REGULATED STORAGE AND SUBSEQUENT TRANSFORMATION OF SPERMATOZOA IN THEFALLOPIAN-TUBES OF AN AUSTRALIAN MARSUPIAL, SMINTHOPSIS-CRASSICAUDATA

The disposition of spermatozoa in the female tract of the dasyurid mar supial, Sminthopsis crassicaudata, was examined before and after ovula tion, by transillumination using differential interference optics. At 6-30 h after mating in females with maturing follicles, spermatozoa we re restricted to special storage crypts that define the isthmus of the Fallopian tube. These spermatozoa were mostly immotile and were spear shaped, except for 50-200 vanguard spermatozoa within anterior crypts that were T shaped due to a rotation of the head on the tail. After o vulation and arrival of eggs in the narrow upper ovarian segment of th e tube, several hundred isthmic spermatozoa migrated to that region, d ispersed singly or in small groups of variable motility with some fest ooned around eggs. Most clearly displayed a T configuration, and subse quently, those remaining in the isthmic crypts also adopted a T shape in situ. The motility of mature epididymal spermatozoa and of those fl ushed from the isthmus before and after ovulation was similar in vitro , and all were spear shaped except for some T-shaped isthmic spermatoz oa recovered after ovulation. However, neither spear- nor T-shaped act ive isthmic spermatozoa ever adhered to the zona pellucida when incuba ted with eggs. We conclude that transport to and storage of spermatozo a in the isthmus of the Fallopian tube, and their later migration up t o the fertilization site and coincident transformation to a T shape, a ll are closely regulated by the ovarian follicular cycle in Sminthopsi s. Only on reaching the upper ovarian segment of the Fallopian tube do Sminthopsis spermatozoa appear to be able to bind to the egg, when th e T shape brings the whole surface area of the acrosome into appositio n with the zona pellucida. These observations are discussed with speci al reference to sperm capacitation, sperm penetration mechanisms, and the different idiosyncratic designs of the sperm head in marsupial and eutherian mammals, respectively.