IMMUNOPHENOTYPIC PROPERTIES AND ESTROGEN DEPENDENCY OF BUDDING CELL STRUCTURES IN THE DEVELOPING MOUSE MAMMARY-GLAND

The initial phase of growth of the parenchymal component of the mouse mammary gland is ductal clongation, which is mainly accomplished by pr oliferating cells in a specialized structure termed end bud. End buds are composed of multiple layers of epithelial cells (so called body ce lls) which are capped by a single layer of morphologically unique cell s termed cap cells. We sought to examine the interrelationship between cap cells and other epithelial cell subclasses using a variety of ant ibodies to different keratin proteins and also antibodies to vimentin, actin and collagen IV. An extensive immunohistochemical characterizat ion of the epithelial components of the developing and differentiating mammary gland demonstrated that cap cells were devoid of any immunohi stochemically - detectable keratins but were positive for collagen IV. In contrast, the majority of cells in the end bud along with the lumi nal epithelial and myoepithelial cells were keratin positive. The body cells of the end bud were the only cells which were positive for anti body to keratin 6, a keratin which previously has been reported to be expressed in proliferating mammary epithelial cells. In addition, estr ogen receptor was localized only to epithelial cells of ducts, alveoli and body cells of end buds, but not to cap cells or myoepithelial cel ls. We interpret these results to suggest that cap cells are not totpo tent stem cells but rather cells specialized in paving the way for duc tal elongation as well as serving as precursors to myoepithelial cells .