Chorionic villi with syncytiotrophoblast, cytotrophoblast and fetal capillaries
Expansion
Chorionic villus, from maternal blood inwards:
- Syncytiotrophoblast: a multinucleate layer with no cell boundaries, in direct contact with maternal blood. Produces hCG, human placental lactogen, progesterone and oestrogen, and performs transport
- Cytotrophoblast (Langhans cells): the proliferative reserve, prominent early and largely lost by term
- Villous stroma with Hofbauer cells (macrophages)
- Fetal capillary endothelium
Change with gestation: the barrier thins as cytotrophoblast disappears and fetal capillaries move peripherally to form vasculosyncytial membranes, greatly improving diffusion at the time of maximum fetal demand.
The syncytial layer is the reason the placenta is not rejected: with no intercellular gaps and minimal MHC expression, it presents little to the maternal immune system.
Pathology worth recognising: hydatidiform mole with hydropic villi and trophoblastic proliferation; placental abruption with retroplacental haematoma; pre-eclampsia with failure of the normal trophoblastic invasion converting spiral arteries into low resistance vessels; and chorioamnionitis with neutrophil infiltration of the membranes.