Changing dynamics of the fibroblast differentiation hierarchy during mammary gland morphogenesis and tumorigenesis [scRNA-Seq I]

Fibroblasts form a major component of the stroma in normal mammary tissue and breast tumors. Here we have applied single-cell transcriptome profiling to characterize fibroblasts in the mouse mammary gland across five different developmental stages and during mammary oncogenesis. In the normal gland, diverse stromal populations were resolved, including lobular-like fibroblasts, committed preadipocytes and adipogenesis-regulatory, as well as cycling fibroblasts in puberty and pregnancy. These specialized cell types appear to emerge from CD34high mesenchymal progenitor cells, accompanied by elevated Hedgehog pathway activity. During late tumorigenesis, diverse cancer-associated fibroblasts were identified in different models of breast cancer, including a population of CD34– myCAFs that were transcriptionally and phenotypically similar to senescent CAFs. Moreover, Wnt9a was demonstrated to be a regulator of senescence in CD34– myCAFs. These findings reflect a diverse and hierarchically organized stromal compartment in the normal mammary gland that provides a framework to better understand fibroblasts in normal and cancerous states.