Classification by Developmental Origin
Four extraembryonic organs form during human embryogenesis. They are classified based on the origin of their epithelial component:
- Derivatives of extraembryonic ectoderm:
- Amnion — epithelium develops from amniotic ectoderm.
- Chorion — epithelium originates from the trophoblast.
- Derivatives of extraembryonic endoderm:
- Yolk sac — epithelial lining originates from hypoblast cells.
- Allantois — forms as an outgrowth of intestinal endoderm into the connecting stalk.
The stromal (connective tissue) component of the wall in each of these organs is always formed by extraembryonic mesenchyme.
Embryogenesis of the Amnion
The process of amnion formation in humans starts on the 7th day (during the first phase of gastrulation) and features an important distinction: unlike in birds and certain mammals, human embryos lack amniotic folds. Formation proceeds through the splitting (cavitation) of the embryonic epiblast.
Initially, small cavities appear between epiblast cells and soon merge into a single amniotic cavity. This creates a primary structure — a vesicle. Its roof consists of amniotic ectoderm, and its floor consists of the embryonic epiblast (embryonic disc).
Subsequently, the roof cells undergo complex movements. For a short time, the roof appears to be formed directly by trophoblast cells due to the separation of the amniotic ectoderm. However, the ectoderm soon reunites, completely isolating the vesicle from the trophoblast (chorion). Connection between them is preserved only in one region — the connecting stalk (amniotic stalk).
Externally, the amniotic ectoderm becomes covered by extraembryonic mesenchyme migrating from the epiblast. The wall becomes bilayered, and as the embryo invaginates into it, the vesicle transforms into a mature amniotic membrane surrounding the fetus.
Evolution of Chorionic Villi
From the very beginning (6th day), the trophoblast forms projections with high lytic activity that break down the zona pellucida and endometrial tissues for implantation. From this point on, it is referred to as the chorion (villous membrane). Villi undergo three stages of structural complexity during development:
- Stage I. Primary (trophoblastic) villi. Present until the end of the 2nd week. They branch actively and consist exclusively of a bilayered epithelium: cytotrophoblast and syncytiotrophoblast.
- Stage II. Secondary (connective tissue) villi. Appear at the beginning of the 3rd week. Extraembryonic mesenchyme grows into the interior of the epithelial projections, forming the connective tissue stroma. The bases of the villi are united by a layer of mesenchyme — the chorionic (mesodermal) plate.
- Stage III. Tertiary (vascularized) villi. Arise at the end of the 3rd week. Blood capillaries form in situ within the villi from mesenchymal cells via vasculogenesis. They integrate with the vessels of the chorionic mesodermal plate.
A mature tertiary villus on cross-section consists of three components: in the center is the connective tissue stroma with blood capillaries; surrounding it is a middle layer of dividing cytotrophoblast cells (cambium); and on the surface is a growing non-cellular mass of syncytiotrophoblast. The villi submerge into endometrial lacunae, where maternal blood from eroded vessels bathes them, facilitating the transition to hematotrophic nutrition.
Yolk Sac and Allantois
The development of the yolk sac proceeds in two stages. Initially, hypoblast cells proliferate along the inner surface of the trophoblast from the edges of the embryonic shield. By fusing, they form the primary yolk sac (exocoelom). At the end of the 2nd week, the hypoblast forms a new, smaller cavity adjacent to the embryonic shield ventrally — the secondary yolk sac. Mesenchyme joins its extraembryonic endoderm, making the wall bilayered. The primary yolk sac degenerates, and its cavity becomes filled with loose mesenchyme (exocoelomic vesicles may occasionally persist). In humans, the yolk sac contains no yolk; instead, it is filled with serous fluid.
The allantois forms on weeks 3–4 as a finger-like outgrowth. It originates from the caudal part of the yolk sac and grows into the connecting stalk. The allantoic wall consists of two layers: extraembryonic endoderm lines the inside, while extraembryonic mesoderm covers it on the outside.