UNUSUAL AMPULLARY SPERM CRYPTS, AND BEHAVIOR AND ROLE OF THE CUMULUS-OOPHORUS, IN THE OVIDUCT OF THE LEAST SHREW, CRYPTOTIS-PARVA

The gametes of the least shrew, Cryptotis parva, were studied in regar d to their maturation and structure, and with particular emphasis on t heir behavior in the fallopian tube, from the time of ovulation until the appearance of two-cell embryos beginning some 9 h after ovulation. Cryptotis spermatozoa are organized according to the conventional eut herian mold, with the exception of a barbed perforatorium and an unusu al plasma membrane density lent by a bristly coat where it overlies th e acrosome rim. In the epididymis they undergo a maturation of the cap acity for motility and an -S-S-related stabilization of the nucleus an d tail organelles, with the cauda housing only approximately 4-5 milli on spermatozoa. Mating involves penile locking and also the deposition of a modest vaginal plug that covers the cervix. The short (4-5 mm) f allopian tube has three regions-a simple isthmus, a relatively narrow ampulla populated throughout by ciliated crypts, and a crypt-free term inal infundibulum-the fertilization site. Unlike the situation in most mammals, the tubal isthmus was devoid of spermatozoa in mated females before and after ovulation, which occurred approximately 13 h post-hC G and produced a mean of 5.7 ova. However, the ampulla then housed sim ilar to 1500 active cells in groups within the ciliated crypts, someti mes together with leukocytes but with few spermatozoa above in the inf undibulum. Within about 1 h after their ovulation from approximately 4 00-mu m follicles, eggs were penetrated while in the infundibulum desp ite the nonexpanded hyaluronidase-resistant state of the cumulus oopho rus. However, on moving down to the ampulla by 2-4 h after ovulation, the dense cumulus around fertilized eggs appeared to proliferate and b egan to disperse coincidentally with secretion of a hyaluronidase-sens itive matrix in which hundreds of motile spermatozoa often became enme shed. This cumulus change also occurred around unfertilized eggs, thou gh more slowly, but not around fertilized or unfertilized eggs culture d in vitro. Thus, cumulus matrix production appeared to be stimulated to an important degree by factors in the oviduct, not by preovulatory gonadotropins as in many mammals. Although cumulus-invested eggs were fertilized readily in vitro, cumulus-free eggs of the same age were ne ver fertilized, and spermatozoa bound to the zona pellucida had intact acrosomes. This and related evidence from other shrews makes it seem likely that the soricid cumulus has an essential role in fertilization and may induce the acrosome reaction.