EXPRESSION OF INHIBIN SUBUNITS AND FOLLISTATIN DURING POSTIMPLANTATION MOUSE DEVELOPMENT - DECIDUAL EXPRESSION OF ACTIVIN AND EXPRESSION OFFOLLISTATIN IN PRIMITIVE STREAK, SOMITES AND HINDBRAIN

Members of the activin family are believed to act as mesoderm-inducing factors during early amphibian development. Little is known, however, about mesodermformation in the mammalian embryo, and as one approach to investigating this we have studied activin and follistatin expressi on during early mouse development. Activins are home- or heterodimers of the beta(A) or beta(B) subunits of inhibin, itself a heterodimer co nsisting of one of the beta subunits together with an a subunit. Folli statin is a single-chain polypeptidewhich inhibits activin function. E xpression of the inhibin alpha chain could not be detected in embryoni c or extraembryonic tissues at any of the stages studied (5.5 to 8.5 d ays) and expression of the beta(A) and beta(B) subunits could only be observed in the deciduum in cells surrounding the embryo. Expression o f follistatin could also be detected in the deciduum, but in a pattern complementary to that of the beta subunits. Embryonic expression of f ollistatin first occurred inthe primitive streak, and at later stages transcripts were detectable in the somites and in rhombomeres 2, 4 and 6 of the hindbrain. These results are consistent with a role for acti vin in mesoderm formation in the mouse embryo, and suggest functions f or follistatin in addition to its role as an inhibitor of activin.