155 Ultrastructural Characterics of Trichilemmal Cysts

M. Nakamura, M. Toyoda, M. Kagoura, S. Higaki, and M. Morohashi. Dept. of Dermatology, Toyama Medical and Pharmaceutical University, Toyama, Japan

The name of trichilemmal cyst (TC), one of the skin tumors with differentiation toward hair structures, was designated on the basis of its keratinization pattern which is analogous to that in the outer root sheath of the hair. Although the histological features of TC have been well documented, only few ultrastructural studies of TC have previously been performed. We examined two cases of TC which occurred on the scalps of 2 Japanese women mainly at the electron microscopic level. Histologically, the tumors were intradermal cysts lined by a squamous epithelium composed of pale keratinocytes that increase in height as they mature and that transform abruptly into a solid eosinophilic-staining keratin without forming a granular layer. The ultrastructural characteristics of TC included an abrupt transition from nucleate to anucleate cells, a few, oval and small-sized keratohyaline granules mixing with increasing numbers of tonofibrils from the peripheral layer towards the center, the aggregation of spherical vesicular particles, especially along the boundary zone between the keratinizing cells and the keratinized cells, desmosomal structures in keratinized cells, and interdigitation of keratin in the uppermost keratinizing cells. The keratinization pattern of TC was unique in that typical soft or hard keratinization processes did not occur. Although the origin of spherical particles is unclear at present, our observation suggested that the spherical particles may be derived from the plasma membrane of the uppermost keratinizing cells and may contribute to the formation of lipid droplets, a feature characteristic of keratin in TC. Trichilemmal keratinization is observed in the trichilemmal sacs which surround the lower ends of catagen or telogen hairs as well as in the outer root sheaths in the follicular isthmus of anagen hairs. Comparison of the ultrastructure of these two known types of trichilemmal keratinization with the keratinization pattern of TC in our patients indicates that TC may differentiate into or originate from the proliferation of the outer root sheath in the follicular isthmus of anagen hairs.