Benign acanthomasAcanthomas are benign cutaneous neoplasms characterized by an expansion of the epidermis. The acanthoma may be composed of clones of cells that displace or compress the preexisting epidermis. In contrast to the reactive acanthosis seen in inflammatory disorders, the rete ridge pat-tern is commonly ablated by the neoplastic tissue of an acanthoma.

Seborrheic keratosesSeborrheic keratoses are acanthomas composed of small polygonal keratinocytes about the size of acrosyryngeal keratinocytes (the cells that make up the intraepidermal portion of the eccrine duct). The cells are typically smaller than the cells of the surrounding epidermis, and they are commonly pigmented. Architectural subtypes of seborrheic keratoses include acanthotic, hyperk-eratotic, reticulated, and clonal. Any of these sub-types may be pigmented, irritated (spindling of cells and squamous eddy formation), or inflamed (usually lymphoid inflammation). Melanoacan-thoma is a distinct subtype of seborrheic keratosis composed of small keratinocytes and dendritic melanocytes.

Seborrheic keratoses produce a characteristic loose lamellar shredded-wheat stratum cor-neum. Exceptions include irritated or inflamed seborrheic keratosis. Instead of the characteristic loose lamellar horn, irritated or inflamed sebor-rheic keratoses produce a compact brightly eosi-nophilic parakeratotic stratum corneum. Adjacent unaffected areas of the seborrheic keratosis still produce the characteristic loose lamellar stratum corneum, and it is common to see remnants of loose stratum corneum above areas of compact stratum corneum. Melanoacanthomas produce a deeply eosinophilic compact parakeratotic

stratum corneum, even when they are not irri-tated or inflamed.

Seborrheic keratoses may express BCL-2, a marker associated with resistance to programmed cell death (apoptosis). Activating point mutations in the gene encoding fibroblast growth factor re-ceptor 3, a tyrosine kinase receptor, are also com-mon in seborrheic keratoses.

Acanthotic seborrheic keratosisKey features Broad sheets of small polygonal keratinocytes

with intervening horn cysts Loose lamellar shredded-wheat or onion-skin

keratin Commonly pigmented

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

Benign tumors and cysts of the epidermis

Dirk M Elston, MD

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Figure 2-1 Acanthotic seborrheic keratosis