8.3: Embryonic Stage

Learning Objectives

1. Define the embryonic period.
2. Identify the stages of embryonic development that occur during prenatal development.
3. Explain the function of the placenta and its connection to the embryo via the umbilical cord.
4. Differentiate between cephalocaudal and proximodistal patterns of growth.
5. Identify the three germ layers.
6. Recognize why the embryonic period is the most vulnerable stage for teratogenic effects.

The Embryonic Period

The embryonic period begins approximately two weeks after conception and lasts until approximately 8 weeks. By the beginning of the third week, the blastocyst has implanted in the uterine wall and is now referred to as an embryo, which is supported by two key membranes: the chorion and the amnion. The chorion, formed from the outermost cells of the fertilized egg, facilitates the exchange of nutrients and gases between the mother and developing embryo. The amnion, or innermost membrane, creates the amniotic sac, which fills with fluid to cushion the embryo and provide a stable environment for growth. As development progresses, the chorion and amnion integrate into the placenta.

The placenta is a temporary organ that forms during pregnancy and performs essential functions, such as transferring oxygen and nutrients from the mother to the developing organism, and removing waste products like carbon dioxide from the organism's bloodstream and fetal waste later in pregnancy. It is attached to the wall of the uterus and connected to the organism through the umbilical cord.

Growth

During the embryonic period, cells continue to differentiate and develop into specialized tissues. Growth during prenatal development occurs in two major directions: from head to tail, called cephalocaudal development, and from the midline outward, referred to as proximodistal development. This means that those structures nearest the head develop before those nearest the feet, and those structures nearest the torso develop before those away from the center of the body (such as hands and fingers). You will see that this pattern continues after birth.

During the embryonic stage, all the major organs and body structures are laid down through a process called organogenesis. What was the innermost mass of cells of the trophoblast, differentiated into three layers:

1. Ectoderm (outermost layer)- becomes the skin, sense organs, brain, and spinal cord
2. Endoderm (innermost layer)- becomes the respiratory system, the digestive system, the liver, and the pancreas
3. Mesoderm (middle layer)- becomes muscles, bones, blood, heart, kidneys, and gonads

Figure \(\PageIndex{1}\): Germ layers of the embryo. Image by Netha Hussain is licensed CC-BY 4.0

The head develops in the fourth week, and the precursor to the heart begins to pulse (Tan & Lewandowski, 2020). In the early stages of the embryonic period, gills and a tail are apparent. However, by the end of this stage, they disappear, and the organism assumes a more human-like appearance. The embryo is approximately 1" in length and weighs about 4 grams at the end of eight weeks. The embryo can move and respond to touch at this time. ²⁴

Figure \(\PageIndex{2}\): A human embryo. Image by Anatomist90 is licensed under CC BY-SA 3.0

About 20% of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities, often before the mother even knows that she is pregnant. The embryonic period is when the developing organism is most vulnerable to the greatest amount of damage if exposed to harmful substances, due to the foundation of all vital systems during organogenesis.

References, Contributors and Attributions

Tan, C.M.J. & Lewandowski, A.J. (2020). The transitional heart: From early embryonic and fetal development to neonatal life. Fetal Diagnosis and Therapy, 47, 373-386. https://doi.org/10.1159/0000501906

24. Lifespan Development: A Psychological Perspective (page 43) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0