Bovine mammary progenitor cells: Current concepts and future directions

Although cell number is positively correlated with milk production, much remains to be learned about the bovine mammary stem cell and progenitor cells. Bovine mammary development is driven by many of the same classic mammogenic hormones studied in murine models, yet histologic features of bovine mammary development differ from that of rodent models. Most notably, terminal end buds, as they have been described for murine models, do not exist in the bovine mammary gland. However, among the most important common features of mammary development in disparate species is the involvement of histologically distinct, lightly staining epithelial cells as putative stem and progenitor cells. Although stem cell research has often focused on mammary development, mammary stem cells seemingly provide the basis for mammary growth and cell turnover in the mature animal. These cells provide an obvious focus for research aimed at increasing the efficiency of milk production. This review addresses recent findings concerning the histology and molecular physiology of putative bovine mammary stem and progenitor cell populations, areas where more study is critically needed, and areas where studies of bovine mammary physiology may present a unique opportunity to better understand mammary physiology in many species.