71. 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 cells 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 a 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?
    (1) A, C and D (2) A, B and E
    (3) B, C and D (4) C, D and E

Evaluation of Each Statement

• A: Primary neurulation involves surrounding cells directing neural plate cells to proliferate, invaginate, and separate from the ectoderm forming a hollow tube.

  • Correct: This describes the process well and is supported by embryology studies.

• B: Secondary neurulation does not arise from aggregation of mesenchyme cells into a solid cord.

  • Incorrect: Secondary neurulation specifically involves mesenchymal cell aggregation into a solid medullary cord that later cavitates.

• C: Sonic Hedgehog (Shh), expressed in the notochord, induces floor plate cells to form medial hinge point in the neural plate.

  • Correct: Shh signaling is critical for floor plate induction and MHP formation during neurulation.

• D: In mammals, secondary neurulation starts at the sacral vertebrae level.

  • Correct: Secondary neurulation typically begins around the sacral or lower lumbar vertebrae region.

• E: In mammals, primary neurulation forms brain regions, secondary neurulation forms CNS from neck to tail.

  • Correct: Primary neurulation generates the anterior CNS (brain and upper spinal cord), secondary neurulation forms the lower spinal cord.

Summary

Statements A, C, D, and E are correct, whereas statement B is incorrect. The aggregate of correct statements therefore fits option:

Final Answer:
(1) A, C and D