Distinctive features of the gametes and reproductive tracts of the Asian musk shrew, Suncus murinus

Abstract

Insectivora are of special interest as the most primitive of the eutherian mammals, but essentially nothing is known of their gamete function. In this respect, the Asian musk shrew (Suncus murinus), investigated in the present study, displays many idiosyncrasies. In the epididymis, the giant acrosome undergoes further stabilization, its unusual resilience being especially evident in a "rim" created by a persistent close alignment of the outer acrosomal and overlying plasma membranes. However, until spermatozoa reached a gland on the vas deferens, no post-testicular change was demonstrable in the sperm head surface, the unusual nature of which was indicated by a dorso-ventral differentiation, by an inability to auto-agglutinate or to bind to the homologous zona pellucida, and by an insensitivity to anti-sperm immunoglobulin IgG in fresh serum. At mating, only about 1 x 10(6) spermatozoa are inseminated as far as the anterior vagina with plug formation. Within the small (6 mm) fallopian tube, the isthmus and ampulla are sharply delineated by their contractile activity and epithelial character; there is evidence of some sperm entry into isthmic crypts and a tendency for ipsilateral ovarian control of sperm transport to the tubal ampulla. The cumulus oophorus does not undergo preovulatory mucification and expansion, is characterized by persistent intercellular gap junctions, and is insensitive to hyaluronidase and trypsin. It is unclear how the compact cumulus is penetrated at fertilization. The giant acrosome contains acrosin and an unusually temperature-dependent cumulytic activity; it is intact in motile ampullary spermatozoa but appears to be discharged before reaching the zona pellucida. Since eggs were not penetrated in the presence of ampullary spermatozoa until 4-10 h after ovulation, Suncus spermatozoa spend a long period in the female before they can fertilize. The determinants of sperm function, including capacitation and the acrosome reaction (AR), may depend on a different set of controls in Suncus, perhaps as a legacy of the resilient giant acrosome. This possibility could be examined in other Crociduran and Soricine shrews selected according to acrosome size.