65. The major structural characteristic of avian gastrulation is the primitive streak, which becomes the blastopore lips of amniotic embryos. Migration through the primitive streak is controlled by Fgf8. What would happen if the Fgf8 protein, which repels migrating cells away from the streak, is over expressed in the primitive streak?
(1) The yolk sac will be deformed.
(2) Wnt signalling will be activated and orientation of the primitive streak will change
(3) Cells of the streak will not form the paraxial mesoderm.
(4) Cells generate mesodermal portions of the embryo
Overexpression of Fgf8 protein, which repels migrating cells away from the primitive streak, would most likely cause the cells of the streak to fail to form the paraxial mesoderm. This is because Fgf8 is essential for promoting cell migration and mesoderm formation, and excessive levels that repel cells would inhibit their normal ingress and movement leading to defects in mesodermal structures including the paraxial mesoderm.
Therefore, the correct answer is:
(3) Cells of the streak will not form the paraxial mesoderm.
Introduction
Fibroblast growth factor 8 (Fgf8) plays a pivotal role during avian gastrulation by regulating the migration of cells through the primitive streak. This cell migration is crucial for mesoderm formation, which contributes to essential tissues such as muscle, bone, and connective tissue. Proper levels of Fgf8 ensure directional movement of mesodermal precursors. However, overexpression of Fgf8 leads to repulsion of migrating cells around the primitive streak, impairing the formation of mesodermal structures, especially the paraxial mesoderm.
Role of Fgf8 in Primitive Streak Cell Migration and Mesoderm Induction
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Fgf8 is expressed in the primitive streak and functions as a signaling molecule promoting cell migration away from the streak and toward specific mesodermal destinations.
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Studies have shown Fgf8 signaling is essential for mesoderm ingression, specification, and migration that forms paraxial, intermediate, and lateral plate mesoderm.
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It acts by modulating cell motility, survival, and differentiation during early embryogenesis.
Effect of Fgf8 Overexpression
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Overexpressing Fgf8 in the primitive streak can disrupt the normal directional migration by repelling cells excessively.
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This repulsion prevents the cells from properly forming the paraxial mesoderm, a precursor to somites and skeletal muscle.
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The inhibition of cell ingression and migration leads to defects in axial tissue formation and overall embryonic morphology.
Other Answer Choices Explained
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(1) Yolk sac deformation is not a primary effect seen with Fgf8 overexpression in the primitive streak.
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(2) Wnt signaling and primitive streak orientation changes are not direct consequences of Fgf8 overexpression; these pathways function independently though they may interact.
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(4) Cells generating mesodermal portions is the normal function of the primitive streak cells, but overexpressing Fgf8 disrupts this process rather than supporting it.
Summary Table: Fgf8 Overexpression Effects on Primitive Streak and Mesoderm
| Option | Description | Correctness | Explanation |
|---|---|---|---|
| (1) | Yolk sac deformation | Incorrect | No direct evidence of this phenotype due to Fgf8[effects]. |
| (2) | Wnt signaling activation and streak orientation change | Incorrect | Separate processes from Fgf8 overexpression effects. |
| (3) | Cells of the streak fail to form paraxial mesoderm | Correct | Excess Fgf8 repels cells, blocking paraxial mesoderm formation. |
| (4) | Cells generate mesodermal portions | Incorrect | Normal function disrupted by Fgf8 overexpression. |
Conclusion
Fgf8 is critical for controlling proper cell migration through the primitive streak during avian gastrulation. Overexpression of Fgf8 causes repulsion of migrating cells, leading to failure in paraxial mesoderm formation. This specifically impairs early mesoderm-derived tissue development, making option (3) the correct choice.
FAQ
Q: What role does Fgf8 play in primitive streak cell migration?
Fgf8 promotes migration and specification of mesodermal cells during gastrulation.
Q: What happens if Fgf8 is overexpressed?
Cells are repelled, preventing proper formation of paraxial mesoderm and impairing embryonic development.
Q: Does overexpression of Fgf8 affect Wnt signaling and streak orientation?
No, these pathways function independently and are not directly altered by Fgf8 overexpression.
This detailed analysis reveals how Fgf8 modulation governs critical cellular behaviors during early development, highlighting the importance of regulated signaling for robust embryogenesis.
