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.