Trichilemmal cysts. Ultrastructural similarities to the trichilemmal sac.

Recently experienced cases of trichilemmal cysts were studied by light and electron microscopy. Although ultrastructural findings in the present case were basically consistent with those reported in the literature, they were in contrast to them in some important respects: (1) a few, oval and small-sized keratohyalin and a large number of membrane-coating granules were observed in the cytoplasm of keratinizing cells beneath the keratinized cells and (2) the junctional structure between the keratinized cells and the keratinizing cells beneath them showed interdigitation. From these findings, it is shown that trichilemmal cysts have close similarities to and might be derived from the trichilemmal sac surrounding the lower end of the catagen or telogen hair.