46. The group of cells of amphibian blastula capable of inducing the organizer is called as
(1) Hensen’s node                                                      (2) Nieuwkoop centre
(3) Dorsal blastopore lip                                  (4) Hypoblast

 

The group of cells in the amphibian blastula capable of inducing the organizer is called the Nieuwkoop center. Located in the dorsal vegetal region of the blastula, the Nieuwkoop center secretes signaling molecules that induce the formation of the Spemann’s organizer in the overlying equatorial cells. This organizer plays a key role in directing the body axis formation and embryonic patterning during gastrulation.

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The Nieuwkoop center is a crucial group of cells located in the dorsal vegetal region of the amphibian blastula stage embryo, particularly studied in Xenopus laevis. These cells possess the remarkable ability to induce the formation of the Spemann organizer, the group of cells responsible for orchestrating the early body plan and dorsal-ventral axis of the developing embryo.

During early development, the Nieuwkoop center secretes important signaling molecules such as Nodal-related proteins, which initiate a cascade of gene expression events in the overlying equatorial cells. This induction triggers those cells to become the Spemann organizer, which itself acts as a signaling hub, secreting BMP inhibitors and other factors to dorsalize adjacent tissues.

The interaction between the Nieuwkoop center and the organizer exemplifies embryonic induction, a process where one group of cells directs the developmental fate of another. The Nieuwkoop center essentially serves as a primary signaling center that defines the dorsal side of the embryo and lays the foundation for axis formation and tissue differentiation.

Unlike structures such as Hensen’s node, which is found in avian embryos, the Nieuwkoop center is specific to amphibians and is distinct from the dorsal blastopore lip (the actual location of the Spemann organizer) and the hypoblast (which primarily arises in birds and mammals).

Understanding the Nieuwkoop center’s role provides insight into how early embryonic signals establish symmetry and initiate complex developmental processes in amphibians. It also highlights molecular mechanisms conserved across vertebrates involved in axis specification and organizer formation.

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Final Answer:
The group of cells in the amphibian blastula capable of inducing the organizer is called the (2) Nieuwkoop center. Located in the dorsal vegetal region, these cells signal to the overlying equatorial cells to become the Spemann organizer, thereby initiating dorsal axis formation and embryonic patterning.