Polarized distribution of coated pits and coated vesicles in the rat lens: an electron microscopy and WGA-HRP tracer study.

The presence and distribution of coated pits (CPs) and coated vesicles (CVs) in the rat lens were studied by thin-section electron microscopy (TEM) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a tracer. TEM revealed that CPs and CVs were approximately 150 nm in diameter, of which the characteristic clathrin coat was approximately 20 nm thick. CPs and CVs were found in both epithelium and superficial fiber cells of the entire lens, and were distributed preferentially along the basal membrane facing the lens capsule. It was estimated that more than 80% of CPs and CVs in the entire epithelium were seen along the basal membrane. The number of CPs and CVs along the basal membrane in the equatorial epithelium (4.4 per 10 microns membrane) was similar to that at the central zone (3.8 per 10 microns membrane), but there was a significant increase along the apical and lateral surfaces of the equatorial epithelium compared to that of the central epithelium, although the overall number was considerably smaller. In the lens fibers, CPs and CVs were usually found within 2-3 superficial layers of fiber cells. The number of CPs and CVs along the basal membrane of young fibers at the post-equatorial region (3.1 per 10 microns membrane) was 3-fold greater than that of the mature fibers at the posterior polar area (1 per 10 microns membrane). Thus, CPs and CVs along the entire basal membrane showed a gradual decrease in number from the anterior (and equatorial) regions to the posterior polar surface of the lens. WGA-HRP experiments showed that approximately 80% of tracer-carrying pits and vesicles were also found along the basal surface of the equatorial epithelium. This study suggests that a polarized distribution of CPs and CVs along the basal surface of epithelium and superficial fiber cells may facilitate receptor-mediated endocytosis of important macromolecules directly from the aqueous humor and vitreous body into metabolically active lens cells.