Trichilemmal keratosis (horn): a light and electron microscopic study.

Three cases of trichilemmal keratosis (horn) were light microscopically examined and all showed numbers of U- or V-shaped epidermal proliferations which keratinized in a fashion either identical or similar to trichilemmal keratinization. Electron microscopy revealed both uneven and linear borders between the keratinized and the keratinizing cells with a few keratohyalin droplets, remnants of desmosomes, no marginal band in the horny layer, perinuclear vacuolation, few spherical bodies in the intercellular spaces (ICS) of the upper epidermis, and widening of the ICS of the lower epidermis. A number of electron dense spherical particles, 40-50 nm in diameter, were observed in nuclei of the upper epidermis. This suggests that ultrastructure of trichilemmal keratosis is similar rather to viral warts than to trichilemmal cysts, although there are close similarities between trichilemmal keratosis and cyst.