Recent reports have revealed that bone marrow (BM)-derived cells can be constituents in a number of organs, especially in remodeling tissue. Using bone marrow transplantation (BMT) technique, we found that BM can serve as a source of both myoepithelial cells and periductal fibroblasts in the mammary gland. The numbers of BM-derived myoepithelial cell were 4.8-fold, and those of periductal fibroblast were 2.4-fold higher in the mice when BMT which was performed at the pubertal stage, as compared with BMT was performed at the postpubertal stage. Treatment with estrogen + progesterone pellet increased numbers of BM-derived myoepithelial cells and periductal fibroblasts, to levels 4.5- and 2.6-fold higher than in placebo mice, respectively. In situ hybridization revealed BM-derived periductal fibroblasts expressed insulin-like growth factor I mRNAs that are known to regulate mammary gland. These results suggest that drastic structural change that is induced by hormonal stimulation increased the recruitment of BM-derived myoepithelial cells and periductal fibroblasts to the mammary gland context. (c) 2006 Elsevier Inc. All rights reserved.