Analysis of Murine Mammary Epithelial Cellular Hierarchy.

Abstract

The murine mammary system is a complex milieu of epithelial cell types that function together to support lactogenesis. The breast tissue is spatially and temporally regulated, with most growth occurring postnatally. The mature mammary gland displays two characteristic architectural features: ducts and terminal end buds. There are different cell types found in these two features that perform independent functions in the mature organ. In general, the mature gland is a bilayer system with a single layer of fibroblast-like myoepithelial cells encompassing an inner luminal epithelial layer. Upon pregnancy, the induction of alveolar secretory cells is initiated, which eventually leads to milk production. Milk is then channeled through the entire system via contraction of the myoepithelium, and eventually dispersed through the nipple. The mouse mammary gland is capable of multiple stages of development: growth, lactation and involution. These stages are regulated by cytokines and hormones depending on the developmental stage of the mouse and the physiological context (i.e. estrus). Numerous studies have shown the mammary epithelium is capable of regeneration upon transplantation. Recent reports have isolated fractions of cells enriched for duct forming ability and self-renewal. In addition, a distinct progenitor population was also identified that lacked the ability to form ducts in vivo but retained bipotentiality in vitro. This thesis builds on previous marker studies of mammary epithelial cells, using existing phenotypes to further enrich the mammary stem cell population. My data shows Thy-1, or CD90, is differentially expressed in mammary cells. In addition, Thy-1+CD24medCD49fhi cells are enriched for in vivo engraftment of ductal epithelium. These cells are approximately 10-fold enriched for duct-forming ability.. Importantly, the Thy-1+CD24medCD49hi phenotype enriched for cells that are capable of self-renewal and retain the full differentiation potential of the parent stem cells.. This data provided a new phenotype for the stem cells (MaSC). The Thy-1-CD24medCD49hi cells are not only diminished for engraftment but have significantly decreased self-renewal, suggesting they are a potential multipotent progenitor population. I have named them mammary multipotent progenitors, or MMPPs. A novel protein staining methodology revealed the Thy-1+CD24medCD49hi and Thy-1-CD24medCD49hi cells have keratin profile similar to myoepithelial cells