IMMUNOHISTOCHEMICAL LOCALIZATION AND MESSENGER-RNA EXPRESSION OF ACTIVIN, INHIBIN, FOLLISTATIN, AND ACTIVIN RECEPTOR IN BOVINE CUMULUS-OOCYTE COMPLEXES DURING IN-VITRO MATURATION

The aim of this study was to investigate whether bovine cumulus-oocyte complexes (COCs) synthesize activin A, inhibin, and follistatin and w hether they contain activin receptor during in vitro maturation. There fore, COCs obtained from small and medium-sized follicles were culture d in M-199 supplemented with 10% fetal calf serum (FCS) and gonadotrop ins for 24 hr. At 0, 6, 12, and 24 hr after the onset of culture, COCs were removed for immunohistochemical staining to detect the expressio n of activin A, inhibin, follistatin, and activin receptor type II pro teins. At 0 and 24 hr, COCs were removed and prepared for reverse-tran scriptase polymerase chain reaction (RT-PCR) to assess the presence of mRNA of these proteins. It appeared that cumulus cells and oocytes ex press activin, follistatin, and activin receptor proteins as well as t heir mRNA. While expression of inhibin mRNA was found exclusively in c umulus cells, the inhibin protein was present in cumulus cells and ooc ytes, Immunohistochemical study both in cumulus cells and in oocytes o ften showed a moderate and strong staining intensity for activin and f ollistatin, respectively. Activin staining underwent little or no chan ge during culture except at 24 hr of maturation, where about 60% of th e oocytes showed no staining. Follistatin immunoreactivity remained st rong in the majority of COCs. At the onset of culture, a spotlike inhi bin staining was observed in the oocyte, which increased after 12 hr a nd was absent at the end of culture, Activin receptor immunoreactivity in cumulus cell membranes and oolemma increased during oocyte maturat ion to maximum values at the end of culture in most of the COCs. It is concluded that the consistent presence of activin and the increase in activin receptor in cumulus cells and oocytes during in vitro maturat ion indicate a paracrine and/or autocrine action for activin on bovine oocyte maturation. This action may be modulated by inhibin and/or fol listatin. (C) 1998 Wiley-Liss, Inc.