Figure 2.

A schematic drawing illustrating the mechanism of spermatid movement along Sertoli cells in the seminiferous epithelium via a cascade of biological events. Spermiation occurs at stage VIII of the epithelial cycle, which coincides with the time for the passage of preleptoene spermatocytes (PS) across the BTB. During these cellular events, the different cascades of biological events (see the numbers in the figure) happen in a highly regulated manner. We hypothesize that germ cell movement involves the continuous disassembly and reassembly of Sertoli–Sertoli and Sertoli–germ cell junctions. These require the presence of cytokines (e.g. TNFα and TGF-β3) and proteases to perturb the junctions by lowering the steady-state protein level of several integral TJ and AJ proteins. Protein kinases and protein phosphatases also mediate tyrosine phosphorylation of the AJ (cadherin/catenin/CK2/cSrc) and FAC-like (integrin/laminin/FAK/cSrc) adhesion complex or an induction of cofilin and/or gelsolin. The net result of these changes leads to a weakening of adhesion complexes at the basal and apical ES. A transient disengagement between TJ and AJ via peripheral adaptors, such as ZO-1 and catenins, is used to preserve the BTB integrity when AJ is perturbed. To assist spermatid movement, upward traction force is generated by the motor proteins, such as myosin VIIa and dynein, at both the apical and basal ES, involving GTPases. Prior to spermiation, integral membrane junction proteins such as nectin-2 and nectin-3 may be internalized by both apical and basal TBC to facilitate the depletion of elongate spermatid (ES) from the epithelium (i.e. spermiation) and the passage of PS across the BTB. Recently, it has been speculated that cleavage of laminin at the apical ES by MMPs can assist the ‘opening’ of BTB via a yet-to-be identified signaling pathway. This model was prepared based on recent findings in the field (see text). SC Nucleus, Sertoli cell nucleus.