INHIBIN AND ACTIVIN IN EMBRYONIC AND FETAL DEVELOPMENT IN RUMINANTS

Inhibin, activin and follistatin are protein hormones with diverse phy siological roles. The involvement of inhibin in the regulation of pitu itary FSH production and secretion in adult males and non-pregnant fem ales is well established. However, it is unlikely that inhibin plays a similar role in pregnancy in ruminants. Inhibin and activin molecules show a high degree of structural similarity to potent growth and diff erentiation factors of the transforming growth factor beta (TGF-beta) superfamily of peptides and their localization in a range of embryonic and fetal tissues indicates that they may thus play a role in develop ment. Furthermore, the demonstration that follistatin is also present in a number of embryonic and fetal tissues and fluids has further impl ications for the actions of activin to which it binds. The role of inh ibin, activin and follistatin in early development has yet to be estab lished since gene knockout experiments have so far proved inconclusive . During mid- and late gestation, high concentrations of inhibin are f ound in the testes and plasma of male fetuses of sheep and cattle. Inh ibin may play a role in regulating pituitary FSH release in late pregn ancy, but the very high concentrations of this hormone in ovine fetal testes and in male fetal plasma compared with that observed in the fet al ovary and female fetal plasma has yet to be explained. The recent o bservation of high concentrations of inhibin, activin and follistatin in amniotic fluid surrounding the fetus is intriguing. Excretion via u rine or lung liquid is partly responsible for the presence of these pr oteins in amniotic fluid. The fetal membranes and the placenta are als o possible sources. It remains to be established whether these protein s constitute an inactive pool of secreted hormone or whether they have other actions in this fetal compartment.