What is the Difference Between Yolk Sac and Fetal Pole

The main difference between yolk sac and fetal pole is that the yolk sac is a small membranous structure occurring outside the embryo, reducing its size, and communicating ventrally with the developing embryo, whereas the fetal pole is an embryo developing into the fetus.

The yolk sac and fetal sac are two types of structures that occur in pregnancy. Generally, the yolk sac and fetal pole are responsible for embryonic development in humans.    

Key Areas Covered

1. What is Yolk Sac
     – Definition, Structure, Function
2. What is Fetal Pole
     – Definition, Structure, Function
3. Similarities – Yolk Sac and Fetal Pole
     – Outline of Common Features
4.  Difference Between Yolk Sac and Fetal Pole
     – Comparison of Key Differences

Key Terms                                                                                                                  

Fetal Pole, Yolk Sac

What is Yolk Sac

The yolk sac is a structure that develops inside the uterus during pregnancy, providing nutrients to the embryo while circulating gas between the mother and the fetus. It develops several important structures. In prenatal ultrasonography, healthcare providers examine the yolk sac’s appearance, location, and size. Generally, the yolk sac is a round or pear-shaped pouch made up of a thin, skin-like membrane. The fetal pole occurs next to the yolk sac. Moreover, the yolk sac occurs inside the gestational sac, which is a large cavity of fluid occurring inside the uterus. 

Figure 1: Yolk Sac

Furthermore, the main function of the yolk sac is to deliver nutrients to the embryo. Other important functions of the yolk sac include the circulation of gases between the mother and the embryo, the development of structures including the umbilical cord, reproductive system, and gastrointestinal system, and the providence of immune function and metabolism. In addition, the yolk sac is visible during the second week of pregnancy.    

What is Fetal Pole

The fetal pole is one of the first stages of pregnancy. It develops into the embryo. Therefore, early prenatal ultrasound can view and measure the fetal pole. It provides information on the gestational age, the location of the embryo, and possible complications. The fetal pole is also called the embryonic pole or the embryo since it develops into the fetus. Generally, the fetal pole’s size determines the embryo’s gestational age. Multiple fetal poles occur in multiple pregnancies. The fetal pole also shows where the pregnancy occurs. Unexpectedly small fetal poles provide data on miscarriage.

Figure 2: Fetal Pole

Moreover, the fetal pole occurs next to the yolk sac, which provides nutrients to the fetus. The fetal pole is round; one end is the head of the embryo, called the crown, while the other end is a tail-like structure called the rump. The length of the fetal pole indicates the duration of the pregnancy. Normally, within five and a half weeks of pregnancy, the fetal pole is visible.         

Similarities Between Yolk Sac and Fetal Pole

• The yolk sac and fetal pole are two structures that occur during pregnancy.
• Both develop different structures related to pregnancy.
• Both occur in early pregnancy.

Difference Between Yolk Sac and Fetal Pole

Definition

The yolk sac refers to a sac lacking yolk in the early embryo of a mammal, while the fetal pole refers to an embryo, one of the first stages of pregnancy.

Structure

The yolk sac is a membranous sac attached to the embryo formed by the cells of the hypoblast, while the fetal pole is a thickening on the margin of the yolk sac.    

Function

Moreover, the yolk sac provides nutrients to the embryo and circulates gases between mother and fetus while the fetal pole develops into the fetus.

Developing Structures

The yolk sac develops the umbilical cord, blood cells, and reproductive organs, while the fetal pole develops embryonic structures.

Conclusion                                                     

In brief, the yolk sac and fetal pole are two types of structures occurring in early pregnancy. The yolk sac is a membranous sac attached to the embryo. The main function of the yolk sac is to provide nutrition to the developing embryo. In addition, it allows gas exchange between the mother and the fetus. However, the yolk sac develops structures such as the umbilical cord, blood cells, and reproductive organs. Meanwhile, the fetal pole is a thickening structure on the margin of the yolk sac. The cells in the hypoblast develop into a fetal pole. And, the fetal pole develops into a fetus. Therefore, the main difference between the yolk sac and the fetal pole is their structure and function.        

References:                

1. Yolk sac in early pregnancy: Meaning & function. Cleveland Clinic. 
2. Fetal pole: Ultrasound, Anatomy & Function. Cleveland Clinic. 

Image Courtesy:

1. “Gray22.” By Henry Vandyke Carter – Own work (Public Domain) via Commons Wikimedia
2. “Ultrasound of embryo at 5 weeks, colored” By Mikael Häggström – Own Work (CC BY 2.5) via Commons Wikimedia