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.