DISTINCTIVE FEATURES OF THE GAMETES AND REPRODUCTIVE TRACTS OF THE ASIAN MUSK SHREW, SUNCUS-MURINUS

Insectivora are of special interest as the most primitive of the euthe rian mammals, but essentially nothing is known of their gamete functio n. In this respect, the Asian musk shrew (Suncus murinus), investigate d in the present study, displays many idiosyncrasies. In the epididymi s, the giant acrosome undergoes further stabilization, its unusual res ilience 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 po st-testicular change was demonstrable in the sperm head surface, the u nusual nature of which was indicated by a dorso-ventral differentiatio n, by an inability to auto-agglutinate or to bind to the homologous zo na pellucida, and by an insensitivity to anti-sperm immunoglobulin IgG in fresh serum. At mating, only about 1 x 10(6) spermatozoa are insem inated as far as the anterior vagina with plug formation. Within the s mall (6 mm) fallopian tube, the isthmus and ampulla are sharply deline ated by their contractile activity and epithelial character; there is evidence of some sperm entry into isthmic crypts and a tendency for ip silateral ovarian control of sperm transport to the tubal ampulla. The cumulus oophorus does not undergo preovulatory mucification and expan sion, is characterized by persistent intercellular gap junctions, and is insensitive to hyaluronidase and trypsin. It is unclear how the com pact cumulus is penetrated at fertilization. The giant acrosome contai ns acrosin and an unusually temperature-dependent cumulytic activity; it is intact in motile ampullary spermatozoa but appears to be dischar ged 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 cont rols in Suncus, perhaps as a legacy of the resilient giant acrosome. T his possibility could be examined in other Crociduran and Soricine shr ews selected according to acrosome size.