Evidence for somatic stem cell niches has been clearly defined in several biological systems. To determine if a stem cell niche occurs in the mouse mammary gland and if it displayed tissue-specific dominance, we mixed mammary epithelial cells from a wild type host with testicular cells isolated from a WAP-Cre/R26R male donor at a 1:1 ratio and inoculated them into the cleared inguinal fat pads of immune-compromised Nu/Nu female hosts The host mice were bred 6-8 weeks later and examined 20-30 days post involution. This approach allowed for growth of mammary tissue, transient activation of the WAP-Cre gene, recombination and constitutive expression of Lac Z from the Rosa 26 promoter. Only cells derived from the male testes contain the LacZ gene and expression will only occur if the WAP-Cre promoter has been active. Both transgenes must reside in the same cell to result in LacZ expression and full pregnancy must be achieved. Activated cells and their progeny were detected by X-gal stain (blue cells). Whole mount analysis was done on the inguinal glands from the injected mice. The presence of blue cells along the mammary ducts indicates that male testicular cells have contributed to the mammary epithelial outgrowth. In addition, the number of mammary cells recovered following transplantation had increased ∼20 fold over the number implanted. Therefore the testicular cells present within the outgrowth will have also expanded indicating a self-renewal capacity. In all, ∼50% (n=9/20) were successful and of these ∼70%(n=7/9) contained blue cells. PCR analysis of the DNA from outgrowths containing blue cells confirmed the presence of both transgenes and Y chromosome-specific alleles. No LacZ-positive cells were observed in epithelial outgrowths collected from non-breeding hosts indicating that prenancy was essential for activation. These results were confirmed in a second experiment. The presence of male cells in significant numbers throughout mammary epithelial outgrowths suggests the following scenario: mammary epithelial cells recombine to assemble growth-competent cellular microenvironments, occasionally chimeras are formed which include male cells; development proceeds. The male cells not only contribute significantly to mammary epithelial growth but also differentiate during pregnancy activating the WAP-Cre gene and concomitantly the LacZ reporter. We conclude that uncommitted testicular cells capable of self-renewal and multiple cell fates, enter mammary epithelium-specific niches and adopt the function of similarly endowed mammary cells. These results imply the tissue-dominance of stem cell niches.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]