FIGURE 2.

Notch signalling promotes and maintains the luminal progenitor cell fate in the mammary gland. The initial stem cell found within the mammary gland (fMaSC) is multipotent and can form both luminal and myoepithelial cells. However, by late embryogenesis, the gland contains two unipotent progenitor cells that form and maintain the luminal and myoepithelial cell layers of the ductal structures within the gland during puberty and adult life and a quiescent multipotent adult MaSC that is only reactivated upon injury (Woodward et al., 2005; Watson and Khaled, 2020). Notch signalling promotes the differentiation of the foetal MaSCs into the unipotent luminal progenitor cell and prevents this cell differentiating into mature luminal epithelial cells to maintain the population (Dontu et al., 2004; Buono et al., 2006; Bouras et al., 2008; Raouf et al., 2008; Lafkas et al., 2013; Šale et al., 2013; Rodilla et al., 2015; Zhang Y. et al., 2016; Lilja et al., 2018). Upon ablation of the luminal epithelial cells in the adult mammary gland, Notch signalling can also promote the conversion of unipotent myoepithelial progenitor cells into unipotent luminal progenitors to repopulate the luminal lineage (Centonze et al., 2020).