Ectoplasmic specializations are actin filament-endoplasmic reticulum complexes that occur in Sertoli cells at sites of intercellular attachment. At sites between inter-Sertoli cell attachments, near the base of the cells, the sites are also related to tight junctions. We studied the characteristics of ectoplasmic specializations from six species using conventional views in which thin sections were perpendicular to the plane of the membranes, we used rare views in which the sections were in the plane of the membrane (en face views), and we also used the freeze-fracture technique. Tissues postfixed by osmium ferrocyanide showed junctional strands (fusion points between membranes) and actin bundles, actin sheets, or both, which could be visualized simultaneously. En face views demonstrated that the majority of tight junctional strands ran parallel to actin filament bundles. Usually, two tight junctional strands were associated with each actin filament bundle. Parallel tight junctions were occasionally extremely close together (∼12 nm apart). Tight junctional strands were sometimes present without an apparent association with organized actin bundles or they were tangential to actin bundles. En face views showed that gap junctions were commonly observed intercalated with tight junction strands. The results taken together suggest a relationship of organized actin with tight junction complexes. However, the occasional examples of tight junction complexes being not perfectly aligned with actin filament bundles suggest that a precise and rigidly organized actin-tight junction relationship described above is not absolutely mandatory for the presence or maintenance of tight junctions. Species variations in tight junction organization are also presented.
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