Fine structure and significance of snakelike chromatin in conjunctival epithelial cells

Abstract

Purpose: In patients with several disorders of the ocular surface, in wearers of contact lenses, and occasionally in patients considered "normal," the epithelial cells of the bulbar conjunctiva present in their nucleus peculiar alterations of the heterochromatin arrangement that, because of the shape this assumes, are named "snakelike chromatin," or "snakes." To obtain some information about the yet unknown etiology of these snakes, the authors investigated them by electron microscopy.

Methods: Identical conjunctival epithelial cells, collected by impression cytology from long-time contact lens wearers, were first identified by light microscopy and then observed by scanning and transmission electron microscopy.

Results: In scanning electron microscopy, cytoplasmic and nuclear components of air-dried cells were seen collapsed on the snake, which became prominent at the surface, proving its high degree of compactness and showing its axial position and characteristic shape in the elongated nucleus. In transmission electron microscopy, the marginal heterochromatin of the affected nuclei was detached peripherally, forming thin chromatin strands directed toward the main nuclear axis and accumulating there into the snake structure. An important component of the nuclear skeleton, the fibrous lamina, was altered or lost, whereas the nuclear envelope itself did not move and remained intact. Stages of the snake alteration considered as advanced showed an almost completely reversed eu- and heterochromatin distribution, and nucleoli were usually no longer seen. In cases with increased epithelial alteration, there occurred various stages of segmentation of nuclei, induced by an atypical accumulation of cytoplasmic filaments, rolling up around the nucleus and constricting it like a cuff.

Conclusions: The presence of a mechanical stimulus is shown in the ultrastructural findings, and these strongly suggest that it is altering the nuclear and cytoplasmic skeleton, producing snakes and their segmentation. Therefore, snakelike chromatin is suggested as an indicator of mechanical stress on the ocular surface.