Concept explainers
To review:
Triggering of Wnt3 upregulation upon amputation in hydra. Based on the fact that if a midsection cut and a more apical cut both result in Wnt activation, the triggering of apoptosis in the first scenario and not in the other.
Introduction:
The decapitated Hydra regenerate their heads through morphallaxis, means, without significant contributions made by cell proliferation or interstitial stem cells. Indeed, Hydra depleted of interstitial stem cells regenerate robustly, and Wnt3 from epithelial cells triggers head regeneration. This process is called morphallactic regeneration (regeneration by cell differentiation).
Explanation of Solution
When hydra have cut at its midsection, cells derived from the interstitial stem cells (neurons, nematocytes, secretory cells, and gametes) undergo apoptosis immediately below the cut site. These cells produce Wnt3 before death triggering release of β-catenin in the interstitial cells beneath them, which causes a wave of proliferation in the interstitial cells as well as remodeling in the epithelial cells. Epimorphic regeneration is a process where a variety of cell types differentiates and self-organize or regenerates to rebuild a missing structure. Generally occurs in salamander and hydra or another process known as epimorphosis (regeneration of tissues or organs through dedifferentiation of existing, differentiated adult tissues). Canonical Wnt signaling is significant in normal budding and in head regeneration.
The different mechanism controls regeneration after midgastric bisection in hydra equipped with both epithelial and interstitial cell lineages. The apoptosis process is rapidly induced and Wnt3 secretion among interstitial cells at the head- (but not foot-) regenerating site. Apoptosis is both necessary and sufficient to induce Wnt3 production and head regeneration, even at ectopic sites. The head inhibitor and head activator (Wnts) are both present in the hypostome of hydra, head inhibition gradient falls off rapidly than the head activator gradient. Inhibition of head activator by the head inhibitor induces the formation of budding zone.
Hydra exhibits morphallaxis, and epimorphic regeneration for triggering of Wnt3 upregulation upon amputation in hydra. Certain head inhibition gradients are present in hydra body which prevents any region of hydra body to form head and induce head formation at a specific location.
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Chapter 22 Solutions
Developmental Biology
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