PDGFA and hydrops fetalis: One example would be skin-derived adipocyte precursor cells, especially since these cells were found to highly express platelet-derived growth factor A (PDGFA), which mediates HF cycling as demonstrated by loss-of-function genetic tools in animal studies, i.e., PDGFA-null mice presented with abnormally thinner hair with smaller DP compared to that of the wild-type controls, hence suggesting a lack and/or delay in the activation of HFSCs (1, 8).