Placenta pathology associated with maturation abnormalities

The placenta is the fastest growing organ of the human body. The placenta grows from a single cell to approximately 5x10 cells in 38 weeks. A good blood supply from maternal blood via the spiral arteries to the placenta is not only very important for the foetus but also very important for normal development of the placenta. The maternal blood is supplied to the placenta by spiral arteries. Approximately 100-150 spiral arteries are formed and 50 to 200 veins are present to return the blood to the maternal circulation (see Pijnenborg on page 185212 in 1 and 2-6). The normal placenta parenchyma is divided into 10-40 lobes or lobules separated by grooves or septa. These interlobular septa usually do not reach the chorionic plate or foetal surface. In the centre of each lobule one or several spiral arteries can be found. From the chorionic plate 60-70 foetal stem vessels can be seen. Each stem vessel supplies a villous tree. The architecture of the villous tree changes dramatically during gestation (see for an excellent overview of the changes during gestation 7). During the first twelve weeks of development the placenta consists of mesenchymal villi. After this period subsequently stem or anchoring villi, immature intermediate, mature intermediate and terminal villi are formed. The terminal villi can be recognised by the presence of syncytio-vascular membranes. The development of syncytio-vascular membranes in the third trimester is very important to provide the foetus with adequate amounts of oxygen. Oxygen reaches the foetal blood by diffusion and the concentration of foetal oxygen is highly dependent upon the distance between maternal and foetal blood. An increase in the diffusion distance by 2 decreases the oxygen content by four. The terminal villi have small capillaries that lie immediately below the syncytio-trophoblast surface, the so-called syncytio-vascular membranes where the actual foeto-maternal exchange takes place. The syncytio-vascular membranes can be recognised by the anuclear surface of the syncytiotrophoblast directly opposite to the vessel wall. Immature intermediate villi can be