Molecular biology of sperm head shaping.

The shaping of the mammalian sperm involves the elongation and condensation of the spermatid nucleus, the development of the acrosome, and the transient appearance of the microtubular manchette. F-actin-containing ectoplasmic hoops of Sertoli cells embrace the upper third of the spermatid head during elongation. During acrosomal biogenesis, proacrosomal vesicles derived from the Golgi apparatus, dock and fuse along the acroplaxome, an F-actin/keratin 5-containing cytoskeletal plate. The acroplaxome consists of a bent plate and a marginal ring encircling the spermatid nucleus. It anchors the developing acrosome to the spermatid nucleus. The manchette, consisting of a perinuclear rings with inserted microtubules, lies subjacent to the marginal ring of the acroplaxome. During spermatid elongation, the two overlapping rings reduce their diameter to fit, in a sleeve-like fashion, the decreasing diameter of the spermatid nucleus. The acroplaxome may provide a planar scaffold to modulate exogenous constriction forces generated by Sertoli cell F-actin hoops during spermatid head elongation. The dynamics of the F-actin cytoskeleton, one of the components of the acroplaxome and Sertoli cell hoops, can be regulated by tyrosine kinases, which target cortactin, an F-actin-associated protein. Tyrosine phosphorylation of cortactin correlates with a reduction in the crosslinking properties of F-actin. Phosphorylated cortactin and tyrosine kinase Fer are present in the acroplaxome, thus supporting a role of this F-actin remodelling pathway during spermatid head shaping. Keratin 5, an additional component of the acroplaxome, may also undergo dynamic reorganization during spermatid head elongation. We postulate that the F-actin/keratin 5 cytoskeleton in the acroplaxome may undergo a dynamic reorganization to modulate exogenous shear forces exerted by Sertoli cell F-actin hoops during spermatid head shaping. The acroplaxome-manchette perinuclear rings may reduce their diameter to balance exogenous constriction forces generated by the embracing Sertoli cell F-actin hoops and guide nuclear elongation.