Physiological factors underlying the formation of ovarian follicular fluid

Abstract

The composition of follicular fluid in Graafian follicles is similar but not identical to ovarian venous plasma. Differences between the two fluids are attributed to a blood-follicle barrier, which restricts the passage of large molecules, and to the existence of a hypothetical active transport mechanism and/or Donnan equilibrium, reflected by the distribution of small permeant ions. The follicle epithelium has been characterized as 'leaky' on the basis of both chemical and electrical criteria. The rate of follicular fluid acccumulation is much greater during preovulatory activation of the follicle (secondary fluid) than at preceding stages (primary fluid), suggesting that gonadotrophic hormones have a major influence on the rate of swelling. The evidence for water transport following an osmotic gradient set up by active transport of Na+ has been inconclusive. The conventional view that fluid forms from transudation of plasma rests on circumstantial evidence and is less likely to account for primary than for secondary fluid. The discussion has focussed on the fluid in the antrum principally because so little is known about the fluids which lie in the narrow extracellular spaces and bathe the granulosa cells and oocyte. A major challenge for future research will be elucidation of the compositions of these microenvironments which, in contrast to the bulky antral fluid which will buffer change, should be sensitive indicators of fluctuating biosynthetic activity and of optimal conditions for cell culture.