2011 Annual Report 1a.Objectives (from AD-416) Develop strategies that promote cell proliferation and replacement in the bovine mammary gland, in order to increase lactational productivity and efficiency. 1b.Approach (from AD-416) We propose to accomplish the research objective by utilizing and extending current knowledge of mammary stem cell biology. Our specific aims are to manipulate mammary stem cell activity to enhance the growth of mammary epithelium and promote the replacement of senescent cells. 3.Progress Report One experiment addressing Specific Aim #1 of the grant is completed and two others are in progress. Holstein heifers were reared at moderate or accelerated rates of gain prior to puberty and their mammary glands were treated with xanthosine. Milk production data for the first lactation were completed. Preliminary evaluation of data suggests that xanthosine treatment alleviates the decline in milk production that typically accompanies rapid rearing of heifers. In addition to the stated objectives of the grant, another experiment was performed to characterize gene expression in putative mammary stem cells and control cells from different regions of the mammary gland. The data provided evidence that bovine mammary stem cells are located in the basal layer of the mammary epithelium and more differentiated progenitor cells are located in suprabasal layers of the epithelium. Several potential and novel markers for mammary stem cells have been identified. These markers can be detected by immunological techniques and are expressed in an abundance and locale that is consistent with identification of mammary stem cells and progenitor cells. Another experiment that is utilizing an in vitro approach to evaluate the impact of xanthosine on mammary stem cells confirms the ability of xanthosine to promote expansion of mammary stem cells. Monitoring activities associated with this project include discussion with NPS and submission of an annual report to NIFA-NRI.