Follicular cysts (ie, epidural inclusion cysts, epidermoid cysts) are nonneoplastic, noninflammatory, sac-like lesions lined by epithelium. Most canine and feline skin cysts are follicular cysts (ie, arise from hair follicles) and include several histologic subtypes that are not cytologically distinguishable. These histologic subtypes have little clinical significance; they are benign and can be completely excised surgically.
On cytologic examination, follicular cysts have abundant keratinocytes (either anucleate or containing karyolytic nuclei) with cholesterol crystals, hair fragments, and activated macrophages (Figure 2). Within the preparation background, follicular cysts can also contain melanin granules that should be differentiated from bacteria. Cyst rupture and immunogenic keratin exposure to the dermis or subcutis can result in foreign body reaction with mild-to-marked infiltrates of neutrophils, macrophages, and multinucleated giant cells. Because, on cytology, a cyst cannot be differentiated from a cyst within a neoplasm (usually benign follicular neoplasm), histopathology is required to assess architecture; cytologic differentials include trichoepi- thelioma, infundibular keratinizing acanthoma, and pilomatrixoma.