Martik Megan L, McClay David R
Department of Biology, Duke University, Durham, NC 27708, USA; University Program in Genetics and Genomics, Duke University, Durham, NC 27708, USA.
Department of Biology, Duke University, Durham, NC 27708, USA; University Program in Genetics and Genomics, Duke University, Durham, NC 27708, USA.
Mech Dev. 2017 Dec;148:3-10. doi: 10.1016/j.mod.2017.06.005. Epub 2017 Jul 3.
Gastrulation is a complex orchestration of movements by cells that are specified early in development. Until now, classical convergent extension was considered to be the main contributor to sea urchin archenteron extension, and the relative contributions of cell divisions were unknown. Active migration of cells along the axis of extension was also not considered as a major factor in invagination.
Cell transplantations plus live imaging were used to examine endoderm cell morphogenesis during gastrulation at high-resolution in the optically clear sea urchin embryo. The invagination sequence was imaged throughout gastrulation. One of the eight macromeres was replaced by a fluorescently labeled macromere at the 32 cell stage. At gastrulation those patches of fluorescent endoderm cell progeny initially about 4 cells wide, released a column of cells about 2 cells wide early in gastrulation and then often this column narrowed to one cell wide by the end of archenteron lengthening. The primary movement of the column of cells was in the direction of elongation of the archenteron with the narrowing (convergence) occurring as one of the two cells moved ahead of its neighbor. As the column narrowed, the labeled endoderm cells generally remained as a contiguous population of cells, rarely separated by intrusion of a lateral unlabeled cell. This longitudinal cell migration mechanism was assessed quantitatively and accounted for almost 90% of the elongation process. Much of the extension was the contribution of Veg2 endoderm with a minor contribution late in gastrulation by Veg1 endoderm cells. We also analyzed the contribution of cell divisions to elongation. Endoderm cells in Lytechinus variagatus were determined to go through approximately one cell doubling during gastrulation. That doubling occurs without a net increase in cell mass, but the question remained as to whether oriented divisions might contribute to archenteron elongation. We learned that indeed there was a biased orientation of cell divisions along the plane of archenteron elongation, but when the impact of that bias was analyzed quantitatively, it contributed a maximum 15% to the total elongation of the gut.
The major driver of archenteron elongation in the sea urchin, Lytechinus variagatus, is directed movement of Veg2 endoderm cells as a narrowing column along the plane of elongation. The narrowing occurs as cells in the column converge as they migrate, so that the combination of migration and the angular convergence provide the major component of the lengthening. A minor contributor to elongation is oriented cell divisions that contribute to the lengthening but no more than about 15%.
原肠胚形成是发育早期特定细胞进行的复杂运动编排。到目前为止，经典的汇聚延伸被认为是海胆原肠延伸的主要因素，而细胞分裂的相对贡献尚不清楚。细胞沿延伸轴的主动迁移也未被视为内陷的主要因素。
在光学透明的海胆胚胎中，利用细胞移植结合实时成像技术在高分辨率下研究原肠胚形成过程中内胚层细胞的形态发生。对整个原肠胚形成过程中的内陷序列进行成像。在32细胞期，将八个大分裂球中的一个替换为荧光标记的大分裂球。在原肠胚形成时，那些最初约4个细胞宽的荧光内胚层细胞后代斑块，在原肠胚形成早期释放出一列约2个细胞宽的细胞，然后在原肠管延长结束时，这列细胞通常会变窄到一个细胞宽。这列细胞的主要运动方向是原肠管伸长的方向，变窄（汇聚）发生在两个细胞中的一个比其相邻细胞向前移动时。随着这列细胞变窄，标记的内胚层细胞通常保持为连续的细胞群体，很少因侧向未标记细胞的侵入而分开。对这种纵向细胞迁移机制进行了定量评估，其占伸长过程的近90%。大部分延伸是由Veg2内胚层贡献的，Veg1内胚层细胞在原肠胚形成后期贡献较小。我们还分析了细胞分裂对伸长的贡献。确定在多棘海胆中，内胚层细胞在原肠胚形成过程中大约经历一次细胞加倍。这种加倍在细胞质量没有净增加的情况下发生，但细胞分裂的定向是否可能有助于原肠管伸长的问题仍然存在。我们了解到，实际上细胞分裂沿原肠管伸长平面存在偏向性定向，但在定量分析这种偏向性的影响时，它对肠道总伸长的贡献最大为15%。
多棘海胆原肠管伸长的主要驱动因素是Veg2内胚层细胞作为一个变窄的细胞列沿伸长平面的定向运动。变窄发生在列中的细胞迁移时汇聚，因此迁移和角度汇聚的结合提供了伸长的主要组成部分。对伸长的次要贡献是定向细胞分裂，其对伸长的贡献不超过约15%。
