What is the Difference Between Chorion and Placenta

The main difference between chorion and placenta is that chorion is the outermost fetal membrane, covering the embryo of mammals, reptiles, and birds whereas placenta is the temporary organ that connects the developing fetus to the uterine wall through umbilical cord in mammals. Furthermore, chorion serves as a protective barrier during the development of the embryo while placenta supplies nutrients and oxygen to the developing embryo, removing its wastes.  

Chorion and placenta are two anatomical structures developed during the development of the embryo. Moreover, in placental mammals, chorion contributes to the formation of the placenta. 

Key Areas Covered 

1. What is Chorion
     – Definition, Structure, Importance
2. What is Placenta
     – Definition, Structure, Importance
3. What are the Similarities Between Chorion and Placenta
     – Outline of Common Features
4. What is the Difference Between Chorion and Placenta
     – Comparison of Key Differences

Key Terms 

Chorion, Embryo, Fetus, Placenta, Placental Mammals, Uterus

What is Chorion 

Chorion is the outer fetal membrane in placental mammals. Basically, chorion develops from an outer fold on the surface of the yolk sac, occurring outside of the zona pellucida. Moreover, it contains two layers: outer trophoblast and inner somatic mesoderm. The amnion is in contact with the latter. Along with the amnion, it forms the amniotic sac. Additionally, chorionic villi arise from the chorion. Furthermore, chorionic villi invade the uterine wall by destroying uterine decidua. Thus, it helps to absorb the nutrients required for the growth of the embryo. 

Figure 1: Chorion in the Amniotic Egg

Furthermore, in monotremes, reptiles, and birds, the chorion is one of the four fetal membranes of the amniotic egg. It occurs inside the egg white. Chorion also occurs in insects. However, in flowering plants, chorion refers to the outer membrane of seeds. 

What is Placenta 

The placenta is a temporary organ which connects the developing fetus to the uterine wall through the umbilical cord. The main function of the placenta is to facilitate the exchange of gases, nutrients, and wastes while involving in thermoregulation with mother’s blood supply. Additionally, it produces hormones that support pregnancy, including human chorionic gonadotropin, estrogen, progesterone, and human placental lactogen. Further, IgG antibodies can pass through the placenta to the fetus, and thus, triggering passive immunity. However, placenta serves as an immunological barrier, protecting the fetus from the normal immune response of the mother. 

Figure 2: Placenta

Moreover, placental mammals have a chorioallantoic placenta, developed from the chorion and allantois. Generally, trophoblast forms the outer layer of the placenta, containing two layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The middle layer is allantois, developing from the embryonic hindgut. Meanwhile, the inner layer is the amnion, which surrounds the fetus. However, marsupials and other non-mammals develop different types of placentas. In contrast, in flowering plants, placenta refers to the part of the ovary where ovules develop; in non-flowering plants where the spores develop. 

Similarities Between Chorion and Placenta 

• Chorion and placenta are two anatomical structures formed during the embryonic development. 
• Both surround the embryo.  
• Also, both are temporary structures. 
• Besides, they are good sources for hematopoietic stem cells for transplantation. 

Difference Between Chorion and Placenta 

Definition 

Chorion refers to the outermost membrane surrounding an embryo of a reptile, bird, or mammal while placenta refers to a temporary organ that connects the developing fetus via the umbilical cord to the uterine wall in placental mammals. Thus, this explains the main difference between chorion and placenta.

Occurrence 

Furthermore, chorion mainly occurs in mammals, reptiles, and birds, while placenta mainly occurs in placental mammals.  

Significance 

While chorion is one of the four fetal membranes, the two parts of the placenta are the maternal placenta and the fetal placenta. 

Development 

Another difference between chorion and placenta is that chorion is a part of the embryo, while the placenta is a combination of the fetus and the tissues of the mother.  

Function 

Moreover, chorion protects the embryo while placenta facilitates nutrient uptake, gas exchange, waste elimination, and thermo-regulation in the fetus.  

Conclusion 

Chorion is one of the four fetal membranes, serving as the outer membrane of the developing embryo. It surrounds the amnion. Generally, it occurs in all mammals, reptiles, and birds. Most importantly, it provides protection to the embryo. On the other hand, the placenta is the temporary organ, which connects the developing fetus to the uterine wall through the umbilical cord. Basically, it is a combination of both maternal and fetal structures in placental mammals. Furthermore, chorion contributes to the formation of the fetal part of the placenta. Further, the placenta is also responsible for the exchange of nutrients, wastes, and gases while involving in thermoregulation of the fetus. Therefore, the main difference between chorion and placenta is their structure and importance. 

References:

1. “Chorion.” Chorion – an Overview | ScienceDirect Topics, Available Here.
2. “PLACENTA.” UWYO, Available Here.

Image Courtesy:

1. “Chicken egg diagram” By KDS4444 – Own work (CC BY-SA 4.0) via Commons Wikimedia   
2. “2910 The Placenta-02” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia