Anchoring device between Sertoli cells and late spermatids in rat seminiferous tubules.

Near the end of spermiogensis, the late spermatids remain attached to the superficial layer of the seminiferous epithelium for an appreciable period of time (i.e., 3 to 4 days). Ths sickle-shaped heads of the spermatids are embedded in an apical process of Sertoli cell cytoplasm which is connected to the rest of the cell by a narrow stalk. In the concavity of the head several long (2-3 mum) and very narrow (50 nm) tubular projections of the spermatid's plasma membrane invaginate the Sertoli cell cytoplasm. These tubular processes terminate by a bulbous swelling which may measure up to 1 mum in diameter. Along the process the plasma membrane of the Sertoli cell is closely apposed to the spermatid's membrane, the intracellular space being only 6-8 nm wide. In the Sertoli cytoplasm immediately surrounding the tubular portion of the structure there is an accumulation of filamentous material, while next to the bulbous extremity there are, at a shrot distance, smooth surfaced cisternae of endoplasmic reticulum. The whole structure was referred to as a tubulobulbar complex. These complexes, of which there are up to 24 per spermatid, appear as these cells complete their migration toward the apex of the Sertoli cells. They disappear just before the release of the spermatids in the lumen of the seminiferous tubule as a result of the fragmentation of the spermatid's plasma membrane followed by a resorption of the Sertoli plasma membrane. Morphological evidence suggests that the Tubulobulbar complexes serve as anchoring devices that retain the spermatids at the surface of the seminiferous epithelium while their dissolution contributes in part to the process of spermiation. Similar tubulobulbar complexes were also formed by the plasma membranes of two adjacent Sertoli cells close to the Sertoli-Sertoli tight junctions near the tubular limiting membrane.