Spermatid translocation in the rat seminiferous epithelium: Coupling membrane trafficking machinery to a junction plaque

In this study, we demonstrate that specialized junction plaques that occur between Sertoli cells and spermatids in the rat testis support microtubule translocation in vitro. During spermatogenesis, Sertoli cells are attached to spermatids by specialized adhesion junctions termed ectoplasmic speciali zations (ESs). These structures consist of regions of the plasma membrane a dherent to the spermatid head, a submembrane layer of tightly packed actin filaments, and an attached cistern of endoplasmic reticulum. It has been pr oposed that motor proteins on the endoplasmic reticulum interact with adjac ent microtubules to translocate the junction plaques, and hence the attache d spermatids, within the epithelium. If this hypothesis is true, then isola ted junctions should support microtubule transport. To verify this predicti on, we have mechanically isolated rat spermatids, together with their attac hed ESs, and tested them for their ability to transport microtubules in vit ro. Most assays were done in the presence of 2 mg/ml testicular cytosol and at room temperature. ESs attached to spermatids supported microtubule tran slocation, In some cases in which motility events were detected, microtubul es moved smoothly over the junction site. In others, the movement was slow but progressive, saltatory and "inch-worm-like." No motility was detected i n the absence of exogenous ATP or in the presence of apyrase (an enzyme tha t catalyses the breakdown of ATP). Our results are consistent with the micr otubule-based motility hypothesis of spermatid translocation.