AN ULTRASTRUCTURAL ANALYSIS OF THE EPITHELIAL-FIBER INTERFACE (EFI) IN PRIMATE LENSES

The purpose of this study was to conduct a comprehensive ultrastructur al analysis of the epithelial-fiber interface (EFI) in normal adult pr imate (Macaque nemestrina and fascicularis; 6-9 years old, n = 10) len ses, Scanning electron microscopy (SEM) was used to initially characte rize the gross size, shape and three-dimensional organization of centr al zone (cz) epithelial cells and the anterior ends of elongating fibe rs beneath these cells. This fiducial information was essential to pro perly orient lens pieces in freeze fracture specimen carriers for the production of replicas with unambiguously identifiable EFI. Transmissi on electron microscopy (TEM) of replicas and thin-sectioned material w ere used to ultrastructurally analyse the cz EFI. TEM thin-sectioned m aterial was also used to ultrastructurally analyse the pregerminative (pgz), germinative (gz) and transitional zone (tz) EFI. Correlative SE M and TEM of cz EFI components revealed that the apical membrane of bo th epithelial and elongating fiber cells were irregularly polygonal in shape, and aligned in parallel as smooth, concave-convex surfaces. Ho wever, whereas epithelial cell apical surfaces had minimal size variat ion, elongating fibers were larger and considerably variable in size, Quantitative analysis of > 10 000 mu m(2) cz elongating fiber apical s urfaces failed to detect any gap junctions defined in freeze fracture replicas as complementary aggregates of transmembrane proteins (connex ons) conjoined across a narrowed extracellular space. However, a compa rable frequency of vesicular events was noted in this region as quanti fied previously in adult and embryonic chick lens. Correlative TEM ana lysis > 1 500 linear micrometers of thin-sectioned EFI from this regio n confirmed the presence of epithelial-epithelial gap junctions, elong ating fiber-elongating fiber gap junctions, and an extreme paucity of epithelial-elongating fiber gap junctions. In contrast, TEM analysis o f > 1 000 linear micrometers of thin-sectioned pgz, gz and tz EFI, con firmed the presence of epithelial-epithelial gap junctions, elongating fiber-elongating fiber gap junctions, numerous epithelial-elongating fiber adherens junctions and a few epithelia-elongating fiber gap junc tions. Thus, the results of this and previous quantitative morphologic al and physiological studies (electronic and dye coupling) demonstrate that there is limited coupling between ct epithelial cells and underl ying elongating fibers, Furthermore, the absence of gap junctional pla ques in cz EFI freeze-fracture replicas and either pentalaminar or sep talaminar profiles in correlative thin-sections, suggests that this li mited coupling could be mediated via isolated gap junction channels. H owever, the results of this and previous quantitative studies further show that a greater degree of coupling exists across the pgz, gz and t z regions of the EFI and that this coupling is likely to be mediated b y gap junction plaques. Finally, this and other studies continue to de monstrate that transcytotic processes play a role in lens physiology a t the EFI. (C) 1995 Academic Press Limited