Acanthotic seborrheic keratosis

Acanthotic seborrheic keratoses are composed of broad sheets of cells with intervening horn cysts or pseudohorn cysts. Horn cysts are completely encased within the acanthoma, whereas pseudohorn cysts open to the surface. Like other seborrheic keratoses, they may become irritated or inflamed.

Hyperkeratotic seborrheic keratosis

The tall stacked stratum corneum is typically much thicker than the epidermis. As in other forms of seborrheic keratosis, the keratin has a loose lamellar “shredded-wheat” appearance unless the lesion has become irritated or inflamed. Papillomatosis is characteristic, but variable in degree. Horn cysts are inconspicuous or absent.

Reticulated seborrheic keratosis (adenoid seborrheic keratosis)

The lesion is composed of thin interlacing strands of epidermis, typically two cells thick. These strands are generally pigmented.

Clonal seborrheic keratosis

Clonal seborrheic keratosis is characterized by islands of small keratinocytes with uniform bland nuclei. The nests are embedded within the epidermis. Sometimes, the nests are large enough that the normal epidermis is reduced to thin strands separating the large nests. Horn cysts are usually absent. The nests may demonstrate pigment. There may be squamous eddies (irritated seborrheic keratosis), lymphocytes (inflamed seborrheic keratosis), or both. In contrast to Bowen’s disease, the cells are uniform and atypia is absent. In contrast to hidroacanthoma simplex, no ducts are present within the clones.

Pigmented seborrheic keratosis

Irritated seborrheic keratosis

Irritated seborrheic keratosis is characterized by the formation of squamous eddies within the epidermis and the presence of spindled keratinocytes. Some areas of the tumor typically still produce a loose lamellar “shredded-wheat” pattern of keratin, and horn cysts composed of loose lamellar keratin are often present. In areas, the keratin becomes compact, eosinophilic, and parakeratotic. A zone of loose lamellar keratin may be seen above the dense eosinophilic keratin. This was produced before the lesion became irritated. It has since been pushed upward.

Inflamed seborrheic keratosis