JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-N02

Signalment (JPC # 1754646): Guinea pig

HISTORY: Dermal mass

HISTOPATHOLOGIC DESCRIPTION: Haired skin: Expanding the dermis and subcutis, compressing the adjacent adnexa and panniculus carnosus, and elevating the overlying epidermis is a 2 cm x 1.5 cm, unencapsulated, well circumscribed, cystic, multilobulated neoplasm composed of polygonal cells forming numerous primitive folliculosebaceous units arranged around a central cyst (primary follicle) on a moderate fibrovascular stroma. Neoplastic cells have indistinct borders and contain a scant to moderate amount of eosinophilic cytoplasm with a round to oval nucleus containing finely stippled chromatin and one to two indistinct nucleoli. Mitoses average 2 per 2.37mm². The primary follicle is lined by attenuated epithelium that undergoes both abrupt and gradual keratinization. The folliculosebaceous units are composed of secondary follicles that undergo varying degrees of differentiation and incompletely recapitulate hair follicles, characterized by a peripheral layer of basaloid cells that mature to attenuated squamous cells centrally or form hair bulbs, with lumina containing amorphous keratin debris and occasionally birefringent hair shafts. The central squamous cells often contain eosinophilic, 2‑4 um, irregularly shaped, brightly eosinophilic granules (trichohyalin granules). Multifocally there is sebaceous differentiation. There is a paucity of hair follicles within the overlying epidermis.

MORPHOLOGIC DIAGNOSIS: Haired skin: Trichofolliculoma, guinea pig, rodent.

GENERAL DISCUSSION:

• Trichofolliculoma is one of the most common skin tumors of guinea pigs, along with trichoepitheliomas, seen predominantly in males
• Benign neoplasm originating from the trichogenic epithelium and may include the pilosebaceous units with active trichogenesis (anagen)
• Rare tumor of dogs and cats; may represent a hamartoma as it resembles the entire follicular or folliculosebaceous unit

PATHOGENESIS:

• Unknown

TYPICAL GROSS FINDINGS:

• Solitary, dome shaped, firm, subcutaneous nodules less than 2 cm in diameter
• Solid or polycystic
• Overlying epidermis is intact, but may have a central pore or depression
• Guinea pig: Commonly in the dorsal lumbar region, though can occur anywhere; hairless, up to 4-5cm, firm; often associated with the scent glands; often ulcerate and produce exudate

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Well circumscribed, unencapsulated, cystic neoplasm consisting of one or several primary hair follicles that are large, dilated, and undergo gradual keratinization
  • May contain one or multiple secondary follicles within the cyst wall; radiate outward from the primary follicle in an aborizing pattern and exhibit various stages of maturation from predominately basaloid hair germ to fully developed follicles containing delicate hair shafts
• Cystic spaces contain keratin, ghost cells, hair shafts, and sebum
• An infundibulum that forms a pore on the skin surface may be present
  • May have variable amount of sebaceous units; if there is a large sebaceous component, it is termed a sebaceous trichofolliculoma
• Rupture incites subacute to chronic inflammation (foreign body reaction to hairs/keratin)

DIFFERENTIAL DIAGNOSIS:

Other reported cutaneous neoplasms in guinea pigs:

• Liposarcoma, lymphoma, cutaneous papilloma, sebaceous gland adenoma, fibrosarcoma, fibroma, and carcinoma

Histologic differential diagnoses:

• Trichoepithelioma: More numerous cysts and less advanced trichogenesis (than trichofolliculoma), and less likely to have arborizing pattern
• Dermoid cysts: Developmental defect; typically occurs on the midline of young animals; the cyst is vertically oriented in the dermis and subcutis; lined by epidermal squamous epithelium that undergoes gradual keratinization with a prominent keratohyaline granular layer (recapitulates normal skin); possess fewer secondary follicles and lacks secondary branching of follicles
• Fibroadnexal hamartoma (adnexal nevus, fibroadnexal dysplasia, focal adnexal dysplasia, folliculosebaceous harmartoma): Focal haphazard and disorganized folliculosebaceous units, associated with increased fibrous tissue; may have direct connection with overlying epidermis

COMPARATIVE PATHOLOGY:

• Uncommon in dogs, rare in cats
• Reported in the rat, man, and a rock hyrax
• Hamsters: Hamster polyoma virus (HaPV) causes:
  • Transmissible lymphoma in young hamsters
  • Hair follicle epitheliomas (I-N01; HaPV replication in the keratinizing epithelium)

REFERENCES:

1. Agnew D, Nofs S, Delaney MA, Rothenburger JL. Xenartha, Erinacoemorpha, Some Afrotheria, and Phloidota. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:524-525.
2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA:Blackwell Publishing; 2016:176-178, 250.
3. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Saunders; 2016:705.
4. Welle MM, Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1145-1146.
5. Wiener DJ. Histologic features of hair follicle neoplasms and cysts in dogs and cats: a diagnostic guide. J Vet Diagn Invest. 2021 May;33(3):479-497.

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