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I appreciate Tami getting this page ready for us. We talk about this all of the time and now we can provide a concise explanation for the problem! (Bob McKee)
Cutaneous cysts are common in dogs and uncommon in other domestic animals. They are benign non-neoplastic lesions characterized by an epithelial wall, with keratinous to amorphous contents. They are subdivided into six types based on histologic appearance.
Epidermal Inclusion cysts
Thought to be acquired lesions arising from displaced fragments of epithelium or occluded pilosebaceous follicles. They may be single to multiple, and there is no age, breed, or sex predilection. The lesions are round, smooth and well circumscribed, firm to fluctuant, and up to 5 cm in diameter, often bluish, and dermoepidermal to subcutaneous in location. They may open and exude a gray to white-brown, cheesy material. In the horse, they most commonly occur at the base of the ear, and may discharge a mucoid material, or unilaterally in the false nostril.
Histologically, they are characterized by an epithelial lining that undergoes maturation and keratinization typical of epidermis, but contains no adnexal structures.
Usually hereditary or congenital lesions, which may be single or multiple. Breed predilections are listed in the chart. There is no sex predilection, and the lesions are seen in young animals. Dermoid sinus, pilonidal sinus of Rhodesian ridgebacks is this type of cyst, and is seen along the dorsal midline of the neck and sacrum.
Histologically, dermoid cysts are characterized by a stratified epithelial lining that contains adnexae (hair follicles, sebaceous glands, etc; and central keratinaceous material.
Follicular cysts (milia)
Develop be retention of follicular glandular products, due to congenital or acquired loss or obliteration of follicular ostia. They are usually small, and white to yellow, and may resemble pustules or calcinosis cutis.
Histologically, they are characterized by a greatly enlarged, dilated, keratin-filled hair follicle with sebaceous or apocrine sweat glands or atrophic secondary hair follicles entering the base of the cyst.
Trichilemmal (pilar) cysts
These are seen in dogs. Grossly, they resemble other cysts.
Histologically, they are greatly enlarged and dilated hair follicles with a proliferative wall exhibiting trichilemmal keratinization, and homogenous keratin filling the cyst cavity. Squamous eddies are frequently present
Also known as conchal fistula, cyst, or sinus; dentigerous cyst, temporal teratoma or odontoma; ear fistula or ear tooth). They occur in the horse, and are thought to represent developmental defects. Lesions are solitary and occur at or near the base of the ear or in relationship to the zygomatic process. They may discharge a grayish mucoid material.
Histologically, they may be epidermoid, dermoid, or dentigerous (containing a tooth or teeth).
These occur in the wattles of goats, most commonly Nubian and Nubian crossbreeds. They are believed to be developmental abnormalities, possibly arising from branchial clefts. They contain a clear fluid, and if aspirated, with reform.
Histologically, they are characterized by a cyst wall composed of a single to double layer of cuboidal to columnar epithelial cells that often exhibit the apical budding typical of apocrine sweat gland secretory epithelium.