Convergent extension
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Convergent extension is the process during organogenesis in which layers of cells intercalate (converge) and become longer (extend).
Involution in the blastomere begins dorsally led by the pharangeal endoderm and head mesoderm. The next tissues to enter are the notochord and somite precursors. Meanwhile the lip of the blastopore extends to have dorsolateral, lateral and ventral sides. IMZ is originally several layers thick. Shortly before their involution through the blastopore lip the layers intercalate and form a long broad layer, this extends the IMZ vegetally. At the same time the superficial cells spread out by dividing and flattening. When the deep cells reach the balstopore lip they involute and initiate a second type of intercalation called convergent extension along the mediolateral axis that integrated several mesoderm streams to form a narrow band. The anterior part of this band migrated toward the animal cap thus the mesoderm stream continues to migrate toward the animal pole and the overlying superficial cells are passively pulled toward the animal pole thereby forming the endodermal roof of the archenteron (lining of the primitive gut). Two major forces appear to drive convergent extension.
1) Genes encoding paraxial protocadherin and axial protocadherin become expressed specifically in the somite forming mesoderm and notochord respectively.
2) Calcium fluxes'. Waves of calcium surge over he dorsal tissues undergoing convergent extension causing waves of contraction along the tissue.
The head region does not undergo convergent extension. Otx2 is expressed in the head region and is a critical gene for head formation involved in forebrain formation. It activates the Xenopus calponin gene that binds to actin and myosin and prevents them contracting as this is not desired in the head region. Mesoderma movement progresses and convergent extension continues to narrow and lengthen the IMZ. Involuting cells contain prospective endodermal roof of the archenteron in its superficia layer and the prospective mesoderm cells including those of the notochord in the deep region.
[[1]] Model of Convergent Extension in Animal Morphogenesis
[[2]] WNT/planar cell polarity (PCP) signaling pathway controls tissue polarity and cell movement.
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