JPC SYSTEMIC PATHOLOGY
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 x 1.5 cm, unencapsulated, well circumscribed, cystic, multilobulated, neoplasm composed of polygonal cells arranged in numerous arborizing primitive folliculosebaceous units 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 HPF. The primary follicle is lined by attenuated epithelium that undergoes both abrupt and gradual keratinization. The arborizing folliculosebaceous units are composed of secondary follicles that undergo varying degrees of differentiation and incompletely recapitulate hair follicles. The secondary follicles are characterized by a peripheral layer of basaloid cells that mature to attenuated squamous cells centrally or form hair bulbs 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.
MORPHOLOGIC DIAGNOSIS: Haired skin: Trichofolliculoma, guinea pig, rodent.
- Trichofolliculoma is the most common skin tumor of guinea pigs
- Rare tumor of dogs and cats; may represent a hamartoma as it resembles the entire follicular or folliculosebaceous unit
- Benign neoplasm originating from the epithelium of the pilosebaceous unit with active trichogenesis
- Inhibition of bone morphogenic protein (BMP) may play a role in tumorigenesis
- Studies in transgenic mice revealed that inhibiting BMP with the antagonist Noggin resulted in development of hair follicle derived tumors
- BMP is an important tumor suppressor and plays a pivotal role in the regulation of skin development
TYPICAL GROSS FINDINGS:
- Solitary, dome shaped, firm subcutaneous nodules less than 2 cm in diameter
- Solid or polycystic
- Commonly in the dorsal lumbar region
- Overlying epidermis is intact, but may have a central pore or depression
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Well circumscribed, unencapsulated neoplasm consisting of one or several primary hair follicles that are large, dilated and undergo gradual keratinization
- +/- hair shafts
- Secondary follicles radiate outward from the primary follicle in an aborizing pattern
- Secondary follicles 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
- There may be an infundibulum that forms a pore on the skin surface
- May have variable amount of sebaceous units
- If there is a large sebaceous component it is termed a sebaceous trichofolliculoma
- There is subacute to chronic inflammation (foreign body reaction to hairs/keratin) when ruptured
- Other follicular neoplasms appear similar grossly
- Trichoepithelioma –more numerous cysts and less advanced trichogenesis (than –folliculoma), and less likely to have arborizing pattern
- Dermoid cysts: Lined by epidermal squamous epithelium that undergoes gradual keratinization with a prominent granular layer; possess fewer secondary follicles and lacks secondary branching of follicles
- Fibroadnexal hamartoma-remarkably similar if there is a prominent sebaceous component of a trichofolliculoma
- Rare in the dog and cat
- Reported in the rat and man
- Hamsters: Hamster polyoma virus (HaPV) causes:
- Transmissible lymphoma in young hamsters
- Hair follicle epitheliomas (HaPV replication in the keratinizing epithelium)
- Gross TL, Ihrke PF, Walder EJ, Affolter VK. Skin Diseases of the Dog and Cat. 2nd ed. Ames, IA: Blackwell Publishing; 2005: 616-625.
- 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.
- Mohr U. International Classification of Rodent Tumors Part I-The Rat: 5. Integumentary System. International Agency for Research on Cancer, Lyon, France, 1993:1.
- Murphy GF, Elder DE. Non-melanocytic tumors of the skin. In: Rosal J, Sobin LH, eds. Atlas of Tumor Pathology. 3rd Series. Fascicle 1. Washington, DC:Armed Forces Institute of Pathology; 1990: 138-139.
- O’Rourke DP. Disease problems of guinea pigs. In: Quesenberry KE, Carpenter JW, eds. Ferrets, Rabbits, and Rodents Clinical Medicine and Surgery. 2nd ed. St. Louis, MO:Saunders; 2004:251.
- Percy DH, Barthold SW. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA:Blackwell Publishing; 2016:250.
- Sharov AA, Mardaryev AN, Sharova TY, Grachtchouk M, Atoyan R, Byers HR, Seykora JT, Overbeek P, Dlugosz A, Botchkare VA. Bone morphogenetic protein antagonist noggin promotes skin tumorigenesis via stimulation of the Wnt and Shh signaling pathways. Am J Path. 2009:175(3).