Oocyte-follicle cell interactions during ovarian follicle development, as seen by high resolution scanning and transmission electron microscopy in humans

The aim of this article is to summarize and update, through an integrated analysis by transmission electron microscopy (TEM) and high resolution scanning electron microscopy (SEM) after osmium-dimethyl sulfoxide-osmium (ODO) maceration, the studies of our research group on the morphodynamics of oocyte-follicle cell associations during follicle development in humans. In resting oocytes, follicular cells project few and short cytoplasmic processes in the perioocytic space. They often form bulbous terminals very close to the oolemma where zonulae adherentes, maculae adherentes, and gap junctions are present. The oolemma mostly appears smooth with short and scanty microvilli. In early growing follicles, follicular cell projections appear as (a) long and tortuous microvilli or (b) large and short extensions. The oolemma shows numerous short microvilli. By TEM, long and thin follicular "intraooplasmic processes" have been seen to penetrate deeply into some oolemma invaginations. In macerated samples, they are observed by SEM to come very close to the nucleus and contact different oocyte organelles. These processes are more likely involved in early oocyte growth. In late growing follicles, oocyte-somatic cell interactions-now established through the interposition of the zona pellucida (ZP)-preserve the general features of early growth stage, with the exceptions of "intraooplasmic processes," which are no more present. In mature follicles subjected to a long ODO maceration, corona cells appear to contact the oocyte through an apical plume of numerous very long "curly hair-like microvilli." Corona cell microvilli, quite likely provide a sort of cytoplasmic skeleton for the ZP and they are possibly involved in (a) release of nutrients or removal catabolites to/from oocyte and vice versa and (b) transfer of substances to build up ZP. In conclusion, among oocyte and somatic cells a structural and functional association is revealed. This association, certainly highly dynamic in vivo, plays a key role in regulating the healthy folliculogenesis to assure a correct and timed oocyte maturation and ovulation.