Moreover, we used the data to delineate molecular and cellular events that precede the emergence of Pc morphogenesis and discovered that OGN+/UCHL1+ cells constitute the progenitor cell population at the earliest phase of Pc formation, thereby revealing that BMP and TGFβ signaling pathways likely initiate the development of Pc and hence HF morphogenesis. This evidence concerns the gene UCHL1 and hydrops fetalis.