Distribution and Function of Organized Concentrations of Actin Filaments in Mammalian Spermatogenic Cells and Sertoli Cells

Abstract

Actin filaments are concentrated in specific regions of spermatogenic cells and Sertoli cells. In spermatogenic cells they occur in intercellular bridges and in the subacrosomal space. In Sertoli cells they are abundant in ectoplasmic specializations and in regions adjacent to tubulobulbar processes of spermatogenic cells. At all of these sites, the filaments are morphologically related to the plasma membrane and+or intercellular membranes, and, as in many other cell types, are arranged in either bundles or networks. In at least two of the locations just indicated (ectoplasmic specializations and intercellular bridges), elements of the ER are closely related to the actin filaments. In tubulobulbar complexes, ER is present but is more distantly related to the filaments. Elements of the ER, when present, may serve a regulatory function. The filaments in ectoplasmic specializations and in regions adjacent to tubulobulbar processes of spermatogenic cells are suspected to be involved with the mechanism by which intercellular junctions are established, maintained, and degraded. In intercellular bridges, actin filaments may serve to reinforce and perhaps regulate the size of the cytoplasmic connections between differentiating germ cells. Filaments in the subacrosomal space may serve as a linking network between the acrosome and nucleus and may also be involved in the capping process. Because of the possibility that the actin filaments discussed before may be related to specific membrane domains involved with intercellular or interorganelle attachment, and that changes in these membrane domains are prerequisite to processes such as sperm release, turnover of the blood-testis barrier, formation of the acrosome, and coordination of spermatogenic cell differentiation, an understanding of exactly how these actin filaments are related to elements in the membrane and how this interaction is controlled is fundamental to our understanding, and perhaps our manipulating, of male fertility. I suspect that working out the molecular organization of these actin filament-containing sites and determining how their organization is controlled will be the major focus of research in this field over the next few years.