Follistatin was originally described as an ovarian polypeptide hormone able to suppress pituitary follicle-stimulating hormone (FSH) secretion in in vitro systems. We have isolated rat ovarian follistatin cDNA clones and used these to investigate the possibility that decidual tissue expresses the follistatin gene. We found that the follistatin gene is highly expressed during early pregnancy in tissue derived from the site of blastocyst implantation. Follistatin mRNA was observed in tissue obtained from the implantation site at days 6, 8 and 10 of gestation, but was not found in the placenta at later stages of gestation. This same pattern of follistatin expression was observed in pseudopregnant animals in which decidualization had been artificially induced, indicating that decidual tissue is the primary source of follistatin mRNA. In situ hybridization was used to examine follistatin gene expression in decidua in greater detail. The mRNA is preferentially found in antimesometrial tissue, and is strictly confined to the endometrial-derived decidual cells. These results suggest that follistatin might function as an important endocrine hormone during implantation of the fetus. nerhaDS bv actina to appropriatelv modulate maternal FSH secretion during early pregnancy..
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