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mRNA, whereas estrogen plus progesterone did not induce Spp1 in LE. These results establish that estrogen regulates Spp1 in mouse LE and are the first to identify macrophages that produce Spp1 within the peri-implantation endometrium of any species. ment. Pregnancy loss due to defects in implantation is a binding ligand, N-linked glycoprotein (SIBLING) family major cause of infertility in humans, and it is of clinicalDOI: 10.1530/REP-06-0068importance to identify the genes as well as the cellular q 2006 Society for Reproduction and Fertilityof genetically related ECM proteins recognized as key players in diverse processes, such as bone mineralization,We suggest that Spp1 at the apical surface of LE provides a mechanism to bridge conceptus to LE during implantation, and that Spp1-positive macrophages within the stroma may be involved in uterine remodeling for conceptus invasion. Reproduction (2006) 132 919–929