Recently, adult stem cells have become a focus of intensive biomedical research, but the
complex regulation that allows a small population of stem cells to replenish depleted tissues is still
unknown. It has been suggested that specific tissue structures delimit the spaces where stem cells
undergo unlimited proliferation (stem cell niche). In contrast, mathematical analysis suggests that
a feedback control of stem cells on their own proliferation and differentiation (denoted Quorum
Sensing) suffices for preserving homeostasis in developing tissues. In the present work we examine
this discrepancy by analyzing a mathematical model of signalling pathways in a mammary stem
cell. Our work shows that the model has a unique equilibrium depending on the environmental
conditions, which is stable for appropriate parameter values. We show how this dependence on
the micro-environment can control the cell decision to either proliferate or differentiate. Our work
further suggests that this description is sufficient for the creation of niche conditions, with no need
of additional structures. The analysis also pinpoints the pathways that may serve as the Quorum
Sensing mechanism.