Developmental regulation of effect of epidermal growth factor on porcine oocyte-cumulus cell complexes: Nuclear maturation, expansion, and F-actin remodeling

Epidermal growth factor (EGF) efficiently stimulates expansion of mouse and rat oocyte-cumulus complexes (OCC). Contradictory data have been published by several laboratories about the ability of EGF to stimulate expansion of porcine OCC. We assumed that these contradictions may have resulted from h eterogeneous conditions used for isolation, culture, and assessment of OCC. The present experiments were designed to test the hypothesis that porcine OCC acquire the ability to synthesize hyaluronic acid (HA) and undergo expa nsion following EGF-stimulation gradually during the growth of follicles. F or this reason, we isolated OCC from follicles of different sizes and asses sed quantity of produced HA and proportions of expanding OCC after stimulat ion by EGF. In addition, we assessed in those OCC changes in morphology of cumulus cells and assembly of F-actin microfilaments, which are necessary f or expansion to occur. Finally, nuclear maturation of EGF-stimulated OCC wa s assessed and its relationship with occurrence of expansion was evaluated. In all experiments, OCC stimulated with FSH were used as positive controls . The results showed that EGF did not stimulate production of HA, rearrange ment of F-actin and expansion in OCC isolated from small follicles (<4 mm i n diameter). OCC isolated from large preovulatory follicles (6-7 mm in diam eter and PMSG-stimulated follicles) underwent efficient expansion when stim ulated by EGF (93% and 100%, respectively). EGF dramatically stimulated tot al production of HA in these OCC and its retention in extracellular matrix of the expanding cumulus. Cumulus cells of the large OCC underwent essentia l changes of their morphology and extensive rearrangement of F-actin microf ilaments following stimulation with EGF. Interestingly, EGF enhanced nuclea r maturation of OCC isolated from both small and large follicles, which sug gest diversity of signaling pathways controlling maturation and expansion. FSH caused cumulus expansion, F-actin remodeling, and enhancement of oocyte nuclear maturation in OCC originated from both small and large follicles, We conclude that EGF can stimulate expansion of porcine OCC in vitro; howev er, only of those isolated from large follicles. This indicates that EGF ma y have a physiological role in regulation of porcine cumulus expansion in p reovulatory follicles, presumably as a mediator of signals elicited by the LH surge.,(C) 2000 Wiley-Liss, Inc.