Mechanisms of Neural Tube Formation

Learning Objectives

• Name the different ways neural tube forms
• Explain primary, secondary, and junctional neurulation processes
• Describe neural tube closure

Primary Neurulation

Primary neurulation occurs in four steps:

1. Elongation and Folding of the Neural Plate

• Neural plate cells above the nodal core start folding.
• Presumptive epidermis is involved.
• Elongation and folding visible in illustrations.

2. Bending of the Neural Plate

• Medial hinge point (MHP) formation occurs.
• Interaction between ectoderm cells and notochord.
• Formation of neural groove at dorsal midline.

3. Convergence of Neural Folds

• Formation of dorsolateral hinge points (DLHPs).
• Neural folds and surrounding epidermis converge.

4. Closure of the Neural Tube

• Paired neural folds form tube-like structure under epidermis.
• Migration of cells to form neural crest cells.

Hinge Points Formation

• Requires change in cell shape (apical constriction).
• Apical part narrows, basal part thickens.
• Sonic Hedgehog (Shh) and BMP signaling influences hinge point formation.
  • Shh induces formation of floor plate.
  • BMP signaling must be intermediate for proper MHP formation.
• Noggin (BMP inhibitor) is crucial for DLHP formation.

Neural Tube Closure

• Occurs non-simultaneously along anteroposterior axis.
• "Zipping up" mechanism used for closure.
• Multiple genetic factors and signaling pathways involved (e.g., cadherins).
• Disorders from defects include spina bifida, craniorhachischisis, and anencephaly.

Secondary Neurulation

• Occurs in posterior region.
• Formation of mesenchyme and medullary cord.
• Cavitation leads to formation of central neural tube cavity.

Comparison of Primary and Secondary Neurulation

• Primary involves bending and convergence.
• Secondary involves cell rearrangement under the epidermal layer.
• Primary occurs in anterior; secondary in posterior.

Junctional Neurulation

• Occurs between primary and secondary neurulation regions.
• Combines aspects of both primary and secondary neurulation.
• Mechanistic details still under study.

Summary

• Discussed mechanisms of neurulation: primary, secondary, and junctional.
• Focused on processes involved in neural tube formation and closure.