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

This paper is concerned with endosalpingeal specialisations that underlie s tabilisation of unique secretions forming microenvironments within the Fall opian tube lumen, not least as they influence the final stages of sperm tra nsport and maturation. In particular, a specific microenvironment at the am pullary-isthmic junction in the presence of the newly-shed oocyte(s), assoc iated granulosa cells and their secretory products is thought to characteri se the site of fertilisation. Attention is also drawn to the viscous glycop rotein in the caudal isthmus before ovulation: it isolates spermatozoa stor ed with suppressed motility from the metabolic stimulation caused by uterin e or ampullary fluid. This viscous secretion strips male antigens from the sperm surface, facilitating their preovulatory binding reactions to organel les of the isthmus epithelium. Peri-ovulatory activation of discrete number s of viable spermatozoa is regulated by follicular progesterone secretion, but seemingly involves local mobilisation of Ca2+ ions into bound gametes o f appropriate membranous maturity. Motility and progression to the site of fertilisation may be further heightened by an influence of catecholamines d iffusing from the myosalpinx; receptors for such molecules are present on t he sperm surface. There is tight control of sperm activation and release cl ose to the time of ovulation, generating initial sperm:egg ratios at the am pullary-isthmic junction of close to unity. However, with establishment of the block to polyspermy in newly activated eggs and lapse of time after ovu lation, control of sperm progression soon relaxes. Indeed, 2-4 cell embryos may contain large numbers of accessory spermatozoa on or in the zona pellu cida.