A study of Sertoli-spermatid tubulobulbar complexes in selected mammals.

Tubulobulbar complexes (TBCs) were found in nine mammalian species (opossum, vole, guinea-pig, mouse, hamster, rabbit, dog, monkey and human) primarily originating from the plasma membrane overlying the acrosome of late spermatids. Fewer complexes (4--10) were noted in these species than has been previously reported for the rat (up to 24). TBCs were not seen emanating from round spermatids or those elongated spermatids located within the deep recesses of the Sertoli cell, but they appeared as the spermatids came to reside much closer to the tubular lumen in preparation for release. TBCs developed in areas deficient or lacking in Sertoli filaments and endoplasmic reticulum (ectoplasmic specialization). In general their structural configuration was similar to that shown in the rat, although minor differences were noted. Fine fibrils were observed connecting the distal portion of the spermatid tube with the Sertoli plasma membrane forming a bristle-coated pit. The length of TBCs from most species studied was 1--2 micrometers, whereas those of the opossum extended 6--8 micrometers into an apical Sertoli process. TBCs were degraded within the Sertoli cell by its lysosomes prior to sperm release, and for most species there was evidence indicating that formation of more than one generation of TBCs occurred. As sperm release approached, TBCs formed preferentially from the leading edge of spermatids with spatulate heads. The Sertoli cell gradually withdrew from around the spermatid head until only the tip of the head was embedded within the Sertoli cell. This region of contact frequently demonstrated TBCs. The proposed functions of TBCs are reviewed and discussed in light of these findings from other species.