1 - Gamete Transport

Abstract

This chapter focuses on mechanisms for transporting gametes in eutherian mammals. In mammals, oocytes are usually fertilized within a few hours of ovulation. Spermatozoa, however, may have to survive for months after the completion of spermatogenesis and release from the testis—and not just within the body of the individual that produced them. They must also survive, sometimes for long periods. The cervix is immunologically competent. For spermatozoa of many species, however, survival is not enough to ensure fertilization. They may be forced to compete with the sperm cells from other males to pass on their genetic complement. For the unfertilized oocyte, problems of survival and competition are probably minor. In rabbits and humans, insemination into the vagina stimulates leukocyte migration into the cervix as well as into the vagina, the leukocytes being primarily neutrophils and macrophages. Sperm transport through the uterus of vaginal depositors is thought to depend significantly on uterine contractions. In uterine semen depositors, spermatozoa may leave the uterus rapidly to enter the uterotubal junction. The uterotubal junction presents a barrier to spermatozoa in most eutherian mammals. After fertilization, mammalian spermatozoa may be phagocytosed by isthmic epithelial cells or may be eliminated, passing into the peritoneal cavity and then being phagocytosed. Anatomically, the lumen in species as disparate as dairy cattle and mice is particularly tortuous and narrow. There are two issues involved in oocyte transport: (1) The capture of the oocyte from the surface of the ovary or from the ovarian bursa and (2) the transport of the oocyte through the ampulla. There have been no studies conducted to demonstrate an absolute requirement for ciliary activity for oocyte transport. Although much remains to be discovered, and some issues remain to be settled, a general model for gamete transport in eutherian mammals can be derived from already known facts.