JPC Systemic Pathology
Signalment (JPC Accession #2634082): Age and breed unspecified, dog
HISTORY: This dog had a solitary, slow-growing dermal mass on the head.
HISTOPATHOLOGIC DESCRIPTION: Haired skin, head: Focally expanding the dermis and subcutis, elevating the overlying epidermis and compressing adjacent adnexa is a 12mm diameter, densely cellular, unencapsulated, well-demarcated, well-circumscribed, multilobulated neoplasm composed of polygonal basaloid cells arranged in variably-sized cords, trabeculae, undulating ribbons, garlands and occasional islands. The cords and trabeculae are separated and surrounded by a dense, often hyalinized collagenous stroma, whereas the ribbons and garlands are separated by moderately dense fibrous to myxomatous stroma. Neoplastic cells have variably distinct cell borders, scant eosinophilic cytoplasm, oval nucleus with finely stippled chromatin, and one variably indistinct nucleolus. Multifocal islands of neoplastic cells have more abundant, finely granular eosinophilic cytoplasm. Neoplastic cells occasionally line small (up to 80um) cystic structures containing flocculent amphophilic material. The mitotic rate averages 2 per 10 HPF. Anisocytosis and anisokaryosis are minimal.
MORPHOLOGIC DIAGNOSIS: Haired skin, head: Trichoblastoma, breed not specified, canine
Signalment (JPC Accession #2015906): Age and breed unspecified, cat
HISTORY: Cystic mass from the neck
HISTOPATHOLOGIC DESCRIPTION: Dermis, neck: Expanding the dermis, replacing adnexal structures, and extending to all borders is a densely cellular, well-demarcated, multilobular, unencapsulated neoplasm composed of two morphologies of polygonal cells arranged in variably sized islands and cords which often form thick-walled cysts, and are separated by a moderately dense fibrous stroma. The first polygonal to vaguely spindloid cell population has indistinct cell borders, a scant amount of eosinophilic cytoplasm, and a single elongate nucleus with dense chromatin and an indistinct nucleolus. Anisocytosis and anisokaryosis are mild. These cells often contain intracytoplasmic black granules (melanin). The second more polygonal population is arranged in islands within the trabeculae and has distinct cell borders, a moderate amount of eosinophilic cytoplasm with a single round nucleus with finely stippled chromatin and a distinct nucleolus. Anisocytosis and anisokaryosis are mild. The mitotic count for both cell morphologies averages 6 per 10HPF. The center of the cysts often contains abundant necrotic neoplastic cells and melanin pigment.
MORPHOLOGIC DIAGNOSIS: Dermis, neck: Solid-cystic and pigmented apocrine ductular adenoma, breed not specified, feline.
- Trichoblastomas are benign neoplasms in dogs and cats and are either derived from or reduplicate the primitive hair germ of embryonic follicular development
- Poodles, Cocker spaniels, Cockapoos, Kerry blue terriers, Bichon frises, Shetland sheepdogs and Siberian huskies predisposed
- Trichoblastomas were previously termed basal cell tumors (dogs) and spindle cell type of basal cell tumor (cats)
- Trichoblastomas have 4 primary subtypes:
- Medusoid variant
- Additional variant in dogs recognized by some authors: Trichoblastoma with outer root sheath (ORS) differentiation
- “Basal cell tumor” is a previously used term that included solid-cystic apocrine ductular adenomas, trichoblastomas, and basal cell carcinomas, along with other epithelial tumors; now they are identified separately
- Solid-cystic apocrine ductular adenomas are benign multilobular dermal masses composed of islands of tumor cells are often oriented vertically within the dermis with frequent contiguity with the epidermis
- They are a common skin tumor in cats, although true incidence is difficult to estimate as they were previously called basal cell tumors.
- Derived from trichoblastic (primitive hair germ) epithelium
- One study of canine trichoblastomas indicated:
- Expression of vascular endothelial growth factor (VEGF) in only the more aggressive forms, which may be used as a criterion for evaluating growth potential
Solid-cystic apocrine ductular adenomas:
- Derived from apocrine glandular epithelium
TYPICAL CLINICAL FINDINGS:
- Trichoblastoma: Do not typically metastasize
- Solid-cystic apocrine ductular adenomas: Slow growing intradermal masses; can be locally aggressive but do not metastasize
TYPICAL GROSS FINDINGS:
- Solitary, firm, alopecic nodules; dome-shaped or polypoid +/- ulceration
- Site predilection for head, neck, and base of ear
- Solitary, firm, dome-shaped; often pigmented
- Most common on cranial half of trunk
Solid-cystic apocrine ductular adenomas – cats:
- Raised, rounded, well-circumscribed
- May be pigmented, cystic, ulcerated, or alopecic
- Most common on neck, head, and dorsal trunk
TYPICAL LIGHT MICROSCOPIC FINDINGS:
Trichoblastoma – histological subtype does not affect prognosis; usually not contiguous with epidermis
- Ribbon type – most frequently seen in the dog; ribbons, garlands, cords embedded in paucicellular fibrous stroma; generally two cells wide; rarely associated with the epidermis; may have extensive collagenous matrix which may appear hyalinized; mitoses variable; rare variants can have trabeculae of keratinocytes enmeshed in loose fibrous stroma resembling follicular fibrous sheaths
- Medusoid variant – cords radiate from a central island (occasionally with microcysts of amorphous keratin) of densely packed cells often have more abundant eosinophilic cytoplasm and associated with a more mucinous stroma and are most frequently observed in the dog
- Trabecular type – lobules and broad trabeculae of basaloid epithelial cells with prominent peripheral palisading and centrally located cells with more abundant cytoplasm; connection with the epidermis is focal or absent; little intralobular collagenous stroma; most common variant in cats
- Granular cell type – similar to ribbon type but composed of larger cells with granular or vacuolated cytoplasm and eccentric nuclei; rare
- Spindle cell type – trabeculae and lobules of elongated basal epithelial cells within poorly collagenous stroma; peripheral palisading is not prominent; often have a broad zone of connection with normal epidermis; melanin may be present in macrophages or within neoplastic cells; most frequently seen in the cat; rare keratinization
- Trichoblastoma with ORS differentiation: rare, well demarcated composed of lobules and trabeculae with cystic degeneration; anastomosing cords merge into islands of cells with markedly vacuolated cytoplasm resembling anagen suprabulbar outer root sheath or pale pink isthmus-type keratinocytes that line cystic zones; often melanized
- Multiple histological subtypes may be found in a single neoplasm
Solid-cystic apocrine ductular adenoma - cats
- Multilobulated or racemiform (grape-like) with islands, solid sheets or cysts lined by undifferentiated basaloid cells; focal epidermal contiguity often present; oriented vertically within the dermis
- Central cystic degeneration and necrosis of tumor lobules often with central necrosis and melanin pigment within tumor cells and macrophages
- Two distinct polygonal cell morphologies: (1) Small, cuboidal, basaloid appearance, concentrated around periphery without palisading, comprise majority of trabecular structure with occasional ductules, (2) larger polygonal cells with rounded margins in center of islands, intracytoplasmic vacuoles with smooth borders consistent with intracellular ductal differentiation; spindling may be present in either population
ADDITIONAL DIAGNOSTIC TESTS:
- A recent article found that canine trichoblastomas are usually positive for CK15 (a hair follicle stem cell marker) likely indicating a follicular stem cell origin
- Trichoepitheliomas and tricholemmomas variably express CK15
- IKAs and pilomatricomas, which originate from portions of the hair follicle away from the bulge, are generally CK15 negative
- Solid-cystic apocrine ductular adenoma
- Immunoreactive for CAM 5.2 (7/8) and CK13, relatively specific markers for glandular epithelia
For trabecular trichoblastoma:
- Isthmic tricholemmoma- no peripheral palisading; tricholemmal keratinization
For granular trichoblastoma:
- Bulb type tricholemmoma- has more islands and trabecular without peripheral displacement of nuclei seen in granular trichoblastoma
- Granular cell basal carcinoma- epidermal contiguity and features of malignancy
For spindle cell trichoblastoma:
- Solid-cystic ductular sweat gland adenoma- can have some spindling, but usually not as much as spindle cell trichoblastoma
For trichoblastoma with ORS differentiation (dogs):
- Canine apocrine ductular adenoma- tubular structures lined by double row of cells; gradual keratinization with keratohyaline granules and lamellations of keratin
For solid-cystic apocrine ductular adenoma (cats):
- Trichoblastoma – presence of follicular papillary mesenchymal bodies
- Feline apocrine ductular adenoma – well circumscribed, small, multilobular, deep dermal neoplasm with numerous tubules and fluid-filled cysts; rarely associated with overlying epithelium
- Basal cell carcinoma – a low grade malignancy that is invasive with fibroplasia and higher mitotic activity; two histologic variants:
- Infiltrative basal cell carcinoma- cords of primitive basaloid cells with no adnexal differentiation, a high mitotic rate and desmoplasia
- Clear-cell basal cell carcinoma- uncommon; large polygonal cells with clear to finely granular cytoplasm and variable mitotic rate
- Melanoma – variable cellular morphology and patterns; may have junctional activity
- Horses: Rare; more common in Morgans or pony breeds; may resemble sarcoids or melanomas
- Rabbits: Trichoblastoma is the most common cutaneous tumor in pet animals
- Brachelente C, Porcellato I, Sforna M, et al. The contribution of stem cells to epidermal and hair follicle tumors in the dog. Vet Dermatol. 2013;24(1):188-194.
- Goldschmidt, MH. Epithelial tumors. In: Meuten DJ, ed. Tumors in Domestic Animals. 4th ed. Ames, IA: Iowa State Press; 2002: 46-47, 58-60.
- Goldschmidt MH, Dunstan RW, Stannard AA, von Tscharner C, Walder EJ, Yager J. Histological Classification of Epithelial and Melanocytic Tumors of the Skin of Domestic Animals. 2nd Series, Vol. III. Washington, DC: Armed Forces Institute of Pathology; 1998: 18-19, 22-23.
- Gross TL, Ihrke PJ, Walder EJ: Epithelial neoplasms and other tumors. In: Gross TL, ed. Skin Diseases of the Dog and Cat. 2nd ed. Ames, IA: Blackwell Science; 2005: 625-634, 672-677.
- Hargis AM, Myers S. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed., St. Louis, MO: Elsevier; 2017:1106, 1119.e6t-1119.e21t.
- Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals.Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 714-716, 718-719.
- Schaffer PA, Wobeser B, Martin LER, Dennis MM, Duncan CG. Cutaneous neoplastic lesions of equids in the central United States and Canada: 3,351 biopsy specimens from 3,272 equids (2000-2010). J Am Vet Med Assoc. 2013;242(1):99-104.
- Von Bomhard W, Goldschmidt MH, Shofer FS, Perl L, Rosenthal KL, Mauldin EA. Cutaneous neoplasms in pet rabbits: A retrospective study. Vet Pathol. 2007;44(5):579-588.