Ultrastructural analysis of bovine trophoblast giant cells during their migration by serial block-face scanning electron microscopy (SBF-SEM).

Bovine trophoblast giant cells (TGC) are continuously formed and migrate towards the uterine epithelium (ME) to fuse with ME cells. To date it is not clear whether TGC differentiate from any uninucleate trophoblast cell and how exactly the migration works, because most data is based on two-dimensional images, generated by conventional transmission electron microscopy (TEM) with little opportunity for serial sections. Since the serial block face scanning electron microscopy (SBF-SEM) technique allows complete reconstruction of specifically selected tissue regions, we aimed to study TGC morphology and life cycle in relation to the feto-maternal interface in a three-dimensional setting. Placentome samples from six cows at different gestational stages were visualized by SBF-SEM (Zeiss Merlin VP Compact SEM with Gatan 3view2XP). The software Microscopy Image Browser was used for image processing, manual segmentation and analysis, while the software package IMOD was used for visualization. Here, TGC were divided morphologically into young, maturing and mature cells. A surprisingly high number of TGC still possessed contact with the fetal basement membrane (FBM). Young TGC had extensive contact to the FBM, while in maturing and mature TGC the contact occupied only small areas. In mid gestation, many TGC held simultaneous contact to the FBM and the ME (41-93%), which declined to 10% near term. To conclude, by adding a third dimension, SBF-SEM technology has questioned previous assumptions about TGC and hybrid cells and provides valuable new information about cell contacts and characteristics of bovine trophoblast cells.