Estrogen responsiveness of bone formation is mediated by the estrogen receptor alpha (ER alpha) in osteoblastic lineage. As osteolblasts arise from the multipotent bone marrow stromal (mesenchymal) cells, this study was undertaken to observe the ER alpha in primary female adult rat bone marrow mesenchymal stem cells (BMSCs). The ER alpha was localized using immunocytochemical analysis in identified primary BMSCs. Then, using real-time PCR analysis, we measured the expression of ER alpha messenger RNA (mRNA) in BMSCs. ER alpha transcripts showed different trends between untreated cultures (control group) and osteogenic-induced cultures (treated group). In the control group, ER alpha mRNA climbed at peak levels at a confluence stage and decreased until day 20, whereas, in the treated group, the ER alpha mRNA kept climbing from a low level until day 20. Thus, the observed developmental expression of ER alpha mRNA correlates with progressive BMSCs growth and osteogenic differentiation and BMSCs may be a primary target cell for estrogen in maintaining bone formation. (c) 2006 Elsevier Inc. All rights reserved.