Figure 32.

Representative images of labyrinth development from its induction at E8.5 to near birth at E17.5 showing the increase in size and intermingling of maternal and embryonic blood spaces. Figures A and B: At E8.5, the formation of the labyrinth (L) is induced when the allantois fuses with the chorionic plate (C), yielding a partially vascularized tissue comprised of trophoblast and embryonic endothelial cells. The trophoblast giant (TG) cells are involved in remodeling of the maternal decidua (D) at this state. Figures C and D: By E12.5, the highly vascularized labyrinth consists of layers of trophoblast separating parallel embryonic vessels containing many embryonal nucleated (immature or “primitive”) red blood cells (nRBCs). Within the labyrinth, the maternal and embryonic blood spaces are separated by three layers of trophoblast cells and by a layer of embryonal endothelial cells. The trilaminar trophoblast includes a single discontinuous layer of mononuclear sinusoidal trophoblast giant cells that line the maternal sinusoids and two continuous layers of syncytiotrophoblast cells that envelop the embryonal endothelial cells. Figures E and F: By E17.5, the vascular channels in the labyrinth become more prominent with the embryonic and maternal vasculature containing higher blood volumes, and the embryonic vessels contain mainly anucleated (mature or “definitive”) red blood cells (aRBCs).