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?
A, C and D
Neural Tube Formation Explained
Neural tube formation, also known as neurulation, is a critical step in the development of the central nervous system. This process transforms the flat neural plate into a hollow tube, which will eventually give rise to the brain and spinal cord. Neurulation typically occurs through two main mechanisms: primary neurulation and secondary neurulation.
Primary Neurulation Process
Primary neurulation involves the folding and closure of the neural plate. The cells within the neural plate are influenced by surrounding tissues, such as the surface ectoderm and the underlying notochord. These influences cause the neural plate cells to:
- Proliferate (increase in number).
- Invaginate (bend inwards) along the midline, forming a neural groove.
- Elevate at the edges, forming neural folds.
- Fuse dorsally, creating a hollow neural tube.
- Separate from the surface ectoderm, which then covers the neural tube.
Statement A correctly describes these events, indicating that surrounding cells direct the neural plate cells to proliferate, invaginate, and separate from the surface ectoderm to form a hollow tube. Thus, Statement A is correct.
Secondary Neurulation Process
Secondary neurulation is a different mechanism that forms the caudal-most portion of the neural tube, typically in the tail region. This process does not involve folding of a neural plate. Instead, mesenchymal cells (often derived from the posterior-most ectoderm and mesoderm) aggregate to form a solid cord. This solid cord then undergoes cavitation, which is the formation of a lumen (a hollow space) within the cord, thus transforming the solid cord into a hollow neural tube.
Statement B claims that in secondary neurulation, the neural tube does not arise from the aggregation of mesenchyme cells into a solid cord. This contradicts the known mechanism of secondary neurulation. Therefore, Statement B is incorrect.
Role of Sonic Hedgehog in Neurulation
The notochord, a rod-like structure situated ventral to the developing neural tube, plays a crucial role in patterning the neural tube. The notochord secretes signaling molecules, including the morphogen Sonic hedgehog (Shh). Shh acts on the ventral part of the neural plate, inducing the differentiation of floor plate cells along the midline. The floor plate cells themselves also become a source of Shh, creating a ventral-to-dorsal signaling gradient. The floor plate, specifically the medial hinge point (MHP) within it, acts as a pivot point during the bending of the neural plate in primary neurulation.
Statement C asserts that the morphogen, Sonic hedgehog, expressed in the notochord, is required for the induction of floor plate cells in the neural plate to form the medial hinge point. This accurately reflects the function of Shh from the notochord in inducing the floor plate and contributing to the formation of the MHP. Thus, Statement C is correct.
Neurulation Levels in Mammals
In mammals, primary neurulation forms the neural tube in the cranial (head) and most of the spinal regions (extending down to the lumbar levels). Secondary neurulation then completes the formation of the neural tube in the caudal region, which corresponds to the sacral and caudal (tailbone) vertebrae.
Statement D states that in mammals, secondary neurulation begins at the level of sacral vertebrae. This is consistent with the regional differences in neurulation mechanisms in mammals. Therefore, Statement D is correct.
Statement E claims that primary neurulation forms brain regions while secondary neurulation forms the rest of the central nervous system from neck to tail. As explained, primary neurulation forms the brain *and* the majority of the spinal cord (down to lumbar levels), not just the brain. Secondary neurulation forms only the caudal-most part (sacral/caudal spinal cord). Thus, Statement E is incorrect.
Summary of Correct Statements
Based on the analysis:
- Statement A: Correct
- Statement B: Incorrect
- Statement C: Correct
- Statement D: Correct
- Statement E: Incorrect
The combination of all correct statements is A, C, and D.
Important Questions from Morphogenesis and organogenesis in animals
The individuals considered in this question are having two haploid sets of autosomes and no Y- chromosome. The X∶A ratio of the individuals, the type of organisms chosen, their primary sex and number of Barr bodies expected in their cells are shown in the table below:
X∶A ratio Organism Primary Sex Number of Barr bodies i. 0.5
ii. 2
A. Human
B. Drosophila
I. Male
II. Female
III. Metafemale
a. Zero
b. One
c. Two
d. Three
Select the option below with all correct matches:
Several marine organism release their gametes into the environment, where sperm attraction and subsequent events lead to successful fertilization. With reference to sea urchins, which one of the following statements is NOT true?
The following statements regarding the generation of dorsal/ventral axis in Drosophila was made:
A. Gurken protein moves along with the oocyte nucleus and signals follicle cells to adopt the ventral fate.
B. Maternal deficiencies of either the gurken or torpedo gene cause ventralization of the embryo.
C. Gurken is active only in the oocyte and Torpedo is active only in the somatic follicle cells.
D. The Pipe protein is made in the dorsal follicle cells.
E. The highest concentration of Dorsal is in the dorsal cell nuclei, which becomes the mesoderm.
Which one of the following combination of the above statements is true?
During wing development in chick, if Apical Ectodermal Ridge (AER) is removed, the limb development ceases, on the other hand placing leg mesenchyme directly beneath the wing AER, distal hindlimb structures develop at the end of the wing, and if limb mesenchyme is replaced by non-limb mesenchyme beneath AER, the AER regresses. This may demonstrate that:
A. the limb mesenchyme cells induce and sustain AER.
B. the mesenchyme cells specify the type: wing or limb.
C. the AER is responsible for specifying the type: wing or limb.
D. the AER is responsible for sustained outgrowth and development of the limb.
E. the AER does not specify the type: wing or limb.
Which combination of above statements is demonstrated by the experiment?
The following statements are made regarding the amphibian early-embryonic development:
A. The Nieuwkoop center cells are mesodermal in origin.
B. Chordin, Noggin and Goosecoid are secreted by the Organizer.
C. The default fate of the ectoderm is to become neural tissue.
D. BMP levels are high in the presumptive dorsal mesoderm.
Which one of the following options represents the combination of all correct statements?
