Three-dimensional cultures of trophoblast stem cells autonomously develop vascular-like spaces lined by trophoblast giant cells.

The maternal blood space in the mouse placenta is lined not by endothelial cells but rather by various subtypes of trophoblast giant cells (TGCs), defined by their location and different patterns of gene expression. While TGCs invade the spiral arteries to displace the maternal endothelium, the rest of the vascular space is created de novo but the mechanisms are not well understood. We cultured mouse trophoblast stem (TS) cells in suspension and found that they readily form spheroids (trophospheres). Compared to cells grown in monolayer, differentiating trophospheres showed accelerated expression of TGC-specific genes. Morphological and gene expression studies showed that cavities form within the trophospheres that are primarily lined by Prl3d1/Pl1α-positive cells analogous to parietal-TGCs (P-TGCs) which line the maternal venous blood within the placenta. Lumen formation in trophospheres and in vivo was associated with cell polarization including CD34 sialomucin deposition on the apical side and cytoskeletal rearrangement. While P-TGCs preferentially formed in trophospheres at atmospheric oxygen levels (19%), decreasing oxygen to 3% shifted differentiation towards Ctsq-positive sinusoidal and/or channel TGCs. These studies show that trophoblast cells have the intrinsic ability to form vascular channels in ways analogous to endothelial cells. The trophosphere system will be valuable for assessing mechanisms that regulate specification of different TGC subtypes and their morphogenesis into vascular spaces.