Does Expansion of the Presumptive Somite-Notochord Boundry Contribute to Xenopus laevis Involution and Elongation?
Dr. Grayson Davis
Arts and Sciences
The vertebrate embryo’s first organ is the notochord, a stiffening rod later replaced by the backbone. Cells fated to become notochord rearrange extensively as the cells of their array converge upon the midline and extend anteriorly. Simultaneously, the interfaces between presumptive somite and presumptive notochord extend. Convergent-extension (CE) theory suggests that coordinated lateral to medial cell migration within the presumptive notochord is a driving force of elongation and involution and perhaps of involution. Large explants of presumptive notochord will elongate, but smaller ones will not, raising the possibility of a contributing interaction with the adjacent presumptive somite. Somite-notochord spreading (SNS) theory proposes that these two populations rearrange to increase their cell-to-cell contact with one another, thereby increasing the length of the interface between them. If so, destroying the cells of the midline (presumptive notochord) but leaving the notochord-somite boundaries intact should not inhibit SNS elongation. However, reducing the number of presumptive notochord cells contributing to CE could reduce the force available for elongation and perhaps its range of success as well. As a test, the central presumptive notochord cells of thirty-two early (stage 10) gastrulae were scratched out with a needle. Seven of those embryos dissociated into loose piles of dying cells. The twenty-five survivors elongated well. Moreover, four of these formed double notochords producing doubled nervous systems. One produced a triple notochord and triple nervous system. These data argue in favor of a contribution of presumptive somite-notochord spreading toward elongation and involution of the embryo.
Robinson, Sophia L. and Walsh-Rock, Elizabeth, "Does Expansion of the Presumptive Somite-Notochord Boundry Contribute to Xenopus laevis Involution and Elongation?" (2016). Symposium on Undergraduate Research and Creative Expression (SOURCE). 516.
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