Human Anatomy
Third to Eighth Week of Development
Third to eighth week of development for MBBS: gastrulation, notochord, neurulation, somites and organogenesis, mapped to NMC codes AN79.1 to AN79.6.
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Third to Eighth Week of Development
Third to eighth week of development for MBBS: gastrulation, notochord, neurulation, somites and organogenesis, mapped to NMC codes AN79.1 to AN79.6.
This chapter covers the third to eighth weeks, the period of gastrulation, notochord formation, neurulation and organogenesis. It links these events to the neural tube defects and other malformations that arise from disrupted development.
High-yield: Third to Eighth Week of Development
- The third week is marked by gastrulation, which converts the bilaminar disc into a trilaminar disc of three germ layers.
- Gastrulation begins with the appearance of the primitive streak in the caudal epiblast.
- Epiblast cells migrate through the primitive streak to form the ectoderm, mesoderm and endoderm.
- The primitive node at the cranial end of the streak gives rise to the notochordal process.
- The notochord forms the midline axis, induces the neural plate and later becomes the nucleus pulposus.
- Neurulation is the folding of the neural plate into the neural tube, the precursor of the central nervous system.
- The neural tube closes first in the cervical region and then zips towards the cranial and caudal neuropores.
- The cranial neuropore closes around day 25 and the caudal neuropore around day 27.
- Failure of neuropore closure causes neural tube defects such as anencephaly and spina bifida.
- The paraxial mesoderm segments into somites, which form the vertebrae, skeletal muscle and dermis.
- The intra-embryonic coelom appears in the lateral plate mesoderm and forms the body cavities.
- Remnants of the primitive streak may give rise to a sacrococcygeal teratoma.
- Folding of the embryo in the fourth week creates the cylindrical body form and encloses the gut.
- By the end of the eighth week all major organ systems are established and the embryo is recognisably human.
- The third to eighth week is the critical period when teratogens are most likely to cause major malformations.
Key events from the third to eighth week
- **Primitive streak:** Appears in the caudal epiblast; gastrulation forms the three germ layers.
- **Notochord:** Midline axis, induces the neural plate, becomes the nucleus pulposus.
- **Neurulation:** Neural plate folds into the neural tube; neuropores close by day 27. Failure gives neural tube defects.
- **Somites:** Segments of paraxial mesoderm that form vertebrae, skeletal muscle and dermis.
NMC competencies in this chapter
- **AN79.1:** Formation and fate of the primitive streak
- **AN79.2:** Formation and fate of the notochord
- **AN79.3:** Process of neurulation
- **AN79.4:** Development of somites and the intra-embryonic coelom
- **AN79.5:** Embryological basis of congenital malformations, nucleus pulposus, sacrococcygeal teratoma and neural tube defects
- **AN79.6:** Diagnosis of pregnancy in the first trimester and the role of teratogens and alpha-fetoprotein
Frequently Asked Questions
What is gastrulation?
It is the process in the third week by which epiblast cells migrate through the primitive streak to form the three germ layers, converting the bilaminar disc into a trilaminar disc.
What does the notochord become?
The notochord forms the midline axis of the embryo and induces formation of the neural plate. In the adult it persists as the nucleus pulposus of the intervertebral discs.
How do neural tube defects arise?
They result from failure of the neuropores to close during neurulation. Failure of the cranial neuropore causes anencephaly and failure of the caudal neuropore causes spina bifida.
What do somites form?
Somites are segmented blocks of paraxial mesoderm that give rise to the vertebrae and ribs, the skeletal muscle of the trunk and limbs, and the dermis of the skin.
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