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EMBRYOLOGY
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CHAPTER 5:Third Week of Development:Trilaminar Germ Disk.
Clinical Correlates
Teratogenesis Associated with Gastrulation
1. Sensitive Period During Gastrulation
The beginning of the third week of development is a highly sensitive stage for teratogenic damage.
Important event occurring at this time:
Gastrulation begins
Why This Stage Is Important
During gastrulation:
Fate maps for different organ systems are established.
Examples of early developing structures:
Eyes
Brain anlage (early brain primordium)
If teratogens damage these early cell populations, serious congenital defects may occur.
2. Example of Teratogenic Effect — Alcohol
Studies in animals show that:
High doses of alcohol during early gastrulation FOR MORE
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The third week of embryonic development (days 15–21) is marked by gastrulation, a critical process that transforms the bilaminar germ disc into a trilaminar germ disc. This results in the formation of three primary germ layers: ectoderm, mesoderm, and endoderm.
Gastrulation begins with the formation of the primitive streak on the surface of the epiblast. Cells migrate inward through this streak in a process known as invagination. The first migrating cells displace the hypoblast to form the endoderm, while subsequent cells form the mesoderm. The remaining epiblast cells differentiate into ectoderm.
Each germ layer gives rise to specific tissues and organs. The ectoderm forms structures such as the skin and nervous system, the mesoderm develops into muscles, bones, and the cardiovascular system, and the endoderm forms the lining of the gastrointestinal and respiratory tracts.
The notochord also develops during this stage and plays a key role in axis formation and neural induction. It is essential for the proper development of the vertebral column.
Clinically, abnormalities in gastrulation can lead to serious congenital defects such as sacrococcygeal teratoma, which arises from remnants of the primitive streak.
These notes provide a structured overview of key processes, germ layer derivatives, and clinical correlations, making them ideal for quick revision and exam success.
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