Fibroblastic colony forming units (CFU-F) assay

CFU-F assay was performed as described previously (Robey et al., 2014). Briefly, FACS sorted SSC subsets were seeded at a clonal density of 10,000 cells per well (1.9 cm²) of a 24-well plate (Corning, NY) in phenol red-free α-MEM (Cat no. 41061029; Thermo Fisher Scientific) plus 10% fetal bovine serum (Atlanta Biologicals, GA) and 1% penicillin/streptomycin (Invitrogen). One half of the medium was replaced after 7 days and cells were stained with methylene blue (Millipore Sigma, Burlington, MA) at day 14 and colonies were counted.