Figure 25.

Representative images of a cross-section through a uterine implantation site at E10.5. Figure A: The labyrinthine placenta is maturing but is not yet fully mature, and positioning of the embryo, yolk sac (YS), and amnion (AM) can be seen in relation to the rest of the placenta; the lack of widespread anchoring allows these structures to shift position in unpredictable ways during tissue embedding. A cross-section of the umbilical artery (UA) can also be seen. Figure B: A higher magnification of this placental cross-section shows multiple distinct layers: the maternal decidua (D), the embryonic junctional zone (JZ), and the embryonic labyrinth (L) at the core adjacent to the chorionic plate (C). The metrial gland (MG) is not placental tissue but is instead a transient maternal immune organ that develops as a pregnancy-specific modification of the mesometrial uterus, opposite each implantation site. Figure C: The labyrinth contains intersecting columns of trophoblast cells (three embryonic cell lineages) that separate embryonic vessels packed with nucleated red blood cells (nRBCs) from maternal sinusoids containing anucleated red blood cells (aRBCs).