The identification of human mammary progenitor cells and understanding their regulation is critical to enumerating the origins and events that control pathologic conditions. Determining the identity and activity of normal primitive cells could lead t a better understanding to the connection between stem cell biology and cancer. Our study is directed towards understanding the mechanisms that underlie human breast development and the regulation of primitive progenitor cells by hormone-growth factor signaling. The limited understanding of human breast development and stem cell biology has largely been due to the lack of appropriate model systems and assays to detect, analyze, and characterize stem cell properties. In recent years, our laboratory has developed and optimized various in vivo and in vitro tools to study the biology and mechanisms governing human breast development We have pioneered and implemented the use of an innovative in vivo model to study human mammary development by exploiting the mouse mammary fat pad of immunocompromised mice as a source of important endocrine signaling events and using grafted human stroma to support the growth and differentiation of the human mammary epithelium. In recent work, we have identified i) four distinguishable epithelial differentiation states (two luminal phenotypes and two basal phenotypes) within human breast tissues that differ on the basis of CD24, EpCAM and CD49f expression, ii) the existence of bipotent progenitors that contribute to structurally distinct elements (ducts and lobule/alveoli), iii) the hormonal combinations that enhance stem/progenitor cell activity within human epithelial cells, and iv) a hormone growth factor mechanism through TBX3 expression can regulate breast stem-like cells. In Aim 1 of this project, we will delineate the epithelial hierarchy in adult human breast tissue. In Aim 2, we will determine how hormone-growth factor signaling regulates human breast progenitor cell activity. Our studies provide innovative insight into the identity of human breast progenitor cells and provide critical molecula underpinnings by which hormones drive human breast progenitor cell morphogenesis that could serve as focal points during the development of pathological conditions.