Differentiation and regions of the chorion
During development, the villous membrane (chorion) differentiates depending on which part of the maternal decidua it adjoins.
- Chorion frondosum (Chorion frondosum): contacts the basal plate (decidua basalis). In this zone, villi actively proliferate, branch, and become penetrated by umbilical blood vessels. This particular region forms the fetal part of the placenta.
- Chorion laeve (Chorion laeve): adjoins the capsular decidua (decidua capsularis). Due to impaired blood supply in this area, the villi undergo atrophy and disappear. The chorion laeve functions as the middle fetal membrane.
Amnion and cavity dynamics
Amnion (amniotic membrane) is an internal structure that, following the formation of the umbilical cord, is divided into two functional regions:
- Placental region: covers the inner surface of the placental disc and adjoins directly to the chorion frondosum. Its main task is the predominant secretion of amniotic fluid.
- Non-placental region: faces the chorion laeve, forms the inner lining of the fetus, and is responsible for the resorption (absorption) of amniotic fluid.
In early ontogenesis, an extraembryonic coelom (chorionic cavity) exists between the amnion and chorion. However, by the 8th–9th week of development, the amniotic cavity increases in volume so rapidly that it completely obliterates the coelom. As a result, the amnion is pressed tightly against the chorion, and the cavity is obliterated.
Histological structure of the meninges
The fetal membranes form a layered structure surrounding the fetus (except at the site of umbilical cord insertion). Viewed from the inside out, the histological composition is as follows:
- Amniotic membrane (Amnion):
- Epithelial layer: simple columnar epithelium on a basement membrane (derived from extraembryonic ectoderm).
- Lamina propria: dense fibrous tissue with fibroblasts and a network of collagen fibers (derived from extraembryonic mesoderm).
- Intermediate layer: loose "mucoid" connective tissue from extraembryonic mesenchyme. Contains mucocytes and a high amount of glycosaminoglycans. Provides a very loose connection between the amnion and chorion, allowing the membranes to separate easily.
- Chorion laeve (connects to the amnion via its connective tissue side):
- Connective tissue layer: dense fibrous tissue (derived from extraembryonic mesoderm).
- Epithelial layer: stratified cytotrophoblast (derived from extraembryonic ectoderm). The outer layer—syncytiotrophoblast—degenerates in the chorion laeve.
- Decidua capsularis (Decidua capsularis) — a derivative of maternal tissues:
- Connective tissue layer: decidualized superficial layers of the endometrial lamina propria.
- Epithelial layer: simple columnar endometrial epithelium.
Structural features of the stroma and epithelium
The connective tissue component (stroma) is formed by loose fibrous tissue. Collagen fibers are present but located relatively sparsely. The cellular composition is richly represented by fibroblasts and specific macrophages — Kashchenko-Hofbauer cells.
To facilitate diffusion, there is an abundant vascular network: capillaries are located peripherally, lying closely against the epithelial cover. The epithelial barrier itself possesses specialization:
- On the apical surface of the symplasts-trophoblast facing the blood lacunae, there are numerous microvilli that repeatedly increase the surface area for metabolic exchange.
- Between the cytotrophoblast cells, as well as at the border with the symplast, there are submicroscopic intercellular canaliculi that facilitate the transport of molecules.
At the beginning of the labor act, these membranes rupture, and amniotic fluid is discharged. At the end of labor, after the delivery of the baby, the membranes detach and pass out together with the placenta and parietal decidual tissue (decidua parietalis).
Briefly: answers to topical questions
What is the structure of the villi of the chorion frondosum at different stages of pregnancy?
The structure of the chorionic villi consistently complicates as pregnancy progresses from simple epithelial protrusions to vascular structures. The process includes three stages:
- Primary villi — form in the 2nd week (starting from the 6th day). They consist exclusively of a two-layered epithelium (inner cytotrophoblast and outer symplast-trophoblast or syncytiotrophoblast).
- Secondary villi — form at the beginning of the 3rd week due to the ingrowth of extraembryonic mesenchyme. Their base is represented by a connective tissue stroma covered by two layers of trophoblast.
- Tertiary villi — appear at the end of the 3rd week as a result of vascularization. Blood capillaries form from mesenchymal cells inside the villous stroma, determining the level of gas exchange.
What structures make up the maternal part of the placenta?
The basis of the maternal part of the placenta is the basal decidua (decidua basalis). Histological examination of the maternal part of the placenta reveals:
- Sections of chorionic villi — numerous areas of the fetal part adjacent to the endometrium.
- Anchoring villi — villi directly attached to the basal plate.
- Connective tissue septa — cords extending from the basal plate and dividing the lacunae.
- Decidual cells — clusters of large cells clearly distinguishable within the thickness of the basal plate.
To easily remember the functions of the amnion compartments, imagine the water cycle: the placental part secretes ("spring on the mountain"), and the non-placental part absorbs ("soil in the valley").
Class and exam questions
Why is the chorion divided into the smooth and villous parts?
This depends on contact with the decidua. Where nutrition is sufficient (adjacent to the decidua basalis), the villi proliferate. Where nutrition is insufficient (at the decidua capsularis), they atrophy, forming the chorion laeve.
What happens to the chorionic cavity during development?
By the 8th–9th week of embryogenesis, the amniotic cavity increases in volume so much that it completely displaces the extraembryonic coelom. The amnion presses against the chorion, and the cavity disappears.
Why does the amnion easily detach from the chorion during labor?
Between them lies an intermediate layer of loose mucous connective tissue rich in glycosaminoglycans. This connection is very loose, which facilitates easy separation of the layers.
What else to study on this topic
- The process of endometrial decidualization and the formation of decidua basalis, capsularis, and parietalis.
- Histogenesis of the extraembryonic mesoderm, ectoderm, and endoderm.
- Ultrastructural features of the symplast-trophoblast and cytotrophoblast.
- Mechanisms of substance transport across the placental barrier.
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