Endocytosis of macromolecules in the lenses of guinea pig and rabbit.

Endocytosis in lenses of rabbit and guinea pig was studied using freeze-fracture electron microscopy and horseradish peroxidase (HRP) as a macromolecular tracer during various periods of lens organ culture. Thin-section and freeze-fracture TEM show that endocytosis occurs predominantly in the epithelial cells of both rabbit and guinea pig lenses before incubation in the HRP. Tracer study in the guinea pig lens reveals that vesicular transport of HRP was most active in the epithelium and elongating young fiber cells where the metabolic activity is high. The endocytic vesicles, smooth in surface and about 0.07 microns in size, were frequently found associated with the lateral epithelial membranes. Isolated vesicles were seen to fuse with lysosomes to form multivesicular bodies or secondary lysosomes. The number of endocytic vesicles shows only a slight decrease after a 4-hr incubation, while secondary lysosomes gradually increase their number and reach the peak at the end of the culture period. The results suggest that endocytosis is a continuous physiologic process, and may play an important role in transporting some useful macromolecules from aqueous humor into the lens cells.