Neurulation and Notogenesis in Embryology

Study Notes

Neurulation is a process where the embryo develops structures that become the nervous system in vertebrates. This involves the transformation of the neural plate into the neural tube.
The embryo at this stage is called the neurula.
The neural plate folds inward to form the neural tube, which develops into the spinal cord and brain.
This eventually forms the central nervous system.

Notogenesis is the development of the notochord, a cellular rod, by epiblasts that form the floor of the amnion cavity.
Notochord development starts during gastrulation (3rd week).
Progenitor cells of the notochord migrate from the primitive node and pit.
The notochord is a cellular rod that forms the first longitudinal midline axis around which vertebral bodies are organized.
It is the basis for the axial skeleton.
It is visible throughout the embryo, and around it are layered concentrations of cells that represent the primordia of future vertebral bodies.

Neurulation occurs in four stages.
- The central portion of the embryonic ectoderm thickens to form a neural plate.
- The neural plate shapes and elongates.
- The neural plate bends around a medial groove, with lateral folds elevating.
- The neural tube closes.

Notochord induces the formation of the central nervous system (CNS).
The notochord signals the ectoderm germ layer above it, causing it to thicken and form the neural plate.
Cells of the neural plate are known as neuroectoderm.
The induction of the neural plate is the initial event in neurulation.

As the neural plate lengthens, its lateral edges elevate to form neural folds.
The depressed midregion forms the neural groove.
Neural folds approach each other in the midline and fuse, forming the neural tube.
The neural tube eventually differentiates into the spinal cord and brain, forming the central nervous system.
The cephalic and caudal ends of the neural tube communicate with the amniotic cavity through cranial and caudal neuropores, respectively.
Cranial neuropore closure occurs around day 25.
Posterior neuropore closure occurs around day 28.

During neural tube formation, cells at the lateral border of the neuroectoderm dissociate from their neighbors.
This cell population is called the neural crest.
Neural crest cells undergo an epithelial-to-mesenchymal transition (EMT).
They actively migrate and displace themselves into the underlying mesoderm.
Neural crest cells are important for contributing to many organs and tissues and are sometimes considered a fourth germ layer.
Neural crest cells give rise to diverse structures, including dorsal root ganglia, Schwann cells, autonomic nervous system components, meninges, sensory ganglia, bones of the face, teeth, lens of the eye, melanocytes, adrenal medulla, and many glands.