372Chapte

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Benign tumors and cysts of the epidermis

DowlingDegos disease/reticulated pigmented anomaly of the flexures

Key features Resembles multiple foci of reticulated seborrheic

keratosis hanging off hair follicles Comedo-like dilated keratin-filled follicular

infundibula

Clonal seborrheic keratosisKey features Clonal islands of small keratinocytes within the

epidermis Bland uniform nuclei

Absence of duct differentiation within clonal nests

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 pig-ment. There may be squamous eddies (irritated seborrheic keratosis), lymphocytes (inflamed seb-orrheic keratosis), or both. In contrast to Bowens disease, the cells are uniform and atypia is absent. In contrast to hidroacanthoma simplex, no ducts are present within the clones.

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b

Figure 2-9 DowlingDegos disease

Figure 2-10 Clonal seborrheic keratosis

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Figure 2-11 Pigmented clonal seborrheic keratosisf0110f0110

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Figure 2-12 Irritated clonal seborrheic keratosis

a