Differentiation and Regions of the Chorion
During development, the villous membrane (chorion) differentiates depending on which part of the maternal decidua it contacts:
- Chorion frondosum (villous chorion): contacts the decidua basalis. In this region, the villi actively grow, branch, and become invaded by umbilical blood vessels. This specific region forms the fetal part of the placenta.
- Chorion laeve (smooth chorion): contacts the decidua capsularis. Due to poor blood supply in this area, the villi undergo atrophy and disappear. The smooth chorion functions as the middle fetal membrane.
Amnion and Cavity Dynamics
The amnion (amniotic membrane) is the inner 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 lies directly adjacent to the chorion frondosum. Its primary role is the predominant secretion of amniotic fluid.
- Non-placental region: faces the smooth chorion, forms the inner lining of the fetal membranes, and is responsible for the reabsorption of amniotic fluid.
In early ontogenesis, an extraembryonic coelom (chorionic cavity) exists between the amnion and chorion. However, by weeks 8–9 of development, the amniotic cavity expands so rapidly that it completely obliterates the coelom. As a result, the amnion is pressed tightly against the chorion, and the cavity disappears.
Histological Structure of the Membranes
The fetal membranes form a stratified structure surrounding the fetus (except at the umbilical cord insertion site). Viewed from inside to outside, 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 containing fibroblasts and a network of collagen fibers (derived from extraembryonic mesoderm).
- Intermediate layer: loose "mucous" connective tissue derived from extraembryonic mesenchyme. It contains mucocytes and abundant glycosaminoglycans, providing a very loose attachment between the amnion and chorion, allowing the membranes to separate easily.
- Smooth Chorion (joins the amnion on 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—the syncytiotrophoblast—is reduced in the smooth chorion.
- Decidua Capsularis — derived from maternal tissues:
- Connective tissue layer: decidualized superficial layers of the endometrial lamina propria.
- Epithelial layer: simple columnar endometrial epithelium.
Features of the Stroma and Epithelium
The connective tissue component (stroma) is formed by loose fibrous tissue. Collagen fibers are present but relatively sparse. The cellular composition is rich in fibroblasts and specific macrophages known as Hofbauer cells.
To facilitate diffusion, there is an abundant vascular network: capillaries are located peripherally, closely apposed to the epithelial lining. The epithelial barrier itself is specialized:
- The apical surface of the syncytiotrophoblast, facing the blood lacunae, features numerous microvilli that vastly increase the metabolic exchange surface area.
- Submicroscopic intercellular canaliculi are located between the cytotrophoblast cells and at the boundary with the syncytium, facilitating molecular transport.
At the onset of labor, these membranes rupture, resulting in the discharge of amniotic fluid. At the end of labor, after the delivery of the baby, the membranes separate and are expelled along with the placenta and decidua parietalis.