Early placenta

The placenta is a temporary organ that in the course of pregnancy allows to exchange substances between the maternal and foetal blood to support development of the foetus. In structure of the placenta both foetal and maternal tissues participate (foetomaternal organ). Foetal part consists of the chorionic plate (membrana chorii – in Fig. 1 depicted in dark green) and chorionic villi (a part of the frondosum – in light green). Maternal part is formed by the decidua basalis (blue): functional zone of the pregnant endometrium forms a bottom of the intervillous space (I). Edges of the placenta are continuous with the foetal membranes (amniochorion). The intervillous space can be found between the foetal and maternal parts and it is filled in with the maternal blood. Principal chorionic villi that are anchored to the basal plate are called the anchoring villi; these villi not reaching the basal plate are called free villi. Chorionic villi ramify extensively; their ramification and numbers are related to a foetus growth. An area of the placenta including a large anchoring villus and surrounding free villi is referred to as the kotyledon; kotyledons are separated incompletely by the placental septa (purple) arising from the decidua basalis.
The basal plate consists of decidual cells (derived from decidual transformation of endometrial fibroblasts). Decidual cells are oval to polygonal, their cytoplasm is light (due to a large amount of glycogen) or pink; their nucleus is oval and contains a well apparent nucleolus. The blood from the spiral arteries (in Fig. 1 in black) comes in contact with the chorionic villi (direction of blood flow is indicated by arrows). The myometrium (brown) is situated under the endometrium. Hyaline degeneration of cells gives rise to the fibrinoid (orange) that covers the structures in the intervillous space. With time a fibrin can be deposited on the fibrinoid. In full term placenta the fibrinoid can also covers  large areas of the villi.
An outer and smooth surface of the chorionic plate is lined by a layer of low cuboidal amniotic cells. The umbilical cord (yellow) is also attached to the plate from this aspect. Two umbilical arteries (red) and one vein (blue) branch in the chorionic plate into chorionic vessels that supply chorionic villi with the blood. The chorionic plate contains the mesenchymal connective tissue that also enters the chorionic villi. The inner surface facing the intervillous space is covered with the trophoblast; an outer layer contains the syncytiotrophoblast under which lies a layer of pale cytotrophoblast cells.

Fig. 1. Scheme of the placenta. Figure legend can be found in the text. A – amniotic cavity; I – intervillous space. Myometrium (in brown). Author: E. Klika et al.; colours: J. Mokrý.		Fig. 2. 3D reconstruction of the chorionic villus of the early placenta. Figure legend can be found in the text. Author: R. Krstic; colours: J. Mokrý.

The most important functional structure of the placenta is the chorionic villus (Fig. 2). Differential diagnosis of the early and full-term placenta is based on microscopic structure of the chorionic villus. In the early placenta the terminal villi are relatively large and are covered by two layers of the trophoblast. An inner layer, the cytotrophoblast (in Fig. 2 shown in light green) is formed by pale cells whose numbers decline after month 5. The cytotrophoblast lies on the basal lamina that separates it from the underlying mesenchyme (not depicted in Fig. 2). An outer layer is formed by the syncytiotrophoblast (dark green); it is a typical syncytium, i.e. their cytoplasm contains a large number of nuclei. This trophoblast layer is continuous during the whole pregnancy. By accumulation of cell nuclei the syncytial knots are formed. A surface of the syncytiotrophoblast that is in contact with the maternal blood is increased by many microvilli (brush border). Inside of the villus there is the mesenchymal connective tissue and blood vessels; the vessels are not abundant and occur in the centre of the villus. In some slides of the early placenta the nucleated red blood cells (erythroblasts - blue) can be seen; maternal erythrocytes in the intervillous space do not contain the nucleus (orange). The connective tissue (white) contains oval macrophages the so called Hofbauer cells (yellow) – their number increases with placenta maturation; they likely participate in remodelling chorionic villi.