Transmission electron microscopy (TEM)
The placentas were cut into approximately 1x1x1 mm pieces. HTR-8/SVneo and JEG-3 cells were washed with ice-cold PBS, digested with trypsin, and then collected via centrifugation. Both tissues and cells were subsequently fixed via a fixative buffer comprising 2% paraformaldehyde and 2.5% glutaraldehyde in 0.1 M phosphate buffer. Postfixation, the samples were embedded and sectioned at a thickness of 0.12 μm and then stained with uranyl acetate solution and lead citrate solution. Visualization of the ultrathin sections was accomplished through a JEOL transmission electron microscope (Zeiss, Germany).