Origin and initial stages of formation
In embryology, the origin of structures is crucial for understanding their functions. The chorion is unique in that it is the only extraembryonic organ that originates exclusively from the trophoblast. By comparison, the embryo itself and all other extraembryonic membranes form from the embryoblast.
The formation of the chorion begins with changes in the single-layered trophoblast wall. The extraembryonic mesoderm starts growing inward toward this wall from the inner side. Simultaneously, specific outgrowths called villi begin to form on the outer surface of the membrane. The appearance of these structures gives the chorion its alternative name — the villous membrane.
Tissue composition (wall histology)
The histological structure of the chorionic wall includes two main layers that are important to memorize for successfully passing the exam:
- Externally (superficial layer): the extraembryonic ectoderm is located here. This tissue is a direct derivative of the trophoblast.
- Internally (deep layer): the extraembryonic mesoderm is located here, which grows toward the trophoblast during the organogenesis stage.
Interesting comparative fact: If you compare the histological structure of the chorionic wall with the amniotic wall, you will notice that the arrangement of these tissues is completely opposite (reversed).
Placentation and chorionic differentiation
After the embryo undergoes implantation, the chorionic villi begin to develop unevenly. This leads to the functional and morphological differentiation of the membrane into two parts:
- Chorion frondosum (villous chorion). On one side of the membrane, the villi begin to proliferate very actively and strongly. They deeply invade the uterine mucosa. This specific region participates in metabolism and leads to the formation of a new complex organ — the placenta. In the placenta, indirect contact is established between the blood of the mother and the developing fetus.
- Chorion laeve (smooth chorion). On the rest of the membrane, the villi regress, and it remains smooth. The significance of this part is that it forms the second (middle) fetal membrane.
Functional morphology
Due to the deep invasion of villi into maternal tissues, the chorion assumes the role of the primary life-support system. The main functions of the villous membrane (as part of the placenta) include:
- Uninterrupted supply of essential nutrients to the fetus.
- Providing the developing organism with oxygen.
- Timely excretion of metabolic waste products generated during active embryonic growth.
Briefly: answers to topical questions
What stages of development do chorionic villi go through?
The development of chorionic villi goes through three consecutive stages characterized by increasing structural complexity:
- Primary villi (Trophoblastic villi) — form during the first to second weeks. They consist exclusively of a bilayered epithelium (cytotrophoblast and syncytiotrophoblast).
- Secondary villi (Connective tissue villi) — form at the beginning of the third week via the ingrowth of extraembryonic mesenchyme inside the epithelial villi. A connective tissue stroma appears in them.
- Tertiary villi (Vascular villi) — arise at the end of the third week as a result of vasculogenesis in situ, when blood capillaries form from mesenchymal cells inside the villi.
Into which cell layers does the trophoblast of the chorionic villi differentiate?
The trophoblast of the chorionic villi differentiates into two epithelial layers:
- Cytotrophoblast (inner layer) — a cellular layer retaining mitotic activity. It is located on the basement membrane.
- Symplastotrophoblast or syncytiotrophoblast (outer superficial layer) — a multinucleated structure formed by the fusion of cytotrophoblast cells. It loses cell boundaries.
What is the composition of the placental (hemochorial) barrier?
In the hemochorial placenta, the blood-placental barrier is formed exclusively by fetal tissues. Maternal tissues are completely excluded from its composition because the chorionic villi destroy the walls of endometrial vessels, and maternal blood spills into the lacunae, bathing the villi.
To avoid confusing the layers, use the "reverse rule": the chorion has ectoderm on the outside and mesoderm on the inside. The amnion is precisely the opposite!
Class and exam questions
How does the smooth chorion differ from the villous (frondosum) chorion?
The chorion frondosum has highly branched villi that invade the uterus and form the placenta. The chorion laeve lacks villi and serves as the middle fetal membrane.
Does normal mixing of maternal and fetal blood occur?
No. The chorion forms the placenta, which ensures strict indirect contact between maternal and fetal blood for safe substance exchange.
What else to study on this topic
- Stages and mechanisms of trophoblast implantation
- Histological structure of villi at different stages of development
- Formation and structure of the placenta
- Structural features of the uterine mucosa during pregnancy
- Comparative characteristics of provisional organs
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