Triennial Lactation Symposium: Bovine mammary epithelial cell lineages and parenchymal development.

Mammary development proceeds from an aggregation of cells in the ventral ectoderm to the establishment of an elaborate tree of alveoli, ducts, and cisternae. However, despite abundant data on endocrine regulation of ruminant mammary growth, we know comparatively little about cell lineages, expression of differentiation markers, and plasticity in mammary cell phenotype. Histologic analyses have revealed cell populations with distinct histochemical profiles, but functional assessment of cell populations during development has been limited to analysis of proliferation and frequency estimations of morphotypes. The lack of transplantation models, limited availability of validated antibodies with reactivity to bovine antigens, and similar technical challenges have generally hindered the pace of discovery, but the application of new technologies such as laser microdissection, transcriptional profiling, and multispectral image analysis are yielding important clues into bovine mammary cell ontogeny and developmental regulation. Our analyses have shown that prepubertal ovariectomy affects epithelial architecture, increases the proportion of cells expressing the estrogen receptor, and increases myoepithelial cell development, all concomitant with a dramatic reduction in the mass of parenchymal tissue. Our observations point to a dual role for ovarian secretions in the control of not only the rate of epithelial development, but also the nature of the parenchymal development. The balance of stimulus and inhibition pathways cooperatively regulates mammary growth. The increased reliance on objective staining analyses and quantitative approaches will ensure broader repeatability, application, and extension of the findings regarding the impact of the ovary and other regulatory entities and factors. Advances in understanding the ontogeny of mammary epithelial cells, coupled with established and increasing knowledge of endocrine factors affecting mammary development, may yield intervention strategies to improve dairy profitability.