Model Answer
0 min readIntroduction
The development of a multicellular organism from a single fertilized egg involves a complex series of coordinated events that establish the body plan, or axis. A key player in this process is the 'organiser', a region within the developing embryo that directs the organization of surrounding tissues. First discovered by Hans Spemann and Hilde Mangold in the early 20th century through their groundbreaking transplantation experiments on amphibian embryos, the organiser is now understood to be a signalling centre crucial for inducing the formation of the neural tube and establishing the dorsal-ventral axis. Understanding the organiser’s function is fundamental to comprehending vertebrate development.
What is an 'Organiser'?
The organiser is a signalling centre in the early embryo that orchestrates the development of surrounding tissues. It is not a pre-formed structure but rather a region that acquires its organising ability through inductive interactions. In amphibians, the organiser is initially located in the dorsal lip of the blastopore during gastrulation. It is composed of cells that release signalling molecules, influencing the fate of neighbouring cells.
Spemann and Mangold’s Experiment
The discovery of the organiser stemmed from the classic transplantation experiments conducted by Hans Spemann and Hilde Mangold in the 1920s using Triturus (newt) embryos. They transplanted the dorsal lip from a blastula to the ventral side of another blastula. The recipient embryo developed a second neural tube, demonstrating that the dorsal lip possessed the ability to induce the formation of new body axes. This region was termed the ‘organiser’. The experiment proved that the information for dorsal development was concentrated in the dorsal lip and could override the default ventral fate.
Molecular Components of the Organiser
The organiser’s activity is mediated by a complex interplay of signalling molecules. Key components include:
- Chordin: A secreted protein that binds to and inhibits Bone Morphogenetic Proteins (BMPs).
- Noggin: Another BMP inhibitor, working synergistically with Chordin.
- Follistatin: Binds to Activin, another member of the TGF-β superfamily, inhibiting its signalling.
- Wnt signalling pathway: Activated in the organiser, contributing to dorsal axis formation.
- Fibroblast Growth Factors (FGFs): Play a role in maintaining organiser activity and inducing neural development.
Role of the Organiser in Axis Formation in Amphibians
Gastrulation
During gastrulation, the organiser induces the formation of the neural tube by inhibiting BMP signalling. BMPs, if unopposed, would promote epidermal fate. By secreting Chordin, Noggin, and Follistatin, the organiser creates a region of low BMP activity, allowing the ectoderm above it to differentiate into neural tissue. The organiser also influences the movements of cells during gastrulation, ensuring proper convergence and extension of the body axis.
Neurulation
Following gastrulation, the organiser continues to play a crucial role in neurulation, the process of neural tube formation. It expands and refines the neural plate, inducing the formation of the neural folds and ultimately the neural tube. The organiser also contributes to the development of the somites, the segmented blocks of mesoderm that form the vertebrae and muscles.
Signalling Pathways Involved
The organiser’s function relies heavily on several key signalling pathways:
- BMP signalling pathway: Inhibition of this pathway is central to neural induction.
- Wnt signalling pathway: Activates genes involved in dorsal axis formation and posterior development.
- FGF signalling pathway: Maintains organiser activity and promotes neural development.
- TGF-β signalling pathway: Regulated by Follistatin, influencing mesoderm development.
Organisers in other Vertebrates
While initially discovered in amphibians, organiser-like structures have been identified in other vertebrates, including birds and mammals. In birds, the organiser is located in the Hensen’s node, and in mammals, it is found in the node of the primitive streak. Although the specific molecular components may vary slightly, the fundamental principle of organiser function – inducing neural development and establishing the body axis – remains conserved.
Conclusion
The organiser represents a fundamental principle in developmental biology, demonstrating how signalling centres can orchestrate the complex processes of axis formation and tissue differentiation. Spemann and Mangold’s experiment laid the foundation for our understanding of inductive interactions and the molecular mechanisms governing embryonic development. Continued research into the organiser’s function promises to provide further insights into birth defects and regenerative medicine, potentially leading to new therapies for spinal cord injuries and other developmental disorders.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.