Although the molecular basis of sperm-oocyte interaction is unclear, recent studies have implicated two chaperone proteins, heat shock protein 1 (HSPD1; previously known as heat shock protein 60) and tumor rejection antigen gp96 (TRA1; previously known as endoplasmin), in the formation of a functional zona-receptor complex on the surface of mammalian spermatozoa. The current study was undertaken to investigate the expression of these chaperones during the ontogeny of male germ cells through spermatogenesis, epididymal sperm maturation, capacitation, and acrosomal exocytosis. In testicular sections, both HSPD1 and TRA1 were closely associated with the mitochondria of spermatogonia and primary spermatocytes. However, this labeling pattern disappeared from the male germ line during spermiogenesis to become undetectable in testicular spermatozoa. Subsequently, these chaperones could be detected in epididymal spermatozoa and in previously unreported "dense bodies" in the epididymal lumen. The latter appeared in the precise region of the epididymis (proximal corpus), where spermatozoa acquire the capacity to recognize and bind to the zona pellucida, implicating these structures in the functional remodeling of the sperm surface during epididymal maturation. Both HSPD1 and TRA1 were subsequently found to become coexpressed on the surface of live mouse spermatozoa following capacitation in vitro and were lost once these cells had undergone the acrosome reaction, as would be expected of cell surface molecules involved in sperm-egg interaction. These data reinforce the notion that these chaperones are intimately involved in the mechanisms by which mammalian spermatozoa both acquire and express their ability to recognize the zona pellucida.

Fourteen steps of spermatid development in the tammar wallaby (Macropus eugenii ), from the newly formed spermatid to the release of the spermatozoon into the lumen of the seminiferous tubules, were recognised at the ultrastructural level using transmission and scanning electron microscopy. This study confirmed that although the main events are generally similar, the process of the differentiation of the spermatid in marsupials is notably different and relatively more complex than that in most studied eutherian mammals and birds. For example, the sperm head rotated twice in the late stage of spermiogenesis : the shape of the spermatid changed from a T-shape at step 10 into a streamlined shape in step 14, and then back to T-shape in the testicular spermatozoa. Some unique figures occurring during the spermiogenesis in other marsupial species, such as the presence of Sertoli cell spurs, the nuclear ring and the subacrosomal space, were also found in the tammar wallaby. However, an important new finding of this study was the development of the postacrosome complex (PAC), a special structure that was first evident as a line of electron dense material on the nuclear membrane of the step 7 spermatid. Subsequently it became a discontinuous line of electron particles, and migrated from the ventral side of the nucleus to the area just behind the posterior end of the acrosome, which was closely located to the sperm-egg fusion site proposed for Monodelphis domestica (Taggart et al. 1993). The PAC and its possible role in both American and Australian marsupials requires detailed examination. Distinct immature features were discovered in the wallaby testicular spermatozoa. A scoop shape of the acrosome was found on the testicular spermatozoa of the tammar wallaby, which was completely different to the compact button shape of acrosome in ejaculated spermatozoa. The fibre network found beneath the cytoplasm membrane of the midpiece of the ejaculated sperm also did not occur in the testicular spermatozoa, although the structure of the principle piece was fully formed and had no obvious morphological difference from that of the epididymal and ejaculated spermatozoa. The time frame of the formation of morphologically mature spermatozoa in the epididymis of the tammar wallaby needs to be determined by further studies.Key words : Marsupial spermiogenesis ; spermatogenesis ; spermatids ; testicular spermatozoa ; acrosome ; tammar wallaby. Spermiogenesis, the remarkable morphological and biochemical transformation of the round spermatid into the elongate spermatozoon, is one of the most complex cell differentiations found in animals. During this process of spermatozoon formation the spermatid reduces its cell volume more than a hundred times, radically changes its shape and produces a number of morphologically elaborate organelles, including the acrosome, the midpiece and the flagellum. The biochemical transformation of the spermatid is equally complex. During this period the proteins of the unique cytoskeletal el...

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We investigated the cycle of the seminiferous epithelium in a marsupial, namely the brushtail possum (Trichosurus vulpecula), using semithin sections of seminiferous tubules embedded in Spurr's resin. Using 14 steps of spermatid development as markers, we were able to class tubular cross-sections into 10 well-defined stages of the seminiferous epithelial cycle. The duration of one cycle was 13.5 days, as determined by injections of [(3)H]-thymidine and autoradiographic examination of the most advanced sperm cells at 2 h and 17 days after injection. The durations of stages I-X were 21.4, 66.4, 54.1, 47.0, 29.8, 28.5, 25.3, 25.0, 12.0 and 15.9 h, respectively, estimated by the relative percentage of occurrence of each stage. It was estimated that the life spans of the main germ cells were as follows: type B spermatogonia, 5.4 days; primary spermatocytes, 16.7 days; secondary spermatocytes, 0.7 days; and spermatids, 21.4 days. The results suggest that the kinetics of spermatogenesis in marsupials show a similar pattern to that in eutherians.