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Chorion

*Chorion*

For medical students2 min readUpdated 2026-10-10

The chorion is the villous extraembryonic membrane that connects the embryo to the maternal organism. It is a unique structure serving as the primary barrier and exchange site, giving rise to the placenta.

Source of developmentTrophoblast (unlike other organs)
External tissueExtraembryonic ectoderm
Internal tissueExtraembryonic mesoderm
RoleIndirect contact between maternal and fetal blood

Origin and Initial Stages of Formation

In embryology, the origin of structures is key to understanding their functions. The chorion is unique because it is the only extraembryonic organ that originates exclusively from the trophoblast. By comparison, the embryo proper and all other extraembryonic membranes develop from the embryoblast.

The formation of the chorion begins with modifications of the single-layered trophoblast wall. Extraembryonic mesoderm grows inward toward this wall from the inner side. Simultaneously, specific outgrowths called villi form on the outer surface of the membrane. The appearance of these structures gives the chorion its alternative name — the villous membrane.

Tissue Composition (Histology of the Wall)

The histological structure of the chorion wall includes two main layers, which are essential to remember for exams:

  1. Externally (superficial layer): Extraembryonic ectoderm, which is a direct derivative of the trophoblast.
  2. Internally (deep layer): Extraembryonic mesoderm, which grows toward the trophoblast during organogenesis.

Comparative note: Comparing the histological structure of the chorion wall with the amnion wall reveals that the arrangement of these tissues is completely inverted.

Placentation and Chorionic Differentiation

Following implantation, the chorionic villi develop unevenly. This leads to functional and morphological differentiation of the membrane into two distinct regions:

Functional Morphology

Through the deep invasion of villi into maternal tissues, the chorion functions as the primary life-support system. The main functions of the villous membrane (as part of the placenta) include:

Mnemonic

To avoid confusing the layers, use the "mirror rule": the chorion has ectoderm on the outside and mesoderm on the inside. The amnion has the exact opposite arrangement!

Frequently asked questions

What stages of development do chorionic villi undergo?

Chorionic villi develop in three sequential stages characterized by increasing structural complexity:

  • Primary villi (Trophoblastic): Formed during the 1st–2nd weeks. Consist exclusively of a two-layered epithelium (cytotrophoblast and syncytiotrophoblast).
  • Secondary villi (Mesenchymal): Formed at the beginning of the 3rd week by the ingrowth of extraembryonic mesenchyme into the epithelial villi, establishing a connective tissue core.
  • Tertiary villi (Vascular): Developed at the end of the 3rd week via vasculogenesis in situ, when blood capillaries form from mesenchymal cells within the villi.
What cellular layers does the trophoblast of chorionic villi differentiate into?

The trophoblast of chorionic villi differentiates into two epithelial layers:

  • Cytotrophoblast (inner layer): A cellular layer retaining mitotic activity, resting on the basement membrane.
  • Syncytiotrophoblast (outer superficial layer): A multinucleated structure formed by the fusion of cytotrophoblast cells, lacking distinct cell boundaries.
What are the components of the placental (hemochorial) barrier?

In the hemochorial placenta, the placental barrier is formed exclusively by fetal tissues. Maternal tissues are completely excluded because chorionic villi erode the walls of endometrial blood vessels, allowing maternal blood to pool in lacunae and bathe the villi directly.

What is the difference between the smooth chorion and the villous chorion?

The villous chorion (chorion frondosum) features highly branched villi that invade the uterus to form the placenta. The smooth chorion (chorion leve) lacks villi and serves as the middle fetal membrane.

Does maternal and fetal blood mix normally?

No. The chorion forms the placenta, ensuring strict indirect contact between maternal and fetal blood for safe metabolic exchange.

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