A scanning electron microscopic study of the developing epithelial scleral papillae in the eye of the embryonic chick.

Eyes of early embryonic chicks possess 14 scleral papillae, derived from the conjunctival epithelium and present as transient structures between seven and 11 days of incubation. These papillae induce the formation of the 14 scleral ossicles, which develop in the adjacent, neural crest-derived ectomesenchyme. Each papilla undergoes a predictable series of developmental changes, divided by Murray ('43) into six morphological stages (M stages 1-6). We have confirmed his staging, and provide a scanning electron microscopic (SEM) evaluation of papilla development. The earliest stage that can be visualized with the S.E.M. is M stage 2. We describe the initial modifications of the surface of papilla cells, the presence of large microvilli and the asymmetrical morphogenesis and growth of the papillae. Papillae are shed by a mechanism that involves elongation of the cells at the base of the papilla. Such moribund papillae consist of necrotic cells coated with fibers.