Dedifferentiation of iris epithelium during lens regeneration in newt larvae.

Early stages in lens regeneration from the pigmented epithelium of the dorsal iris were studied in larval Notophthalmus viridescens by means of transmission electron microscopy. Normal iris epithelium is composed of two layers of low cuboidal cells. packed with melanosomes and surrounded by a basal lamina. Scattered desmosomes attach adjacent cells. Following lens removal, the intercellular spaces enlarge and the epithelial cells increase in size. Some irregular microvilli from these cells extent into the intercellular spaces. Macrophages invade the iris epithelium and phagocytize melanosomes discharged from the pigmented cells. These invading macrophages have numerous microprojections and are often separated from the surface by a very thin layer of iris epithelial cell cytoplasm. In the iris cells, nucleoli become more prominent and granular, polyribosomes increase greatly in number, melanosomes gradually disappear, mitochondria become more numerous, and mitotic activity is greatly augmented. Fine cell processes of adjacent interdigitate near the external surface, where numerous micropinocytotic vesicles can be seen. Over the external surface, the basal lamina may be disrupted or duplicated in places where pseudopodia project from iris cells or a macrophage has entered an intercellular space. It is lacking on the lumenal surface. Sloughed membranes are often found in these intercellular spaces.