Case 180427-1

Ten-year-old, female spayed Labrador Retriever.

This mass was first noted on the right muzzle about a year ago on physical examination. It has been slowly growing since then, and was 0.5 cm in diameter on presentation for removal. The owner occasionally expresses a small amount of blood and caseous material from the mass. A fine needle aspirate of the mass revealed a mixed inflammatory population.

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One bisected section of haired skin extending to deep dermis is examined, in which a well-demarcated, unencapsulated mass composed of central, ectatic and branching hair follicles that focally open through the skin via a pore and are surrounded by abundant, closely packed, mature sebaceous glands. Follicles are lined by stratified squamous epithelium with abrupt keratinization, including prominent keratohyalin granules and luminal orthokeratosis. Follicular lumina are packed with abundant, lamellar, keratin. The dermis surrounding folliculosebaceous units is infiltrated by large numbers of lymphocytes, plasma cells, and neutrophils. The mass extends to 0.1 cm from the lateral margins and 0.2 cm from the deep margin.

N/A.

Haired skin, right muzzle: Trichofolliculoma

Hair follicles are divided into three levels. The upper portion, or infundibulum, of the follicle extends from the entrance of the sebaceous duct to the follicular ostium. The middle portion, or the ishthmus, extends from the insertion of the arrector pili muscle to the level of the sebaceous duct. The lower, or inferior segment of the follicle extends from the mesenchymal papilla to the insertion of the arrector pili muscle.

Trichofolliculomas are likely a hamartoma, and resemble the entire follicular or folliculosebaceous unit and maintain fairly normal interrelationships between the various tissue types. These benign tumors are uncommon in dogs and rare in cats. They are usually presented as a solitary, dome-shaped nodule which is most commonly less than 2 cm in diameter. Histologically, they are a well-circumscribed but unencapsulated dermal nodule composed of one or several primary follicles that are large, dilated, and keratinize through a granular cell layer. A differential diagnosis for trichofolliculomas is a dermoid cyst, however the follicles which radiate from the central cyst of a trichofolliculoma are more numerous, larger, and often show tertiary branching.

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