Morphological selection of human spermatozoa in vivo and in vitro.

Light microscopic assessment of human spermatozoa in post-coital samples of cervical mucus revealed a significant improvement in the general sperm morphology between the semen and the cervix. Further analysis showed that the excluded spermatozoa were more likely to be those with midpiece or tail defects that impaired motility. Significant changes were also found when the morphology of spermatozoa recovered from the uterus and Fallopian tubes following AIH was compared with the semen used for insemination: in the semen, uterus and oviducts there were respectively 53, 77 and 71% spermatozoa with completely normal morphology, and 12, 3 and 0.6% spermatozoa with defects of the midpiece and/or tail (as assessed by surface replica electron microscopy). Selection of spermatozoa in vitro by allowing them to swim upwards through a nickel mesh also reduced the number of spermatozoa with abnormal morphology, particularly of the midpiece and tail. It is concluded that the apparent selection of morphologically normal spermatozoa is not a direct function of the female tract, but that the spermatozoa can effect their own selection because of their differential motility.