Developmental Lineage of Mammary Stem Cells

MaSCs have been studied in detail in mouse development, and have been much less characterized in humans. In the mouse, MaSCs are defined as those cells that are able to generate a functional mammary gland when transplanted in vivo, but there is evidence that expression of certain markers in mouse MaSCs changes during different stages of mammary gland morphogenesis e.g. during pregnancy. Most data available on human MaSCs is derived from adult stem-like cells in human mammary glands that are able to differentiate in vitro into both myoepithelial and ductal (luminal) epithelial cells and thus are also known as mammary bipotent progenitors (MBiPs). It is uncertain how much these adult human MaSCs/MBiPs differ from embryonic and fetal MaSCs. Multiple reports agree on MaSCs residing within the basal epithelial cell subpopulation in both humans and mice, but the definitive list of markers that can be used to isolate pure MaSCs from within the basal epithelial subset has not been defined (reviewed in Phillips and Kuperwasser 2014).

Mouse MaSCs are first apparent in mammary placodes. Lef1 (Lymphoid Enhancer Binding Factor 1) is a transcription factor that binds to beta-catenin (Ctnnb1) after activation by Wnt/β-catenin signaling. The Lef1:Ctnnb1 complex plays a critical role in controlling the transcription of genes involved in mouse mammary placode formation by driving epithelial cell proliferation and differentiation during the early stages of mammary gland development (reviewed in Lindvall et al. 2007). WNT ligand Wnt10b plays an important role in the embryonic stages of mouse mammary gland development, including placode and bud formation (reviewed in Watson and Khaled 2020, Slepicka et al. 2021). Besides Wnt10b, Wnt6 and Wnt10a are also implicated in mouse mammary bud formation (reviewed in McNally and Stein 2017). Wnt4 plays a role in mouse mammary gland development during puberty and pregnancy (reviewed in Slepicka et al. 2021). Wnt4 expression is regulated by progesterone, as it is expressed in progesterone receptor-positive cells (reviewed in Tanos et al. 2012), and it acts as a paracrine mediator of progesterone signaling in the mammary gland (reviewed in McNally and Stein 2017). FGF signaling also plays a role in mammary placode formation, in particular Fgf10-mediated activation of Fgfr2b (reviewed in Mailleux et al. 2002, McNally and Stein 2017), but the FGF receptor Fgfr1 and ligands Fgf4, Fgf8, Fgf7, and Fgf17 are also expressed in the developing placode (reviewed in McNally and Stein 2017). Other signaling pathways studied in mouse that contribute to development of mammary glands and maintenance and differentiation of MaSC include Hedgehog signaling (reviewed in Lewis and Veltmaat 2004), NOTCH signaling (Bouras et al. 2008, reviewed in Edwards and Brennan 2021), and estrogen and progesterone signaling (Feng et al. 2007, reviewed in Tanos et al. 2012). The involvement of FGF10 in generation and maintenance of MaSCs appears to be conserved in humans.

In normal adult human breast epithelium, mammary stem cells are rare and are located in the ductal part of terminal ductal lobular units.

Recent studies indicate that the MaSC/MBiP cell population is heterogeneous, and it is likely that both early and late MaSCs exist with not completely overlapping sets of markers (Scheele et al. 2017, reviewed in Visvader and Stingl 2014 and Slepicka et al. 2021).