Freshly isolated E18/E19 membranes were fixed and embedded as described previously.15 (link) Membrane cryosections of 10-µM thickness were stained for the expression of proliferin, periostin, and α-fetoprotein (AFP) antibodies (Santa Cruz Biotechnology). Nuclei were counterstained with ProLong Gold antifade reagent with 6-diamidino-2-phenylindole (DAPI) (Thermo Fisher Scientific). Images were captured on Leica Microsystems CTR6500 and further analyzed using NIH-ImageJ software.