Recent studies have demonstrated that bone morphogenetic proteins (BMPs) play fundamental roles in female fertility. This is particularly evident in terms of the ovary. One major question that is just beginning to be addressed is the role of BMPs in the non-pregnant uterus. To help fill this gap, we used in situ hybridization to investigate the expression of BMP family members in the rat uterus over the estrous cycle. We found that the endometrial/uterine cycle is accompanied by the expression of several components of the BMP pathway - including ligands, receptors and antagonists. The mRNAs encoding BMP receptors are expressed in the epithelial (BMP-RIA, -RIB and -RII), periluminal stroma (BMP-RIA and -RII) and smooth muscle cells (BMP-RIA and -RII). The expression of all three receptors showed clear cyclic variations. The mRNAs encoding BMP ligands were highly expressed in the periluminal stroma (BMP-2 and -7) and glandular epithelium (BMP-7). The expression of BMP-2, but not BMP-7, was cyclical. Notably, the periluminal stroma expressed noggin mRNA. In the blood vascular system, BMP-4, -6 and -RII mRNAs were expressed in myometrial endothelial cells. Interestingly, follistatin, noggin, and BMP-4, -6 and -7 mRNAs were expressed in eosinophilic leukocytes, suggesting unexpected roles for eosinophil-derived BMPs in uterine function. We conclude that BMP ligands, receptors and antagonists are expressed in spatially and temporally restricted patterns that are consistent with a physiological role for these regulatory molecules in promoting uterine cellular processes including cell proliferation, differentiation and apoptosis during the cycle.

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