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The basis of naming of neoplasms is their phenotype - what they look like, or the features that allow you to identify the mature cells they resemble.
Primary epithelial neoplasms of skin are classified as resembling epidermis or adnexa - follicles and glands of skin. Subclassification then uses the convention of adenoma and adenocarcinoma for mature cells of the glands, epithelioma for combinations of basaloid/reserve cells and mature types, acanthoma for cells of the stratum spinosum, and papilloma for combinations of basal and spinosal cells of the epidermis.
The majority are benign and require marginal excision to cure local disease.
The common types are listed below.
The naming conventions of epidermal tumors are to use 'basal' for cells of the stratum basale, and acantho or squamous for cells of the stratum spinosum
Acanthoma
An acanthoma has cells resembling the stratum spinosum.
Basal cell tumor
This is a well circumscribed tumor of cellx that resembles the cells of the stratum basale
Basal cell carcinoma
This is an invasive tumor of cells that resemble the cells of the stratum basale
Basosquamous tumor
Basosquamous tumors have many basal cells and spinosal cells in regular combination/differentiation. Stratum corneum is usually not represented.
Papilloma
A papilloma is a combination of basal cells and maturing cells with representatives of all layers of the epidermis - basal, spinosal and corneal layers. They are then subdivided into viral and non viral types based on evidence of viral cytopathic effect or identification of virus in the lesion.
Squamous cell carcinoma
A squamous cell carcinoma is a neoplasm resembling the stratum spinosum that is invasive and potentially metastatic.
Follicular tumors have a naming convention based on prefixes and suffixes. The prefix 'tricho-' or 'pilo-', or a particular part (infundibular, isthmus or inferior) to refer to follicular tumors. The suffix is based on the cell type - -blastoma for undifferentiated cells, -lemmoma for the glycogenated cells and -matricial for the bulb.
The naming convention is based on the phenotype of the cells, which reflects the normal anatomy.
The Nomina Histologica Veterinaria lists the histological terms of the hair. It does not include the divisions that are used in naming tumors of hair follicles. These are indicated here.
The normal anagen hair follicle is divided into the infundibulum, isthmus, inferior portion. The telogen follicle only has infundibulum and the upper portion of the isthmus.
The infundibulum is the part of the external root sheath (commonly called the outer root sheath - ORS) and it is the portion from the osteum of the follicle to the upper end of the cornified internal root shealth (also called the inner root sheath - IRS). This is where the duct of the sebaceous gland enters the follicle. It is identical to epidermis and has keratohyalin granules in the stratum granulosum.
The isthmus is the portion from the bottom of the infundibulum (where the sebaceous duct enters) to the noncornified portion of the internal root sheath. This is where the arrector pili muscle attaches. There are no granules here - no keratohyalin granules thus no granular layer (stratum granulosum). Cornification occurs in a process called tricholemmal keratinization. It is divided into the upper and lower portions. The upper portion has no internal root sheath. In telogen follices, the upper portion of the isthmus is where the tricholemmal keratin anchors the club hair. In the lower portion of the isthmus, the external root sheath has glycogenated ketatinocytes. The isthmus is where the stem cells of the follicle are located.
The inferior portion of the follicle has 2 components - the suprabulbar portion and the bulb. Trichohyalin granules are present in both locations. Trichohyalin granules are from the internal rooth sheath. The suprabulbar portion has glycogenated cells of the external root sheath, and trichohyalin granules. The internal rooth sheath is not cornified. The bulb is comprised of matricial cells, trichohyalin granules and the dermal papilla.
Infundibular keratinising acanthoma
This neoplasm resembles the infundibular portion of the follicle. It has a central cyst filled with lamellar keratin in concentric layers and a pore to the surface. The epithelium has differentiation to the spinosal layer (stratum spinosum), granular layer (stratum granulosum) and corneal layer (stratum corneum) and compact orthokeratotic keratin and a basal layer peripherally. Projections or trabeculae of basal cells extend peripherally into loose collagen. These may keratinize to form secondary cysts. Chondroid and osteoid may form and surround the epithelium and in some, chondroid and osteoid dominates.
Tricholemmoma
These are follicular tumors with differentiation to the isthmus or inferior portions of the hair follicle. Lemma means husk - and refers to the outer layers of the follicle. They resemble the external root sheath, and often are glycogenated. They lack granules - neither keratohyalin nor trichohyalin granules. When they produce keratin it is dense, compact and hypereosinophilic without granules, similar to the keratin of the club of the telogen follicle.
There are 2 types - isthmic and inferior tricholemmoma. The isthmic type is comprised of small eosinophilic keratinocytes and cysts are filled with tricholemmal keratin. Inferior tricholemmoma has no or little keratinization and the cells have a clear, glycogenated cytoplasm and a prominent basement membrane around each cluster of cells.
Trichoblastoma
These are follicular tumors with basaloid cells only. They are subdivided according to appearance and arrangement. They may have aggregates of spindle cells resembling the dermal papilla. Solid, trabecular, ribbon, medusoid, clear cell, granular cell, with tricholemmal differentiation, and spindle cell variants are reported.
These do not have cornification.
The ribbon and medusoid type has basaloid keratinocytes arranged in branching columns that are 2 or 3 cells thick. The nuclei tend to be arranged perpendicular to the axis of the column. The stromal component is abundant in dogs and sparse in cats.
The trabecular type has a pattern of broad trabeculae
The spindle type is mostly spindle cells. In cats these are in contact with the epidermis. The pattern is islands, nests and trabeculae
There is a type with differentiation to the external root sheath, have nests and islands of cells with a central cyst. The cytoplasm of the cells amy have a glycgenated cytoplasm. Were called cystic basal cell tumors.
Granular cell trichoblastoma has a ribbon pattern but the cells are granular or vacuolated.
Pilomatricoma
These are follicular tumors with nodules forming cysts containing keratin. The cells are basaloid, resemble the bulb, and their cytoplasm is scant, and the nuclei are large and hyperchromatic. Many have trichohyalin granules of the internal root sheath. The keratin is dense, compact and eosinophilic and has nuclear 'ghosts' throughout it.
Most are benign, bnut there are malignant pilomatricomas described. These are invasive.
Trichepithelioma
These are follicular tumors with differentiation to all parts of the follicle - infundibular, isthmus and inferior segments. The keratin produced reflects these different components. It typically forms small or large nodules or cystic structures.
Trichofolliculoma
These may be hamartomas, as they form a mass comprised of distinct follicles
Normal glands have a duct and acinus. Ducts are bilayered and may be keratinised. The acinus has a single layer of cells, and there may be a basal cell component, and or a myoepithelial component, depending on the gland.
Convention is to describe glandular neoplasms as intraductal, ductal, cystic, tubular, papillary, cribriform, solid, micropapillary, comedo, anaplastic and others!. Complex tumors have combinations of epithelium and myoepithelium.
Apocrine Adenoma
Apocrine glands are simple tubular glands with a simple bilayered duct. Adenomas are well circumscribed and well differentiated tumors that tend to have cystic lumens, thus most are simple cystic tumors. Some are solid with small tubulare structures in their wall.
Apocrine adenocarcinoma
Apocrine carcinomas are invasive tumors with desmoplasia and have a full range of subtypes including tubular, papillary, solid and rarely anaplastic types.
Eccrine adenoma
Eccrine adenomas are very rare in domestic mammals. They resemble apocrine tumors but instead of an eosinophilic cytoplasm, these are usually clear to pale eosinophilic.
Eccrine adenocarcinoma
Eccrine adenocarcinomas are invasive and often desmoplastic. They resemble apocrine tumors is their various formations.
Sebaceous adenoma
Sebaceous glands produce sebum and their arrangement and structure is different to apocrine glands - they are solid clusters of cells with a basaloid or reserve layer and differentiation centrally to a mature sebocyte with a lipid vacuolation of their cytoplasm. Adenomas are well circumscribed and mature.
Sebaceous epithelioma
Sebaceous epitheliomas are just like sebaceous adenomas except the number of reserve cells is about 50% of the cells or more. There are therefore much fewer mature cells.
Sebaceous adenocarcinoma
Adenocarcinomas are invasive tumors and their cell component is mostly reserve cells or baloon cells with a large lipid vacuole.