DETECTION OF A POPULATION OF LONG-LIVED CELLS IN MAMMARY EPITHELIUM OF THE MOUSE

The fate of dividing mouse mammary epithelial cells was followed by us e of tritiated thymidine (3H-Tdr) autoradiography. Loss of label consi stent with halving kinetics was observed at various times after inject ion; however, heavily labelled cells were frequently observed at two w eeks and later, when none was expected. The grain count over these hea vily labelled cells was often comparable with that 1 h after 3H-Tdr in jection. Extensive serial sectioning revealed that the heavily labelle d cells were often single cells surrounded by many unlabelled cells or that their label was in stark contrast (in excess of 20 reduced silve r grains) to the surrounding group of cells whose label was just above background (a maximum of 3 grains). In addition, by injecting mice at different stages of oestrus, we demonstrated that these long-lived ce lls, although influenced by oestrus, replicated independently of the o estrogen peal;, Our data support a model for mouse mammary epithelium that has a single 'stem' cell positioned within a group of its progeny to form a discrete proliferative unit. This model requires many such stem cells within the mammary epithelium and is consistent with simila r models proposed for other tissues.