SHH and hydrops fetalis: Initiation of HF involves a series of signaling between the epidermal cell and the dermal papilla, such as Wnt/beta-catenin, EDA/EDAR/NF-κB, Noggin/Lef-1, Ctgf/Ccn2, Shh, BMP-2/4/7, Dkk1/Dkk4 and EGF [30, 31].