Transforming the neural plate into a neural tube is an important event towards the formation of central nervous system, in which the following sub-events might take place:

A. In primary neurulation, the cell surrounding the neural plate direct the neural plate cells to proliferate, invaginate and separate from the surface ectoderm to form a hollow tube.

B. In secondary neurulation the neural tube does not arise from the aggregation of mesenchyme cells into the solid cord.

C. The morphogen, Sonic hedgehog, that is expressed in notochord, is required for induction of floor plate cells in the neural plate to form the medial hinge point.

D. In mammals, secondary neurulation begins at the level of sacral vertebrae.

E. In mammals, the primary neurulation forms brain regions while the secondary neurulation takes care of forming rest of the central nervous system from neck to tail.

Which one of the following options represents the combination of all correct statements?

Correct option is A

Neurulation is the process by which the neural plate folds into the neural tube, forming the central nervous system (CNS). It consists of primary and secondary neurulation, each with distinct mechanisms.

Analysis of Statements:

1. Statement A: Correct 

   • Primary neurulation involves the invagination and folding of the neural plate to form a hollow tube.
   • The cells surrounding the neural plate help direct this process, influencing cell proliferation and invagination.
   • This description correctly outlines the process, making it correct.

2. Statement B: Incorrect 

   • Secondary neurulation involves the aggregation of mesenchymal cells into a solid cord, which later hollows out (cavitates) to form the neural tube.
   • Since this statement says the neural tube does not arise from mesenchymal cells, it is incorrect.

3. Statement C: Correct 

   • Sonic Hedgehog (Shh), secreted by the notochord, is crucial for neural tube patterning.
   • It induces floor plate formation in the neural tube, which later contributes to ventral spinal cord development.
   • This statement correctly describes the role of Shh in neural tube induction, making it correct.

4. Statement D: Correct 

   • In mammals, secondary neurulation begins at the sacral vertebral level, forming the lower spinal cord.
   • This occurs as mesenchymal cells condense into a medullary cord, which later cavitates into a hollow tube.
   • Since secondary neurulation indeed starts at the sacral level, this statement is correct.

5. Statement E: Incorrect 

   • While primary neurulation forms the brain and upper spinal cord, secondary neurulation forms the lower spinal cord, but it is not strictly limited to the neck-to-tail region.
   • Neck structures still form via primary neurulation, making this partially incorrect.

Final Selection:

Since A, C, and D are correct, the correct answer is:
Option 1 (A, C, and D).