Primary Neurulation

• The neural plate cells proliferate, invaginate, and separate from the surface ectoderm, resulting in the formation of a hollow neural tube.

• This process generates the anterior neural tube, comprising the brain and much of the spinal cord up to the hindlimb level.

• Statement A is accurate: The surrounding cells guide the neural plate cells through these morphogenetic movements.

Secondary Neurulation

• In secondary neurulation, a solid cord forms by the aggregation of mesenchymal cells and later undergoes cavitation to create a hollow neural tube.

• This process shapes the caudal neural tube, primarily the lumbar, sacral, and tail regions of the spinal cord.

• Statements B, C, and D are correct within the context of vertebrate embryology and species differences:

  • Birds exhibit primary neurulation up to the hindlimb region (C).

  • In mammals, secondary neurulation typically begins near the sacral vertebrae (D).

Clinical Implications

• Anencephaly (Statement E) arises from failure to close the anterior neural tube, causing the forebrain to remain exposed to amniotic fluid, which leads to severe developmental defects. This underscores the importance of proper neural tube closure in brain development.

Summary

All statements (A, B, C, D, and E) correctly describe various aspects of neural tube formation and its clinical significance in vertebrates, addressing differences in mechanisms across species and regions of the neural tube.

Final Answer:
(1) A, B, C, D and E