Biogenesis of the posterior-tail plasma membrane domain of the mammalian spermatozoon: targeting and lateral redistribution of the posterior-tail domain-specific transmembrane protein CE9 during spermiogenesis.

We used immunoperoxidase histochemistry and confocal immunofluorescence microscopy to examine the events involved in the compartmentalization of CE9 to the posterior-tail plasma membrane domain during spermatogenesis in the rat. We identified two major episodes of spermatogenesis during which CE9 appeared to accumulate in relatively large amounts intracellularly within elements of the secretory pathway. The first episode encompassed cells from preleptotene through early pachytene primary spermatocytes and was evident as intense intracellular labeling of the endoplasmic reticulum and the Golgi complex. The second episode encompassed spermatids in steps 8-12 of spermiogenesis and was evident as intense intracellular labeling of the Golgi complex and smaller vesicular structures observed within the cytoplasm of the spermatid. Between these two episodes, CE9 was detected in considerably reduced amounts. Although present within the Golgi complex and the acrosomic system throughout much of the first half of spermiogenesis, CE9 was not detected on the tail of the spermatid until steps 8-9 of spermiogenesis. Although detected initially in relatively small amounts along the entire length of the tail beginning at steps 8-9, there was no evidence for the presence of relatively large amounts of CE9 on the tail or anywhere else on the surface of the spermatid until after step 11 of spermiogenesis. Between step 11 and steps 13-14 of spermiogenesis, CE9 was observed to accumulate in relatively large amounts on the whole tail coincident with its apparent loss from the Golgi complex. CE9 was observed to then undergo further compartmentalization to the posterior-tail domain sometime between steps 13-14 of spermiogenesis and spermiation. Our results suggest that CE9 is synthesized and enters the secretory pathway throughout much of spermatogenesis, but that the site of accumulation of CE9 varies considerably as a function of development. With respect to the biogenesis of the posterior-tail plasma membrane domain, our results suggest that CE9 is targeted from the Golgi complex to the plasma membrane of the whole tail during mid to late spermiogenesis and then redistributes laterally into the posterior-tail domain coincident with the caudal migration of the annulus late in spermiogenesis. This proposed pathway has a number of important implications for the logistical capabilities of the mammalian spermatid.