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 immerse into the maternal basal decidua (decidua basalis).
A special "mucous" connective tissue is located between the chorion and the amniotic membrane. Its extracellular matrix is rich in glycosaminoglycans. The main task of this layer is to ensure the connection between the amnion and the chorion. At the same time, it is worth noting that functionally this connection remains quite loose and does not feature high 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.
In relation to the chorionic plate:
- Stem (supporting) villi — originate directly from the chorionic plate.
- 2nd and 3rd order branches — form as offshoots from the stem villi.
In relation to maternal tissue:
- Free villi — are 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 the stem villi themselves and their large branches can act in this role. They grow to the basal decidua and anchor in it.
The histology of the contact zone has a striking feature: at the site of contact with decidua basalis, the surface epithelial layer (syncytiotrophoblast) completely disappears. Instead, cells of the inner cytotrophoblast layer begin to form specific "columns". They actively invade the maternal tissue and act as biological anchors. These cellular structures are designated by the term peripheral cytotrophoblast.
Villar 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 lying on the basement membrane. This is a cambial reserve whose cells retain the ability to mitotic division.
- Syncytiotrophoblast — the surface layer, representing a multinucleated structure. It is formed by the fusion of cells of the underlying cytotrophoblast.
In the dynamics of pregnancy development, the structure of the villi undergoes changes. Both epithelial layers gradually thin out. At the same time, the nuclear density visually increases in the syncytiotrophoblast — the cytoplasm volume 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 is formed as a result of local breakdown of the villous epithelium combined with clotting processes of maternal blood plasma.
Briefly: answers to topical questions
What blood vessels pass inside the stem chorionic villi?
Inside the chorionic villi are blood capillaries: in tertiary villi, they form in situ from mesenchymal cells and connect with the vessels of the mesodermal chorionic plate.
Umbilical arteries in the placenta break down into capillaries of the chorionic villi; metabolism with maternal blood in the lacunae occurs in them, the blood becomes arterial and then collects into the unpaired umbilical vein.
What hormones does the syncytiotrophoblast secrete?
The syncytiotrophoblast of the chorionic villi secretes female sex steroid hormones. The placenta takes on this function compensatorily, since the mother's ovaries are inactive during pregnancy.
The secreted hormones include:
- Estrogens — their active synthesis starts from the middle of pregnancy, and a sharp increase in concentration before childbirth prepares the organism for the baby's birth.
- Progestins — are synthesized almost throughout pregnancy (synthesis stops before childbirth), ensure the maintenance of pregnancy by suppressing the contractile activity of the myometrium, and are used by the fetus as a substrate for corticosteroid production.
How to remember the layers of villous epithelium: Cyto- are internal Cells (they divide like cambium), and Symplasto- is Fusion (the surface layer without cell boundaries, with multiple nuclei).
Class and exam questions
What is a cotyledon?
This is a structural lobule of the placenta that includes a single stem villus with all its ramifications (branches of the 2nd and 3rd order).
What is the difference between free and anchoring villi?
Free villi float in blood lacunae and provide metabolism, while anchoring villi reach the decidua and physically attach the fetal part of the placenta to it.
What is Langhans fibrinoid formed from?
This is a product of mixed origin: it is formed from clotted maternal blood plasma and breakdown products of the chorionic villous epithelium.
What else to study on this topic
- Microscopic organization and layers of the chorionic plate
- Mechanism of formation of peripheral cytotrophoblast cell columns
- Structure of lacunae and maternal blood circulation
- Ultrastructural changes of the syncytiotrophoblast by the end of pregnancy
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