• Relatively common in dogs, less frequent in cats.
• Age: most commonly seen in adult animals.
• They can occur in any location, but most commonly arise on the head and trunk.
• Usually occur as single masses, less frequently as multiple (e.g. German Shepherd Dog and Pekingese). A small pore can occasionally be found on the surface of the lesion. Masses can be ulcerated.
• Usually confined to the dermis. Larger cysts may extend into the subcutaneous adipose tissue.
• They are benign lesions; however, rupture of the cystic wall and exposure of the keratin to the surrounding tissues can elicit an endogenous foreign body reaction characterized by a neutrophilic to pyogranulomatous inflammation.
Cytological features
• Aspirates usually exfoliate a moderate to large amount of specimen.
• Background: clear or pale basophilic. It may contain cholesterol crystals and hair shafts.
• High numbers of anucleated squamous epithelial cells exfoliate singly or in large groups. Amorphous dense keratin can also be observed.
• Neutrophils, macrophages and/or multinucleated giant inflammatory cells can be seen in inflamed cysts.
Variants
• Follicular cysts can be classified as infundibular, isthmic, matrical and hybrid, based on the type of lining epithelium of the cystic wall. However, this classification does not have any clinical implication and the general term of follicular cyst is most frequently used.
• Infundibular cyst
Usually exfoliates numerous anucleated squamous epithelial cells. These are intact and with well defined margins. They vary from being lightly to moderately basophilic. Occasionally, they may be embedded in a watery basophilic background of keratinic material.
• Isthmic cyst
Aspirates usually contain pale, amorphous and homogeneous keratin.
• Matrical cyst
Characterized by the exfoliation of variable numbers of ghost cells (description of ghost cells can be found in the pilomatricoma section in Chapter 8).
• Hybrid cyst
This type of cyst contains a mixture of the anucleated squames and keratinized material typical of the other cysts described above.
• Infundibular keratinizing acanthoma (no ghost cells seen)
• Trichoepithelioma
• Pilomatricoma
• As cytologically a follicular cyst cannot be definitively differentiated from an infundibular keratinizing acanthoma and trichoepithelioma, a general diagnosis of ‘keratinized lesion’ or ‘follicular lesion’ is usually preferred, and follicular cyst is listed as a differential diagnosis.
• A disseminated form of follicular cysts has been described in a case series that included five dogs and one cat. Numbers of lesions were more than 20 and exceeded 150–200 in three of the cases.
• Dermoid cysts are congenital anomalies that occur in young dogs and cats. They arise on the midline. They contain lamellar keratin, yielding cytological findings similar to an infundibular cyst. Over-represented canine breeds include Rhodesian Ridgeback and Boxer.
Further reading
Adedeji, A.O., Affolter, V.K. and Christopher, M.M. (2017) Cytological features of cutaneous follicular tumours and cysts in dogs. Veterinary Clinical Pathology 43(2), 143–150.
White, A., Stern, A., Campbell, K. and Santoro, D. (2013) Multiple (disseminated) follicular cysts in five dogs and one cat. Veterinary Record 173(11), 269.
7.2 (Sweat Gland) Apocrine Cyst
Non-neoplastic lesion lined by a single layer of apocrine secretory epithelial cells.
• Relatively common in dogs; rare in cats.
• Age: 6 years or older.
• In dogs, it predominantly occurs on the head, legs, neck and trunk. In cats, it is often observed on the head.
• Cysts are usually solitary, occasionally multiple (apocrine cystomatosis). They are variably sized, well defined and fluctuant. They may have a blue tint when viewed through the overlying skin. Cyst content is usually clear and watery, but is occasionally brown and gelatinous due to inspissation.
• It is a benign lesion and carries a good prognosis.
• Over-represented breeds:
• Dogs: Old English Sheepdog and Weimaraner.
• Cats: Persian cat.
Cytological features
• Background: generally clear. It may contain cholesterol crystals.
• Aspirates may be acellular or contain low numbers of macrophages.
• Cuboidal apocrine epithelial cells rarely exfoliate (for morphology, refer to ‘Sweat gland adenoma and carcinoma’ in section 8.5, Apocrine Gland Tumours).
• Following trauma, a variable degree of haemorrhage can occur within the cyst. In this case, macrophages may display erythrophagia or contain haemosiderin granules and/or haematoidin crystals.
Variants of apocrine cyst are the ceruminous gland cysts in the inner pinnae and ear canal. These are more common in adult cats, especially in Abyssinian and Persian cats. These lesions appear as multiple, often numerous, nodules or vesicles. They are often dark in colour and may be mistaken clinically for melanocytic or vascular neoplasms.
7.3 Fibroadnexal Hamartoma
Developmental anomaly of the pilosebaceous unit. It may contain apocrine glands.
• Reported in dogs and accounting for 1.7–2.7% of all cutaneous lesions.
• It is observed in middle-aged or older dogs.
• It may be secondary to chronic trauma and/or scar tissue formation, both resulting in entrapment and subsequent distortion of the adnexal structures. A primary defect of the pilosebaceous units cannot be excluded.
• Hamartomatous lesions can originate from follicles, collagen or from sebaceous glands. However, the coexistence of more adnexal structures is usually observed, hence the term fibroadnexal hamartoma.
• Solitary, firm, circumscribed and nodular to polypoid, dermal mass of variable sizes. It may extend to the subcutaneous tissue. The lesion may also be pigmented, alopecic and ulcerated.
• Fibroadnexal hamartoma is observed more frequently on the distal legs, especially on pressure points. Head and trunk may also be affected.
• It is a benign lesion and carries a good prognosis.
• Over-represented canine breeds: large breed dogs, in particularly Labrador Retriever but also Basset Hound, Maremma Sheepdog and Bracco Italiano.
Cytological features
• Cellularity: generally very low.
• Background: clear to lightly basophilic, with variable degree of haemodilution. Keratin bars are frequently seen.
• Several of the following components may simultaneously be observed:
• Small clusters of mature sebocytes (for morphology, refer to ‘Sebaceous adenoma’ in section 8.4, Sebaceous Tumours).
• Small clusters of sweat gland apocrine epithelial cells (for morphology, refer to ‘Sweat gland adenoma and carcinoma’ in section 8.5, Apocrine Gland Tumours).
• Variable numbers of spindle-shaped stromal cells.
• Mixed inflammation may be present.
• Sebaceous adenoma/hyperplasia (when the sebaceous component prevails)
• Follicular hamartoma
• Trichofolliculoma
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