Classification by sources of development
Four extraembryonic organs are formed in human embryogenesis. They are usually classified according to the source of origin of their epithelial component:
- Derivatives of extraembryonic ectoderm:
- Amnion — the epithelium develops from the amniotic ectoderm.
- Chorion — the epithelium originates from the trophoblast.
- Derivatives of extraembryonic endoderm:
- Yolk sac — the epithelial lining originates from hypoblast cells.
- Allantois — forms as an outgrowth of the gut endoderm into the connecting stalk.
The stromal (connective tissue) part of the wall of 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 (in the first phase of gastrulation) and has an important feature: unlike in birds and some mammals, humans lack amniotic folds. Formation proceeds through splitting (cavitation) of the embryonic epiblast.
First, small cavities appear between the epiblast cells, which soon merge into a single amniotic cavity. A primary structure — a vesicle — is formed. Its roof is represented by the amniotic ectoderm, and its floor by the embryonic epiblast (embryonic disc).
Subsequently, the roof cells undergo complex movements. For a short time, the roof is represented directly by trophoblast cells due to the divergence of the amniotic ectoderm. However, the ectoderm soon closes again, completely isolating the vesicle from the trophoblast (chorion). The connection between them is maintained only in one zone — in the region of the connecting stalk.
Externally, the amniotic ectoderm is covered by extraembryonic mesenchyme migrating from the epiblast. The wall becomes two-layered, and the vesicle itself, as the embryo invaginates into it, transforms into a full-fledged amniotic membrane surrounding the fetus.
Evolution of chorionic villi
From the very beginning (6th day), the trophoblast forms protrusions with high lytic activity that destroy the fertilization membrane and endometrial tissues for implantation. From this moment, it is called the chorion (villous membrane). Villi go through three stages of structural complication in their development:
- Stage I. Primary (trophoblastic) villi. Exist until the end of the 2nd week. They actively branch and consist exclusively of a two-layered epithelium: cytotrophoblast and symplasts-trophoblast.
- Stage II. Secondary (connective tissue) villi. Appear at the beginning of the 3rd week. Extraembryonic mesenchyme grows inside the epithelial protrusions, forming the connective tissue stroma. The bases of the villi are united by a layer of mesenchyme — the chorionic (mesodermal) plate.
- Stage III. Tertiary (vascular) villi. Arise at the end of the 3rd week. Blood capillaries are formed from mesenchymal cells inside the villi by in situ vasculogenesis. They integrate with the vessels of the chorionic mesodermal plate.
A mature tertiary villus in cross-section consists of three components: in the center — connective tissue stroma with blood capillaries, around it — a middle layer of dividing cytotrophoblast cells (cambium), and on the surface — a growing non-cellular mass of syncytiotrophoblast. The villi are immersed in endometrial lacunae, where they are bathed by maternal blood from destroyed vessels, which ensures the transition to a hematotrophic type of 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 closing, 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 from below — this is the secondary yolk sac. Mesenchyme joins its extraembryonic endoderm, making the wall two-layered. The primary sac degenerates, its cavity is filled with loose mesenchyme (exocoelomic vesicles sometimes remain). In humans, the yolk sac does not contain yolk; it contains serous fluid.
Allantois forms in the 3rd–4th week as a finger-like outgrowth. It extends from the posterior part of the yolk sac and grows into the connecting stalk. The allantoic wall consists of two layers: extraembryonic endoderm is located inside, and extraembryonic mesoderm covers it from the outside.
Briefly: answers to topical questions
What functions does the yolk sac perform in humans?
In humans, the yolk sac (saccus vitellinus) functions temporarily in the early stages and then regresses. Its functions:
- Hematopoietic — blood formation begins for the first time in the wall of the sac; blood islands are discovered as the first foci of hematopoiesis and vasculogenesis.
- Generative — temporary residence of primordial germ cells (gonocytes).
- Excretory.
- Immunoregulatory.
- Metabolic.
- Synthetic.
- Trophic — practically absent, since mammals have no yolk, and in humans the sac does not contain yolk and is filled with serous fluid.
What functions does the allantois perform in humans?
The allantois (allantois) in humans performs guiding and excretory functions in the first weeks of embryogenesis. Its main tasks:
- Vascular ("guideline vector") — serves as a conductor along which blood vessels grow from the embryo to the developing placenta.
- Excretory — ensures the collection of metabolic waste products until the full functioning of the placenta begins.
After fulfilling these functions, the organ rapidly undergoes reduction.
What functions does the amnion perform?
The amnion creates and maintains a fluid environment for the development of the embryo/fetus. Its functions:
- Secretion and resorption of amniotic fluid.
- Creation of an internal aqueous environment for fetal development; amniotic fluid surrounds the fetus on all sides.
- Mechanical protection of the fetus — the amnion is part of the fetal membranes, the general function of which is mechanical protection of the fetus.
For classification by sources, remember: structures that "wrap around" the embryo from the outside (amnion and chorion) are of ectodermal origin. Structures associated with the gut tube ventrally (yolk sac and allantois) originate from the endoderm.
Class and exam questions
What tissue components make up the wall of any extraembryonic organ?
The wall is always formed by two components: specific epithelium (depending on the source of development) and connective tissue — extraembryonic mesenchyme.
How do tertiary chorionic villi differ from secondary ones?
Tertiary villi are vascular: blood capillaries are formed in them from mesenchymal cells, whereas secondary villi contain only avascular connective tissue stroma.
What is the specific feature of amnion formation in humans?
In humans, the amniotic vesicle is formed by splitting (cavitation) of the epiblast cell mass, without the formation and growth of amniotic folds characteristic of some other animals.
What else to study on this topic
- Mechanism of epiblast cavitation and cell movements during amnion formation
- Histological structure and cellular composition of the tertiary chorionic villus
- Process of in situ vasculogenesis in secondary chorionic villi
- Lytic activity of the trophoblast during endometrial destruction
- Formation of uteroplacental circulation and hematotrophic nutrition
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