What is the Difference Between Syncytiotrophoblast and Cytotrophoblast

The main difference between syncytiotrophoblast and cytotrophoblast is that syncytiotrophoblast contains a specialized layer of epithelial cells, whereas cytotrophoblast contains undifferentiated cells.

Syncytiotrophoblast and cytotrophoblast are two cell populations that occur in the villous trophoblast. Trophoblast contains the first cells differentiated from the fertilized egg, forming a large part of the placenta.

Key Areas Covered

1. What is Syncytiotrophoblast
– Definition, Structure, Function
2. What is Cytotrophoblast
– Definition, Structure, Function
3. Similarities Between Syncytiotrophoblast and Cytotrophoblast
– Outline of Common Features
4. Difference Between Syncytiotrophoblast and Cytotrophoblast
– Comparison of Key Differences

Key Terms 

Cytotrophoblast, Syncytiotrophoblast

What is Syncytiotrophoblast

Syncytiotrophoblast is a continuous layer of specialized cells to epithelial cells. It has direct contact with the maternal blood, covering the entire surface of the villous tree. Syncytiotrophoblast’s surface area at 28 weeks gestation is about 5 square meters. It can develop up to 11–12 square meters. Terminal villi develop into functional units of maternal-fetal oxygen exchange of oxygen and nutrients. Also, syncytiotrophoblast is responsible for maternal-fetal communication through biomolecular interactions. Additionally, syncytiotrophoblast is an important endocrine organ, producing hormones and growth factors. They regulate the placental and fetal growth and development.

Figure 1: Primary Chorionic Villi

Furthermore, hCG, estrogen and progesterone, hPL, human placental growth hormone, insulin-like growth factors, and endothelial growth factors are the types of hormones and growth factors produced by the syncytiotrophoblast.  

What is Cytotrophoblast

Cytotrophoblast is the layer of cells that occur beneath the syncytiotrophoblast. The cells of the cytotrophoblast are undifferentiated stem cells for syncytiotrophoblast. During villous formation and development, the cytotrophoblast cells continuously differentiate into syncytiotrophoblast. Also, uterine spiral arteries occur in the cytotrophoblast. They invade the cytotrophoblast in a process called placental vascular remodeling in the early implantation stage.

Figure 2: Secondary Chorionic Villi

Moreover, spiral arteries undergo a loss of elasticity and an increase in luminal diameter. Consequently, they become low-resistant vascular channels.  

Similarities Between Syncytiotrophoblast and Cytotrophoblast

• Syncytiotrophoblast and cytotrophoblast are two types of cells that differentiate from trophoblast.
• They occur in the first stage of pregnancy.
• They develop into a large part of the placenta that provides nutrients to the embryo.

Difference Between Syncytiotrophoblast and Cytotrophoblast

Definition

Syncytiotrophoblast refers to the placental barrier between maternal and fetal blood that allows the exchange of nutrients and gases and also represents the endocrine tissue of the human placenta. In contrast, cytotrophoblast refers to the inner cellular layer of the trophoblast of an embryonic placental mammal that gives rise to the plasmodial syncytiotrophoblast covering the placental villi. 

Type of Cells

Syncytiotrophoblast contains specialized epithelial cells, while cytotrophoblast contains undifferentiated stem cells. 

Localization

Syncytiotrophoblast faces maternal blood, occurring between cytotrophoblast and maternal blood, while cytotrophoblast occurs at the fetal interface between the basement membrane and the syncytiotrophoblast layer.

Nucleation

Syncytiotrophoblast contains multinuclear cells, while cytotrophoblast contains mononuclear cells.

Polarity

Syncytiotrophoblast contains fully polarized cells, while the polarity of the cytotrophoblast cells depends on the monolayer confluence and trimester.

Monolayer

Syncytiotrophoblast is a monolayer, while cytotrophoblast is a monolayer during early gestation and becomes discontinuous in the second trimester. 

Function

Syncytiotrophoblast uptakes and transfers nutrients, while cytotrophoblast contains maternal spiral arteries allowing the blood to flow through the placenta.

Conclusion

In brief, syncytiotrophoblast and cytotrophoblast are two cell types in the placenta. Syncytiotrophoblast contains a specialized layer of epithelial cells. It faces maternal blood. It contains fully polarized cells. Also, it is a monolayer of cells containing multinucleated cells. The primary function of the syncytiotrophoblast is to uptake and transfer nutrients. In comparison, cytotrophoblast contains undifferentiated cells that are mononuclear. The polarity of the cells depends on the trimester. Cytotrophoblast is also a monolayer during gestation and becomes discontinuous in the second trimester. The primary function of the cytotrophoblast is to contain maternal spiral arteries allowing the blood to flow to the placenta. Therefore, the structure and function are the main differences between syncytiotrophoblast and cytotrophoblast. 

References:

1. Wang Y, Zhao S. Vascular Biology of the Placenta. San Rafael (CA): Morgan & Claypool Life Sciences; 2010. Chapter 4, Cell Types of the Placenta.

Image Courtesy:

1. “Gray36” By Henry Gray – Own work (Public Domain) via Commons Wikimedia
2. “Gray37” By Henry Gray. – Own Work (Public Domain) via Commons Wikimedia