Chorion Frondosum and Connective Tissue
The central structural element of the fetal part is the chorion frondosum (Chorion frondosum). It is formed by the chorionic plate, from which villi extend and plunge into the maternal decidua basalis.
A special "mucous" connective tissue lies between the chorion and the amniotic membrane. Its extracellular matrix is rich in glycosaminoglycans. The main task of this layer is to provide a connection between the amnion and the chorion. Notably, this functional connection remains quite loose and lacks high mechanical strength.
Cotyledons and Classification of Villi
The structural and functional unit of the placenta is the cotyledon (lobe). It represents a single stem villus together with all its branches. Normally, there are 10 to 20 large lobes and up to 200 small rudimentary ones.
The entire villous system is classified according to two main criteria.
Relative to the chorionic plate:
- Stem (anchoring) villi — originate directly from the chorionic plate.
- Branches of the 2nd and 3rd order — form as branches from the stem villi.
Relative to the maternal tissue:
- Free villi — located in lacunae filled with maternal blood. They float in this space and are responsible for intensive metabolism between mother and fetus.
- Anchoring villi — perform the function of physical fixation.
Anchoring Villi and Fixation Mechanism
Anchoring villi are necessary for the tight attachment of the fetal part of the placenta to the maternal part. Both stem villi and their large branches can serve this role. They grow to reach the decidua basalis and anchor within it.
The histology of the contact zone has a striking feature: at the site of contact with the decidua basalis, the superficial epithelial layer (syncytiotrophoblast) completely disappears. Instead, cells of the underlying cytotrophoblast begin to form specific "columns." They actively invade the maternal tissue and act as biological anchors. These cellular structures are designated as peripheral cytotrophoblast.
Villous Epithelium (Trophoblast) and Fibrinoid
The epithelial cover of the chorionic plate and villi develops from the trophoblast and consists of two layers:
- Cytotrophoblast — the inner cellular layer resting on the basement membrane. This is a stem cell reserve whose cells retain the capacity for mitotic division.
- Syncytiotrophoblast — the superficial layer, representing a multinucleated structure. It is formed by the fusion of cells from the underlying cytotrophoblast.
During pregnancy, the structure of the villi undergoes changes. Both epithelial layers gradually thin out. At the same time, the nuclear density in the syncytiotrophoblast visually increases — the volume of cytoplasm decreases, and there are more nuclei per unit area.
In the second half of pregnancy, Langhans' fibrinoid begins to deposit on the surface of the villi. This is a non-cellular fibrin-like mass of mixed origin. It forms as a result of local breakdown of the villous epithelium combined with maternal blood plasma clotting processes.