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Seborrheic keratoses in five patients were rubbed and biopsied at varying intervals after injury. Microscopic examination revealed acute and chronic patterns of histologic change. Hemorrhage, hyalinization of dermal papillae, and necrosis of epithelial tips were conspicuous early changes. Specimens taken more than 48 h after rubbing showed a spectrum of changes which included: loss of epithelial mass, expansion and interconnection of keratin cysts, thinning of the epithelium, proliferation of epithelial strands from the residual epithelium, increase in size of epithelial cells, and evidence of hair follicle relationships including trichostasis spinulosa and, in one specimen, hair germ proliferation. Dermal lymphocytic infiltration was variable and only rarely involved the epithelium. These observations demonstrate a patterned response of the seborrheic keratosis to trauma, and also indicate a relationship between seborrheic keratosis and the hair follicle.

In 54 of 325 seborrheic keratoses, trichostasis spinulosa was found within the keratinous cysts and between papillomatous surface keratin, in addition to being present in contiguous hair follicles. Trichostasis in seborrheic keratoses and trichostasis spinulosa are both more frequent in older age groups. Trichostasis spinulosa represents a marker of vellus hair follicles and indicates that some seborrheic keratoses incorporate and may be partially derived from the cells of the infundibular portion of the follicle.