Full-term placenta

General structure of the placenta and is parts was described in a topic „Early placenta“. Full term placenta differs from the early one by the structure of their chorionic villi. Intervillous space of the full-term placenta is filled in with a high number of tiny terminal villi (Fig. 1). Their surface is covered with a thin but continuous layer of the syncytiotrophoblast (dark green). Redistribution of cell nuclei of the syncytiotrophoblast gives rise to syncytial knots, while the other parts can become very thin. The cytotrophoblast is reduced and usually there are only few rare cells left (light green). Under the basal lamina (not depicted) of the trophoblast there is the mesenchymal connective tissue (white) the amount of which is reduced. It contains Hofbauer cells (yellow). Inside of the villi there are many blood capillaries (red) reaching the trophoblast (i.e., they appear at the edges of the villi). Endothelium contacting the syncytiotrophoblast reduces the distance for diffusion and exchange of gases and nutrients between the foetal and maternal blood. Foetomaternal barrier is in some places reduced to syncytiotrophoblast and endothelium. Foetal erythrocytes (inside of the capillaries of chorionic villi) have the same morphology as maternal erythrocytes (orange) in the intervillous space. A typical feature of the full-term placenta is presence of fibrinoid (fibrin, tissue debris, placental secretion) that covers partially the chorionic villi (but also the chorionic plate and decidua basalis). Fibrinoid hinders a transport of nutrients across the placental barrier.
A detailed description of macroscopic structure and function of the placenta was given in the lecture.

Fig. 2. 3D reconstruction of chorionic villus in the full-term placenta. Illustration legend can be found in the text.
Author: R. Krstic; colours: J. Mokrý.