October 2016



Signalment (JPC # 1957907):  Age and breed unspecified dog


HISTOPATHOLOGIC DESCRIPTION (Slide A):  Haired skin and subcutis:  Infiltrating the dermis and subcutis, elevating the epidermis, separating and surrounding adnexa, skeletal muscle fibers and collagen bundles, and extending to the margins of the submitted tissue is an unencapsulated, poorly-circumscribed neoplasm composed of sheets of neoplastic mast cells.  Neoplastic cells are round with distinct cell borders, moderate amounts of amphophilic cytoplasm that occasionally contain fine basophilic granules, and generally centrally located, round nuclei with coarsely stippled chromatin and 1 to 2 variably distinct nucleoli.  The mitotic count averages 1 per 10 400X hpf.  Scattered throughout the neoplasm are moderate to high numbers of eosinophils and fewer lymphocytes, plasma cells, and hemosiderin-laden macrophages.  Multifocally, there are ectatic lymphatic vessels (edema) and blood vessels are congested. 

(Slide B) Luna mast cell stain:  Neoplastic cells contain numerous metachromatic granules.

MORPHOLOGIC DIAGNOSIS:  Haired skin and subcutis:  Mast cell tumor, low grade (grade II), breed unspecified, canine

Signalment (JPC # 2924576):  13-year-old standardbred gelding

HISTORY:  A subcutaneous mass on the right flank

HISTOPATHOLOGIC DESCRIPTION (Slide C):  Subcutis (per contributor):  Infiltrating, separating, and surrounding collagen bundles is an unencapsulated, poorly- circumscribed, relatively paucicellular neoplasm composed of sheets of neoplastic mast cells.  Neoplastic cells are round with distinct cell borders, moderate amounts of amphophilic cytoplasm that often contain fine basophilic granules, and centrally located, oval nuclei with coarsely stippled chromatin and indistinct nucleoli.  There is moderate anisocytosis and anisokaryosis.  The mitotic rate is 1 per 10 hpf.  There are multiple large, irregular, up to 5 mm diameter areas of necrosis composed of brightly eosinophilic cellular and karyorrhectic debris, neoplastic mast cells, and multifocal brightly eosinophilic, hyalinized bands of collagen (collagenolysis), that are bordered by deeply basophilic fragmented mineral, and dense fibrous connective tissue, admixed with fine, beaded amphophilic mucin, epithelioid macrophages, and multinucleated giant cells.  Scattered throughout the neoplasm, there is hemorrhage, moderate numbers of hemosiderin-laden macrophages, and occasional eosinophils.

MORPHOLOGIC DIAGNOSIS:  Subcutis (per contributor):  Mast cell tumor, standardbred, equine

SYNONYMS:  Mastocytoma, cutaneous mastocytosis


Normal Mast Cell Production and Function:





Subgroup of poorly differentiated equine cutaneous MCT in which aberrant KIT expression is associated with high proliferative rate and potential aggressive behavior Cats:



A Grenz zone is typically present; may be epitheliotropic



Grading Systems:


  Grade 1

Grade 2

Grade 3


Well-differentiated; medium-sized; distinct granules

Moderate; fine, distinct granules

Poorly- differentiated;

indistinct or no granules



round, condensed chromatin; no nucleoli

Mild pleomorphism;

round to indented

scattered chromatin;

occasional nucleoli;

binucleated and giant cells

Large; pleomorphic; fine chromatin; 1 or more nucleoli;

binucleated and multinucleated giant cells are common

Mitotic index average/hpf





Superficial dermis→ interfollicular

Deep dermis to SQ

SQ to deeper tissues

Aggressive features


Occasional edema; necrosis

Edema; necrosis common


Lymph node cytology:




Other round cell neoplasms (for grade III usually, since they are poorly differentiated):

Horse (for eosinophilic inflammation):



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