Tricholemmoma
A benign follicular neoplasm characterized by external root sheath
differentiation, with a predominance of either the inferior segment of the
external root sheath (inferior type) or the isthmus segment of the external root
sheath (isthmic type).
Gross Appearance:
The tumor cannot be distinguished from trichoblastomas on clinical
evaluation of the animal.
Histology:
Inferior type
•Composed of islands of neoplastic cells separated by a fine fibrillar collagenous stroma
•Prominent eosinophilic basal lamina zone
•Peripheral cells are palisaded and have abundant pale eosinophilic cytoplasm
•Central cells have abundant deeply eosinophilic cytoplasm
•Cells show little pleomorphism or mitotic activity
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Isthmic type
•Composed of epithelial aggregates
•May be an association with the epidermis
•Cords of epithelial cells radiate out from and often join islands of epithelial cells, which show central tricholemmal keratinization
•Large islands may appear cystic
•Cells have moderate amount of lightly eosinophilic, glassy cytoplasm with small euchromatic nuclei
•Tumors may be heavily melanized
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