Although HF development occurs predominantly during the embryonic life, over- and/or under-activation of signaling pathways, such as sonic hedgehog (shh), notch, TGF-β, and Wnt/β-catenin, which regulate epithelial–mesenchymal interaction in HF, play roles in periodic regeneration of HF and, hence, hair cycling in adult [22]. Here, SHH is linked to hydrops fetalis.