The chorion is an external membrane that surrounds the developing embryo and is one of the four extraembryonic membranes that arise during gestation. This specialized layer forms the outer boundary of the amniotic sac, providing the initial interface between the embryo and the maternal uterine tissue. The chorion is important in the development of all vertebrates, particularly in placental mammals where it originates from the outer layer of the blastocyst, known as the trophoblast. It facilitates the growth and survival of the developing offspring.
Development and Structure of the Chorion
The formation of the chorion begins with the differentiation of the blastocyst’s outer layer, the trophoblast, which is responsible for implanting the embryo into the uterine wall. This layer quickly gives rise to two distinct cell populations: the inner cytotrophoblast cells and the outer, fused, multi-nucleated syncytiotrophoblast layer. The syncytiotrophoblast directly contacts and invades the maternal uterine lining, known as the decidua.
This invading trophoblast tissue rapidly forms numerous microscopic, finger-like projections called chorionic villi, which significantly increase the surface area for exchange. The initial projections, or primary villi, appear around 13 to 15 days of development and consist only of a core of cytotrophoblast cells covered by the syncytiotrophoblast. Within a few days, the extraembryonic mesoderm penetrates the core of these projections, transforming them into secondary villi, which are present by around day 21.
The tertiary villi, the fully functional structures, form when blood vessels from the embryonic circulation grow into the mesodermal core, typically around the fifth or sixth week of gestation. These villi contain the fetal capillaries and are bathed by maternal blood, creating the specialized exchange unit of the placenta. The villi near the decidua proliferate to form the bushy chorion, which contributes to the placenta, while those opposite the implantation site regress, forming the smooth chorion. Eventually, the chorion fuses with the amnion to create the chorioamniotic membrane, which encloses the fetus and the amniotic fluid.
The Chorion’s Role in Fetal Support
The fully developed chorion is the fetal component of the placenta, acting as the primary site for the transfer of gases, nutrients, and waste products between the maternal and fetal bloodstreams. This exchange occurs across the chorionic villi, which are suspended in the intervillous space, an area filled with maternal blood supplied by the uterine spiral arteries. Maternal blood directly bathes the syncytiotrophoblast layer, ensuring maximal contact for biological exchange.
Oxygen and carbon dioxide transfer occurs primarily through simple diffusion across the thin membrane separating the two circulations. Similarly, nutrients like glucose, the fetus’s main energy source, are transported across the trophoblast layer, often requiring specialized protein transporters. This entire exchange process involves three distinct steps: delivery via maternal blood, transfer across the chorionic tissue, and uptake into the fetal circulation.
Beyond facilitating material transfer, the syncytiotrophoblast layer of the chorion also functions as an endocrine factory, producing hormones essential for maintaining the pregnancy. The most widely recognized of these is Human Chorionic Gonadotropin, or HCG, which is the substance detected by home pregnancy tests. HCG is responsible for maintaining the corpus luteum, a structure in the ovary that secretes progesterone during the first trimester, thereby sustaining the uterine lining and preventing menstruation.
The chorion also serves a protective function as a barrier at the maternal-fetal interface, regulating the immune environment to prevent the mother’s immune system from rejecting the genetically foreign fetus. Trophoblast cells play a role in this immunological tolerance by modulating the activity of maternal immune cells at the implantation site. This helps to foster a localized immune response that accepts the fetal tissue.
Chorion in Medical Diagnosis and Health
The chorion’s accessible tissue makes it a target for a common prenatal diagnostic procedure known as Chorionic Villus Sampling, or CVS. This procedure is typically performed earlier in pregnancy, between 10 and 13 weeks of gestation, to test for chromosomal abnormalities and certain genetic conditions. A small sample of the chorionic villi is collected, as this tissue shares the same genetic makeup as the developing fetus.
The tissue sample is obtained either by inserting a needle through the mother’s abdomen (transabdominally) or a catheter through the cervix (transcervically) under ultrasound guidance. CVS provides definitive genetic results earlier than other common prenatal tests, allowing for timely decision-making regarding the pregnancy. It is often recommended for individuals who have had abnormal screening results, a history of genetic disorders, or are of advanced maternal age.
Abnormal development of chorionic tissue can lead to serious health issues, categorized as gestational trophoblastic diseases. The hydatidiform mole, or molar pregnancy, is a condition involving the abnormal, excessive proliferation of the trophoblast cells, often resulting in an absence of a viable fetus. These abnormal growths produce extremely high levels of HCG and are characterized by swollen, fluid-filled chorionic villi.
In rare cases, the abnormal trophoblast tissue can develop into choriocarcinoma, a form of gestational trophoblastic neoplasia. This tumor arises from the cytotrophoblast and syncytiotrophoblast cells and can metastasize rapidly to other organs. Because choriocarcinoma is a cancer of the chorionic tissue, it continues to produce HCG, and monitoring the HCG level is a primary method for diagnosis and tracking the effectiveness of treatment.