Ultrastructural characteristics of trichilemmal cysts: report of two cases

Two trichilemmal cysts (TC) on the scalps of two Japanese women were examined at the electron microscopic level. The ultrastructural characteristics of the TC included an abrupt transition from nucleate to anucleate cells, a few oval, small, keratohyaline granules mixed with increasing numbers of tonofibrils from the peripheral layer towards the center, spherical particles with lipid droplets, desmosomal structures in keratinized cells, and the interdigitation of keratin in the uppermost keratinizing cells. The keratinization pattern of the TC was unique in that typical soft or hard keratinization processes did not occur. Trichilemmal keratinization is observed in the trichilemmal sacs that 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 the 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.