Benign epidermal tumors

Tumor consists of basaloid (= roundish, sligtly paler, regular with relatively large nuclei) keratinocytes). Horn cysts with abrupt keratinization are often present (basaloid cells of the tumor suddenly mature into regular keratin lamellae. These lamellae have no parakeratosis and in HE staining are usually slightly bluish. Another common signs are akanthosis, hyperkeratosis and squamous eddies (especially in irritated form).

Acanthotic form is characterised by thickened epidermis with variable acanthosis; horn cysts are common

Melanoacanthotic form see melanoacanthoma

Reticular form consists of multiple, interconnected epithelial bands, often only two cell layers thick.

Clonal form forms nests of regular, basaloid cells within the epidermis

Hyperkeratotic form characterised by prominent hyperkeratosis.

Irritated type of seborrhoic keratosis is characterised by multiple squammous eddies. These eddies are small, without cellular atypia and are present in large numbers. Inverted follicular keratosis is similar or identical process.

Small, bud-like acanthotic proliferations; hyperpigmentation of the basal layer; variable increase of melanocytes; solar degeneration of the corium.

Regular acanthosis, hyperpigmented epidermis, sometimes increased amount of hair follicles.

Small papillomatous lesions with some hyperkeratosis and no parakeratosis (resembles small seborrheic keratosis).

Hyperkeratosis, no parakeratosis, papillomatosis (thin papillae), slight acanthosis.

Hyperkeratosis, no parakeratosis, papillomatosis and acanthosis on the periphery, central thinning of the epidermis. Often dense lymphocytic infiltrate is present within the dermis.

Slight hyperkeratosis, papillomatosis, often colonization with Pityrosporon (Malassezia) orbiculare.

Fine, low papilomatosis, slight orthokeratotic hyperkeratosis (parakeratosis is minimal or none). Keratinocytes are round, sometimes eosinophilic. In some cases increased pigmentation of the basal layer.

Papular or verrucous lesion. Irregular acanthosis with acantholysis and dyskeratosis.

Prominent acantholysis (resembles m. Hailey-Hailey or pemphigus), but the process is localized; orthokeratotic.

Acanthotic proliferation of benign appearance with multiple, tiny squamous eddies.

Small, acanthotic lesion (often fan shaped acanthosis); superficial areas often desintegrate; the keratinocytes contain large, prominent, basophilic intracytoplasmic inclusions.

Slight hyperkeratosis, keratinocytes are two times as large as in normal epidermis.

The histology corresponds to the epidermolytic hyperkeratosis.

Acanthosis, prominent hyperkeratosis.

Hyperkeratotic papule, formed by central keratinous plug surrounded by proliferating, atypical epidermal invagination. Dysplastic epidermis shows changes corresponding to the well differentiated squamous carcinoma, but cytoplasm is usually more eosinophilic. Mitotic activity is usually high, especially in early lesions. Epithel shows focal necroses and neutrophilic or eosinophilic microabscesses. At the base of the lesion there is usually dense lymphocytic infiltrate, often with admixture of eosinophils. At periphery the tumor shows limited infiltrating behavior.

Slight spongiosis, pale, PAS positive keratinocytes (normal basal layer), acanthosis, parakeratosis, neutrophils within the epidermis, sometimes neutrophilic microabscesses, dilated capillaries.

Pale, PAS positive, alcian blue and mucicarmin positive cells containg mucus are scattered in the basal layer, sometimes higher.

Irregular acanthosis of somewhat pale epithelium (infundibular); variable inflammatory infiltrate, some chronic process nearby.

The epidermis is thickened, the basal layer is prominent, keratinocytes are basophilic, palisading. Differential diagnosis against superficial bazalioma is sometimes difficult.