AN ENTIRE FUNCTIONAL MAMMARY-GLAND MAY COMPRISE THE PROGENY FROM A SINGLE-CELL

Any epithelial portion of a normal mouse mammary gland can reproduce a n entire functional gland when transplanted into an epithelium-free ma mmary fat pad. Mouse mammary hyperplasias and tumors are clonal domina nt populations and probably represent the progeny of a single transfor med cell. Our study provides evidence that single multipotent stem cel ls positioned throughout the mature fully developed mammary gland have the capacity to produce sufficient differentiated progeny to recapitu late an entire functional gland. Our evidence also demonstrates that t hese stem cells are self-renewing and are found with undiminished capa cities in the newly regenerated gland. We have taken advantage of an e xperimental model where mouse mammary tumor virus infects mammary epit helial cells and inserts a deoxyribonucleic acid copy(ies) of its geno me during replication. The insertions occur randomly within the somati c genome. CzechII mice have no endogenous nucleic acid sequence homolo gy with mouse mammary tumor virus; therefore all viral insertions may be detected by Southern analysis provided a sufficient number of cells contain a specific insertional event. Transplantation of random fragm ents of infected CzechII mammary gland produced clonal-dominant epithe lial populations in epithelium-free mammary fat pads. Serial transplan tation of pieces of the clonally derived outgrowths produced second ge neration glands possessing the same viral insertion sites providing ev idence for self-renewal of the original stem cell. Limiting dilution s tudies with cell cultures derived from third generation clonal outgrow ths demonstrated that three multipotent but distinct mammary epithelia l progenitors were present in clonally derived mammary epithelial popu lations. Estimation of the potential number of multipotent epithelial cells that may be evolved from an individual mammary-specific stem cel l by self-renewal is in the order of 10(12)-10(13). Therefore, one ste m cell might easily account for the renewal of mammary epithelium over several transplant generations.