Somite-Notochord Spreading in Xenopus laevis
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lecture / presentation description
The vertebrate embryo's first organ is the notochord, a stiffening rod later replaced by the backbone. Cells fated to become notochord first rearrange themselves extensively as their array changes from wider than long to much longer than wide. The cause of this presumptive notochord elongation was investigated in gastrulating frog embryos. Somite-notochord spreading (SNS) theory suggests that presumptive notochord cells and neighboring presumptive somatic cells will rearrange to increase their cell-to-cell contact with one another. If so, destroying the middle but leaving the notochord-somite boundaries intact should not inhibit gastrulation. This is in opposition with the theory of convergent extension (CE), which suggests that directed cellular migration of notochord cells toward the midline drives the gastrulation process. If so, destroying the target of directed migration (the midline) should inhibit elongation. When 29 embryos had the central presumptive notochord cells scratched out with a needle, there was a 76% survival and a 100% elongation of those 76%. Moreover, three embryos formed double notochords, producing doubled nervous systems. Such data argues against centralized CE theory and in favor of the double-sided SNS theory.