EXPRESSION OF MESSENGER RIBONUCLEIC-ACIDS ENCODING THE INHIBIN ACTIVIN SYSTEM DURING MID-GESTATION AND LATE-GESTATION RAT EMBRYOGENESIS/

We recently demonstrated that inhibin/activin alpha-, beta A-, and bet a B-subunit messenger RNAs (mRNAs) are localized in a variety of embry onic rat tissues from 12-20 days post coitum (pc) and reported localiz ations consistent with possible growth effects of activin during rat e mbryogenesis. In the present study, we examined the tissue-specific di stribution of mRNAs encoding all known players of the inhibin/ activin system. In situ hybridization with radiolabeled RNA probes specific f or mouse activin receptors (ActRII and ActRIIB), rat follistatin, and rat inhibin/activin subunits was used to examine the spatiotemporal ex pression of these molecules in adjacent sections of postimplantation r at embryos (8-20 days pc) as well as in midgestation placenta and uter ine tissues (8-12 days pc). With the exception of the dorsal root gang lion and salivary gland, alpha- and beta B-subunit mRNAs were found ex clusively in reproductive tissues (brain, pituitary, and/or gonads). b eta A-Subunit mRNA signal was observed in the brain and gonads as well as in a variety of other tissues during embryogenesis. ActRII mRNA wa s found exclusively in neuronal tissue from 14 days pc until birth. Ac tRIIB mRNA was also found in brain, spinal cord, and ganglion, but usu ally appeared earlier in development than the ActRII message. ActRIIB message was also expressed in a number of other tissues, in some cases along with beta A-subunit mRNA. In these tissues, ActRIIB expression was confined to epithelial and endothelial cell types. Follistatin mes sage was observed in all tissues (except the heart and vessels) locali zing beta A-subunit and/or ActRIIB but not in the same cell type. Outs ide the embryo, beta A-subunit mRNA was localized in the decidua capsu laris during midgestation, whereas ActRIIB message was found in placen ta as early as 9 days pc. Expression of follistatin message was appare nt in decidua from 8-11 days pc, then disappeared from this tissue and was abundant in myometrium at 12 days pc. These data suggest that: 1) inhibin and activin regulate aspects of the fetal reproductive system , whereas activin A may regulate the growth and differentiation of man y embryonic tissues; 2) ActRII and ActRIIB serve different roles durin g development of the rat embryo; and 3) follistatin is in a position t o modulate the effects of activin during postimplantation rat embryoge nesis.