Submitted to: Experimental Biology and Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 9, 2007
Publication Date: November 1, 2007
Citation: Capuco, A.V. 2007. Identification of Putative Bovine Mammary Epithelial Stem Cells by Their Retention of Labeled DNA Strands. Experimental Biology and Medicine. 232:1381-1390.
Interpretive Summary: Adult stem cells in the bovine mammary gland are capable of producing the differentiated cell types within that tissue for the lifetime of the animal and are ultimately responsible for growth and maintenance of the bovine mammary epithelium. In this study, we used a method for identifying adult stem cells in mammary gland of Holstein calves. The method employed was based upon the histological detection of bromodeoxyuridine-labeled DNA within epithelial cells of the mammary gland 5 weeks after administration of bromodeoxyuridine. The ability to retain labeled DNA is considered a characteristic of stem cells. These label-retaining cells were shown to be slowly cycling and appeared capable of responding to estrogen stimulation, as the majority of these cells expressed the estrogen receptor protein necessary for sensing estrogen. Estrogen is critical for inducing mammary growth and development. Using this labeling strategy, it will be possible to study the biology of bovine mammary stem cells.
Stem cells characteristically retain labeled DNA for extended periods due to their selective segregation of template DNA strands during mitosis. In this study, proliferating cells in the prepubertal bovine mammary gland were labeled using five daily-injections of 5-bromo-2-deoxyuridine (BrdU). Five weeks later, BrdU-labeled mammary epithelial cells were still evident. The percentage of BrdU-labeled epithelial cells was greatest in lower region of the mammary gland, near the gland cistern, and decreased toward the periphery of the parenchymal region, where the ducts were invading the mammary fat pad. Increased numbers of BrdU-labeled epithelial cells in basal regions of the gland are likely a consequence of decreased proliferation rates and increased cell cycle arrest in this area. In peripheral regions of mammary parenchyma, the percentage of heavily labeled epithelial cells averaged 0.24%, a number that is consistent with estimates of the frequency of stem cells in mouse mammary gland. Epithelial label-retaining cells represent a slowly proliferating population of cells, as 5.4% of heavily labeled cells were positive for the nuclear proliferation antigen, Ki67. Furthermore these putative stem cells can likely respond directly to mitogenic stimulation by estrogen, as 57% of these BrdU-labeled epithelial cells were estrogen receptor-positive. Continuing studies will address the usefulness of this technique to identify bovine mammary stem cells and facilitate studies of stem cell biology.