Histophysiology of the Fallopian tubes in relation to sperm binding, release, and completion of capacitation

This paper is concerned with endosalpingeal specialisations that underlie stabilisation of unique secretions forming microenvironments within the Fallopian tube lumen, not least as they influence the final stages of sperm transport and maturation. In particular, a specific microenvironment at the ampullary-isthmic junction in the presence of the newly-shed oocyte(s), associated granulosa cells and their secretory products is thought to characterise the site of fertilisation. Attention is also drawn to the viscous glycoprotein in the caudal isthmus before ovulation: it isolates spermatozoa stored with suppressed motility from the metabolic stimulation caused by uterine or ampullary fluid. This viscous secretion strips male antigens from the sperm surface, facilitating their preovulatory binding reactions to organelles of the isthmus epithelium. Peri-ovulatory activation of discrete numbers of viable spermatozoa is regulated by follicular progesterone secretion, but seemingly involves local mobilisation of Ca2+ ions into bound gametes of appropriate membranous maturity. Motility and progression to the site of fertilisation may be further heightened by an influence of catecholamines diffusing from the myosalpinx; receptors for such molecules are present on the sperm surface. There is tight control of sperm activation and release close to the time of ovulation, generating initial sperm:egg ratios at the ampullar-isthmic junction of close to unity. However, with establishment of the block to polyspermy in newly activated eggs and lapse of time after ovulation, control of sperm progression soon relaxes. Indeed, 2-4 cell embryos may contain large numbers of accessory spermatozoa on or in the zona pellucida.