Definition and Role of the Chorion in Embryonic Development


In the process of embryonic development, the chorion plays a vital role in supporting and protecting the developing embryo. It is an essential structure found in many vertebrates, including mammals, birds, and reptiles. In this article, we will explore the definition and role of the chorion, shedding light on its significance in the early stages of embryonic development.

Definition of the Chorion

The chorion is a membrane that surrounds and encloses the embryo in the early stages of development. It is derived from the outermost layer of cells of the embryo, known as the trophoblast. The chorion is present in amniotes, which are a group of animals that possess an amniotic egg or have a similar reproductive strategy.

Structure of the Chorion

The structure of the chorion varies among different species, but it generally consists of several layers that provide protection and support to the developing embryo. The main components of the chorion include:

  • 1. Trophoblast: The trophoblast is the outermost layer of cells of the embryo. It gives rise to the chorion and plays a crucial role in implantation, the process by which the embryo attaches to the uterine wall. The trophoblast is responsible for the formation of structures such as the placenta, which facilitates nutrient and gas exchange between the mother and the developing embryo.
  • 2. Extraembryonic Membranes: The chorion is part of a group of extraembryonic membranes that surround the embryo. These membranes include the amnion, yolk sac, and allantois. The chorion is the outermost membrane and is in direct contact with the uterine wall or the eggshell, depending on the species.
  • 3. Chorionic Villi: Chorionic villi are finger-like projections that extend from the chorion into the uterine wall. They increase the surface area available for nutrient and gas exchange between the mother and the developing embryo. Chorionic villi contain blood vessels that facilitate the transport of oxygen, nutrients, and waste products.
  • 4. Chorionic Membrane: The chorionic membrane is a thin, transparent layer that encloses the embryo. It provides a protective barrier against physical damage and pathogens while allowing for the exchange of gases and nutrients.

Role of the Chorion in Embryonic Development

The chorion plays several important roles in embryonic development, ensuring the survival and proper growth of the developing embryo. Some of its key functions include:

  • 1. Protection: The chorion acts as a protective barrier, shielding the developing embryo from physical harm and preventing the entry of harmful microorganisms. It helps maintain a stable and safe environment for the embryo to grow.
  • 2. Gas Exchange: The chorion facilitates the exchange of gases, such as oxygen and carbon dioxide, between the embryo and the external environment. Oxygen is essential for cellular respiration, while carbon dioxide is a waste product that needs to be eliminated.
  • 3. Nutrient Transport: Through the chorionic villi, the chorion enables the transport of nutrients from the mother to the developing embryo. It facilitates the exchange of essential substances, including glucose, amino acids, vitamins, and minerals, which are necessary for the embryo’s growth and development.
  • 4. Waste Removal: The chorion also plays a role in removing waste products generated by the developing embryo. Metabolic waste, such as urea and carbon dioxide, is transported across the chorionic membrane and eliminated from the embryo’s circulation.
  • 5. Hormone Production: In some species, the chorion is involved in hormone production. It secretes hormones that regulate various aspects of pregnancy, such as maintaining the uterine lining and supporting the growth of the placenta.


The chorion is a crucial membrane that surrounds and protects the developing embryo during the early stages of embryonic development. It is derived from the trophoblast and forms part of the extraembryonic membranes. The chorion plays a vital role in providing a safe and supportive environment for the embryo, facilitating gas exchange, nutrient transport, waste removal, and hormone production. Understanding the structure and function of the chorion is essential for comprehending the intricate process of embryonic development in different species.


  • 1. Is the chorion present in all vertebrates?

No, the chorion is primarily found in amniotes, which include mammals, birds, and reptiles. Amniotes possess an amniotic egg or have a similar reproductive strategy, which requires the presence of the chorion.

  • 2. Does the chorion have any role after the embryonic stage?

In mammals, the chorion plays a significant role in the formation of the placenta, whichfacilitates nutrient and gas exchange between the mother and the developing fetus throughout pregnancy.

  • 3. How does the chorion attach to the uterine wall?

The trophoblast, which gives rise to the chorion, undergoes a process called implantation. During implantation, the trophoblast cells invade the uterine lining and establish a connection with the maternal blood vessels. This allows for the exchange of nutrients and waste products between the mother and the developing embryo.

  • 4. Can abnormalities in the chorion affect embryonic development?

Yes, abnormalities in the chorion can have significant effects on embryonic development. For example, if the chorionic villi do not properly form or function, it can lead to nutrient deficiencies and impaired growth of the embryo. Additionally, abnormalities in the chorion can contribute to complications such as placental insufficiency or preeclampsia in pregnancy.

  • 5. Are there any medical procedures that involve the chorion?

Yes, chorionic villus sampling (CVS) is a medical procedure that involves the sampling of cells from the chorion for genetic testing. It is typically performed during the first trimester of pregnancy to diagnose genetic disorders or chromosomal abnormalities in the developing fetus.


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  • 2. Sadler, T. W. (2019). *Langman’s Medical Embryology* (14th ed.). Wolters Kluwer.
  • 3. Gilbert, S. F. (2000). *Developmental Biology* (6th ed.). Sinauer Associates.
  • 4. Cunningham, F. G., Leveno, K. J., Bloom, S. L., Spong, C. Y., Dashe, J. S., Hoffman, B. L., Casey, B. M., Sheffield, J. S., & McIntire, D. D. (2018). *Williams Obstetrics* (25th ed.). McGraw-Hill Education.
  • 5. Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2019). *The Developing Human: Clinically Oriented Embryology* (11th ed.). Elsevier.


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