Fig. 4.

Transgenic MECs expressing the oncogene ΔNβcat colonized mammary fat pads more efficiently than control cells. MECs were isolated from [ΔNβcat]-transgenic and control mice and counted; then several inocula containing different numbers of cells were transferred to cleared fat pads to test their progenitor potential. The fraction of cleared mammary fat pads colonized by cells was scored. For experiment 1, cells were transferred after the cell preparation was substantially reduced to single cells by enzymatic dissociation, a procedure that tends to impair viability. For experiment 2, cells were transferred in the clusters typical of our standard preparation of MECs. The results were substantially the same and showed that the progenitor activity in MEC populations from [ΔNβcat]-glands was higher than normal. Numbers in parentheses represent the number of glands colonized per total number of glands transplanted. The morphogenesis of outgrowths from 50,000 cell inocula is also shown.