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Neural Tube Formation and Development Overview

Overview of Neural Tube Formation

  • Neural tube formation is a critical process in embryonic development, essential for the formation of the brain and spinal cord.

Figure 15.3: Neural Tube Formation in the Chick Embryo

A. Stages of Neural Tube Formation
  • Early Neural Plate: A flat sheet of cells, similar to a piece of paper—this is the neural plate and serves as the foundation for the brain and spinal cord.
  • Neural Plate Folding: The edges of the neural plate fold upwards to create a 'U' shape, forming a groove (neural groove).
  • Late Neural Tube: The edges of the 'U' shape come together and fuse, creating a hollow tube (the neural tube).
B. Transverse Section (Cross-Section View)
  • Neural Plate Stage: Appears flat with a slight indent in the center.
  • Folding Stage: The plate’s edges lift, forming a 'U' shape with the notochord beneath.
  • Neural Tube Stage: The sides meet and close at the top, resulting in the neural tube.
C. How It’s Formed?
  • The notochord emits signals to the overlying cells, triggering them to differentiate into neural plate cells, which subsequently fold into the tube.

Figure 15.4: Primary and Secondary Neurulation

Primary Neurulation (Upper part of CNS)
  1. Begins with a flat neural plate.
  2. Plate folds into a 'U' shape.
  3. Edges fuse to create a neural tube.
Secondary Neurulation (Lower part of CNS)
  1. A solid rod of cells (not a flat sheet).
  2. The rod hollows out to form a tube, occurring in the lower body where the lower spinal cord is formed.

Figure 15.6: Neural Tube Closure in Mice and Humans

A. Mouse Embryo
  • Closes at three main sites:
    1. Hindbrain.
    2. Forebrain/midbrain boundary.
    3. At the rostral neuropore (top of the head).
    • The tube zips up from these closure points.
B. Human Embryo
  • Similar closure process but includes an additional closure point at the lower spine due to longer spinal cord.
C & D. Cross-Sections
  • Mice: Neatly closed neural tube.
  • Humans: Variations in surrounding tissue structures due to unique anatomy.
E. Human Development Timeline
  • 22 Days: Start of neural tube closure.
  • 23 Days: Most of the tube is closed; initial shape of brain and spinal cord formed.
  • Mention of spina bifida as a result of incomplete closure.

Figure 15.7: Molecules Involved in Neural Tube Formation

  • BMP and Noggin: Key signals from surface ectoderm. BMP inhibits neural tissue formation, while Noggin blocks BMP to allow neural plate formation.
  • Shh (Sonic Hedgehog): Signals from notochord directing neural plate cells to become the floor plate of the neural tube.
  • MHP and DLHP: Hinge points aiding in the folding process to form the tube.
    • MHP: Middle hinge point.
    • DLHPs: Dorsal lateral hinge points.

Simplifying the Big Picture

  • Neural Tube Formation: Starts as a flat neural plate, folds into a tube via primary and secondary neurulation.
  • Importance of Proper Closure: Failure can lead to conditions like spina bifida or anencephaly.
  • Chemical Signaling: BMP, Noggin, and Sonic Hedgehog play crucial roles during this development phase.

Next Steps

  • Continued focus on neural tube defect causes, including environmental factors like lack of folic acid.
  • Detailed exploration of further neural tube formation and neural tissue organization.