Ovarian control of very low sperm/egg ratios at the commencement of mammalian fertilisation to avoid polyspermy

Abstract

This essay considers the means whereby sperm/egg ratios close to unity are generated during the initial stages of fertilisation in placental mammals. Pre-ovulatory graafian follicles and their contents are seen to be key structures orchestrating the events of sperm progression and coordinating the subsequent meeting of male and female gametes. Three levels of control over the numbers of spermatozoa activated and released from the functional reservoir in the caudal region of the fallopian tube isthmus are proposed. A primary control would be obtained by means of a countercurrent transfer of ovarian follicular progesterone from the ovarian vein into the tubal branch of the ovarian artery. The concentration of progesterone so transferred would be proportional to the number of preovulatory follicles, and thus to the number of eggs to be shed, and would act progressively to reduce sperm binding to the endosalpinx of the caudal isthmus. Differential timing of the release from epithelial binding may be a crucial means of achieving the initial low sperm/egg ratios. a secondary regulation of the release of graded numbers of viable spermatozoa towards the ampullary-isthmic junction of the fallopian tubes would be by means of molecular messages derived from the mucified oocyte-cumulus complex shortly before and after the time of ovulation. Third would be reorientation of sperm trajectories by molecular gradients within the cumulus cell mass to direct competent spermatozoa to those oocytes as yet unpenetrated. Together these differing levels of control would impose low sperm/egg ratios during the initial stages of fertilisation, such strict quantitative regulation of male gametes lasting at least until the block to polyspermy is fully established and the vitellus is no longer at risk from further sperm penetration.