Adjunct Associate Professor of Epidemiology and Biostatistics
Introduction
Melanocytes are derived from neural crest cells. Precursor cells migrate from the neural crest to the epidermis in utero. The melanocytes develop dendritic cell processes and produce intracytoplasmic pigment granules. They send their dendritic processes up in between the keratinocytes and transfer the melanin to the keratinocytes.
Normal melanocytes inhabit the basal layer of the skin and the bulb of the hair follicle.
The two terms below are used to classify some of the different types of melanomas.
Junctional activity: the proliferation of nests or th èques (aggregations) of melanocytes along the dermo-epidermal junction.
Compound tumor: indicates that there is a junctional and an intradermal component to the tumor.
Melanocytic neoplasms can either be benign or malignant. To distinguish benign from malignant in dogs the following should be evaluated:
- the anatomic location of the tumor
- the cytologic features
- mitotic activity
- cell morphology
All histopathologic sections should be bleached prior to evaluation as the nuclear morphology may be obscured by the intracytoplasmic melanin granules.
Melanocytoma
Introduction
Definition: A benign tumor arising from melanocytes either in the epidermis, dermis, or adnexa, primarily the external root sheath of the hair follicles.
Synonyms: Cutaneous Melanoma, Dermal Melanoma, Benign Melanoma
Epidemiology
| Sex | N | Percent |
| Female |
637
|
17% (21%)
|
| Female Spayed |
1254
|
32% (33%)
|
| Male |
1084
|
28% (25%)
|
| Male Castrated |
881
|
23% (21%)
|
| ( Normal Population %) |
|
|
| Breeds at Increased Risk |
N | Probability | OR | 95% Confidence Interval |
|
| Vizsla |
70
|
<0.0001
|
7.5
|
5.83
|
9.61
|
| Irish Terrier |
7
|
0.0002
|
6.7
|
3.06
|
14.52
|
| Miniature Schnauzer |
306
|
<0.0001
|
6.4
|
5.67
|
7.23
|
| Standard Schnauzer |
137
|
<0.0001
|
5.0
|
4.21
|
6.00
|
| Bedlington Terrier |
6
|
0.0021
|
4.9
|
2.12
|
11.13
|
| Australian Terrier |
8
|
0.0016
|
3.9
|
1.89
|
7.89
|
| Silky Terrier |
11
|
0.0002
|
3.9
|
2.09
|
7.08
|
| Chesapeake Bay Retriever |
34
|
<0.0001
|
3.6
|
2.54
|
5.08
|
| Airedale Terrier |
65
|
<0.0001
|
3.6
|
2.76
|
4.57
|
| Shar-Pei |
75
|
<0.0001
|
3.4
|
2.67
|
4.27
|
| Doberman Pinscher |
323
|
<0.0001
|
3.3
|
2.96
|
3.73
|
| Irish Setter |
87
|
<0.0001
|
3.3
|
2.66
|
4.11
|
| Rhodesian Ridgeback |
31
|
<0.0001
|
3.2
|
2.25
|
4.64
|
| Bloodhound |
11
|
0.0018
|
3.0
|
1.63
|
5.46
|
| Giant Schnauzer |
13
|
0.0013
|
2.8
|
1.59
|
4.83
|
| Norfolk/Norwich Terrier |
5
|
0.0482
|
2.6
|
1.07
|
6.38
|
| Brittany |
49
|
<0.0001
|
2.6
|
1.92
|
3.42
|
| Golden Retriever |
538
|
<0.0001
|
2.1
|
1.94
|
2.34
|
| Bouvier des Flandres |
15
|
0.0139
|
2.0
|
1.22
|
3.42
|
| Rottweiler |
156
|
<0.0001
|
1.9
|
1.59
|
2.20
|
| Cairn Terrier |
32
|
0.0027
|
1.8
|
1.26
|
2.56
|
| Chow Chow |
26
|
0.0090
|
1.7
|
1.17
|
2.57
|
| Scottish Terrier |
41
|
0.0158
|
1.5
|
1.10
|
2.04
|
| Breeds at Decreased Risk |
N | Probability | OR | 95% Confidence Interval |
|
| German Shepherd |
113
|
0.0001
|
0.7
|
0.58
|
0.84
|
| Mixed Breed |
673
|
<0.0001
|
0.7
|
0.62
|
0.73
|
| Labrador Retriever |
184
|
<0.0001
|
0.6
|
0.49
|
0.66
|
| Lhasa Apso |
24
|
0.0029
|
0.6
|
0.38
|
0.84
|
| English Setter |
7
|
0.0355
|
0.5
|
0.22
|
0.98
|
| German Shorthaired Pointer |
11
|
0.0039
|
0.4
|
0.25
|
0.81
|
| Great Dane |
12
|
0.0017
|
0.4
|
0.25
|
0.78
|
| West Highland White Terrier |
16
|
0.0002
|
0.4
|
0.26
|
0.70
|
| Miniature Poodle |
42
|
<0.0001
|
0.4
|
0.31
|
0.58
|
| Cocker Spaniel |
74
|
<0.0001
|
0.4
|
0.33
|
0.53
|
| Collie |
13
|
0.0002
|
0.4
|
0.23
|
0.69
|
| Greyhound |
6
|
0.0102
|
0.4
|
0.17
|
0.84
|
| English Springer Spaniel |
23
|
<0.0001
|
0.4
|
0.25
|
0.56
|
| Pug |
6
|
0.0056
|
0.4
|
0.16
|
0.81
|
| Beagle |
29
|
<0.0001
|
0.4
|
0.25
|
0.51
|
| Toy Poodle |
5
|
0.0041
|
0.3
|
0.14
|
0.78
|
| Weimeraner |
5
|
0.0004
|
0.3
|
0.11
|
0.64
|
| Border Collie |
3
|
0.0093
|
0.3
|
0.08
|
0.82
|
| Shih Tzu |
15
|
<0.0001
|
0.3
|
0.15
|
0.43
|
| Cardigan Welsh Corgi |
2
|
0.0213
|
0.2
|
0.06
|
0.94
|
| Basset Hound |
9
|
<0.0001
|
0.2
|
0.12
|
0.44
|
| Pomeranian |
3
|
0.0007
|
0.2
|
0.07
|
0.65
|
| Shetland Sheepdog |
15
|
<0.0001
|
0.2
|
0.12
|
0.33
|
| Keeshond |
3
|
0.0004
|
0.2
|
0.06
|
0.61
|
| Boston Terrier |
3
|
<0.0001
|
0.1
|
0.05
|
0.44
|
| Norwegian Elkhound |
1
|
0.0062
|
0.1
|
0.02
|
0.88
|
| Maltese |
2
|
<0.0001
|
0.1
|
0.03
|
0.45
|
| Jack Russell Terrier |
2
|
<0.0001
|
0.1
|
0.03
|
0.42
|
| Husky |
5
|
<0.0001
|
0.1
|
0.04
|
0.21
|
| Bichon Frise |
2
|
<0.0001
|
0.1
|
0.02
|
0.28
|
| Dalmatian |
2
|
<0.0001
|
0.1
|
0.02
|
0.26
|
| Old English Sheepdog |
1
|
<0.0001
|
0.1
|
0.01
|
0.37
|
| Site | Percent |
| Head |
28.5%
|
| Forelimb |
20.5%
|
| Hindlimb |
12.4%
|
| Thorax |
9.8%
|
| Abdomen |
7.9%
|
| Back |
6.8%
|
| Multiple |
5.3%
|
| Neck |
4.1%
|
| Perineum |
2.2%
|
| Scrotum |
1.3%
|
| Tail |
1.2%
|
Clinical Presentation/Physical Exam Findings
- Size varies for these tumors; they can be smaller than 0.5 cm up to several centimeters in diameter.
- These tumors are often solitary, dome-shaped, and pigmented.
- They appear smooth and uniform.
- Ulceration of the overlying epidermis is uncommon but may be seen occasionally.
- The lesions have only a small amount of hair on the surface of the skin.
- Some, regardless of the name, can lack melanin and are pale to dark red.
Tumor Pathology
Gross Findings
- The mass of the tumor resides in the dermis, but often extends into the subcutaneous tissue.
- Edges are well-demarcated from the surrounding dermis and adipose tissue
- This is especially evident in highly pigmented tumors.
- Colors can vary from jet black to slate gray; the melanin-free tumors can appear pale to dark red.
- Some portions of the tumor may contain pigmented and non-pigmented areas interspersed with each other (variegated).
Microscopic Findings
Histopathology
- In the lower part of the epidermis there may be nests of neoplastic cells but
- Some neoplasms are completely confined to the dermis.
- The amount of melanin within the cytoplasm of the neoplastic cells is variable.
- Some may be completely melanin-free.
- The cell morphology consists of one or more of the following cell types:
- small spindle cells
- large spindle cells
- epithelioid
- polygonal
- round
- There is littlecytologic or nuclear atypia and few mitotic figures (<3 per 10 HP fields).
- In the dermis there may be areas of neuroidal differentiation
Balloon Cell Melanocytoma: This uncommon variant is composed of large, round and epithelioid cells that have abundant pale, finely granular-to-foamy cytoplasm. Melanin granules can be very difficult to find in these cells.
Clinical Behavior
The site of origin and histopathologic appearance is what distinguishes melanocytomas from the more serious malignant melanoma. Wide surgical excision is curative for the cutaneous form.
Melanoacanthoma
Introduction
Definition: A tumor with features of a compound melanocytoma and a benign epithelial neoplasm.
These tumors are rare.
Tumor Pathology
Microscopic Findings
- Within these nodular masses, the neoplastic follicular epithelium contains polygonal melanocytes arranged into numerous nests.
- There is a dermal component similar to that found in melanocytomas.
References
- Goldschmidt, M.H., & Hendrick, M.J. (2002). Tumors of the skin and soft tissue. In D.J. Meuten (Ed.), Tumors in domestic animals 4 th ed (pp. 45-119). Iowa: Iowa State Press
- Goldschmidt, M.H., & Shofer, F.S. (1998). Skin tumors of the dog and cat. Woburn, MA: Butterworth-Heinemann
- Gross, T.L., Ihrke, P.J., & Walder, E.J. (1992). Veterinary dermatopathology: A macroscopic and microscopic evaluation of canine and feline skin disease. (pp. 327-485). St. Louis, Missouri: Mosby Year Book
- Smith, S.H., Goldschmidt, M.H. and McManus, P.M. (2002) A comparative review of Melanocytic neoplasms. Veterinary Pathology (39:651-678)
- World Health Organization (1998). Histological classification of epithelial and melanocytic tumors of the skin of domestic animals (2 nd series, vol 3). Washington, DC: Armed Forces Institute of Pathology
- Yager, J.A. & Wilcock, B.P. (1994). Color atlas and text of surgical pathology of the dog and cat. Ontario, Canada: Mosby Year Book
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