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A combination of exogenous contractile forces generated by a stack of F-actin-containing hoops embracing the apical region of the elongating spermatid nucleus and an endogenous modulating mechanism dependent on the spermatid-containing acrosome-acroplaxome-manchette complex may play a cooperative role in the shaping of the spermatid head. In addition, the manchette is a key element in the transport of vesicles and macromolecules to the centrosome and developing spermatid tails as well as in nucleocytoplasmic transport. The proposed model of spermatid head shaping is based on: 1) currently known structural and molecular components of the F-actin hoops, the main cytoskeletal element of the Sertoli cell ectoplasmic specializations; 2) the molecular features of acrosome biogenesis; 3) the assembly of a subacrosomal cytoskeletal plate called the acroplaxome; and 4) the spatial relationship of the acrosome-acroplaxome complex with the manchette, a transient microtubular/actin-containing structure. During acrosome biogenesis, the acroplaxome becomes the nucleation site to which Golgi-derived proacrosomal vescicles tether and fuse. The acroplaxome has at least two functions: it anchors the developing acrosome to the elongating spermatid head. It may also provide a mechanical scaffolding plate during the shaping of the spermatid nucleus. The plate is stabilized by a marginal ring with junctional complex characteristics, adjusting to exogenous clutching forces generated by the stack of Sertoli cell F-actin-containing hoops applied to the elongating spermatid head. A tubulobulbar complex, formed by cytoplasmic processes protruding from the elongating spermatid head extending into the adjacent Sertoli cell, is located at the concave side of the spermatid head. The tubulobulbar complex might provide stabilizing conditions, together with the actin-afadin-nectin-2/nectin-3 adhesion unit, to enable sustained and balanced clutching exogenous forces applied during the elongation of the spermatid head.

The acrosome-acroplaxome-manchette complex and the shaping of the spermatid head