Nonrandom distribution of coated pits and vesicles in the connective tissue cells of the trabecular meshwork of rabbit.

Coated pits (CPs) and coated vesicles (CVs) are distributed nonrandomly along the surface of the connective tissue cells of the rabbit trabecular meshwork. Morphometric and statistical analyses of the distances between consecutive structures reveal a tendency to cluster, which is apparently higher among CVs than among CPs. The spatial relationship between CVs and CPs is also demonstrated by the analysis of their association (presence/absence) in each cell. The data suggest the hypothesis that each cluster is formed by structures in the same stage of maturation. In addition to the recent demonstration of the clustering of CPs in transformed cells in vitro (Pfeiffer et al. 1980), our findings indicate that: (a) clustering is characteristic of both CVs and CPs; (b) it also occurs in normal tissue cells; (c) it represents a further peculiarity, which confirms the specific nature of receptor-mediated endocytosis; (d) it reflects, at a higher level, the clustering of the receptor molecules, which are responsible for the high selectivity of the endocytic process.