Kishi Kanae, Hayashi Momoko, Onuma Takeshi A, Nishida Hiroki
Department of Biological Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
Department of Biological Sciences, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
Dev Biol. 2017 Aug 1;428(1):245-257. doi: 10.1016/j.ydbio.2017.06.008. Epub 2017 Jun 8.
Mechanisms for morphogenetic processes that generate complex patterns in a reproducible manner remain elusive. Live imaging provides a powerful tool to record cell behaviors. The appendicularian, Oikopleura dioica, is a planktonic tunicate that has a rapid developmental speed, small number of cells (less than 3500 cells in a juvenile), and a transparent body. The trunk epidermis, called the oikoplastic epithelium (OE), has elaborate cellular arrangements showing a complex pattern to secrete so-called "house" made of extracellular components. The OE is characterized by invariant number, size, and shape of the monolayer epithelial cells. Pattern formation is achieved during 5h of larval development without growth of the body, making this a suitable system for live imaging of a two-dimensional (2D) sheet. First, we subdivided the OE and defined several domains by cellular resolution, and systematically gave names to the constituent cells, since there is no variation among individuals. Time-lapse imaging of the epidermal cells revealed region-specific pattern formation processes. Each identified domain served as a compartment into which distribution of descendant cells of founder cells is restricted. Regulation of orientation, timing, and the number of rounds of cell divisions, but not cell death and migration, was a critical mechanism for determination of final cell arrangement and size. In addition, displacement of epithelial sheet plates was observed in the Eisen domain. Stem-cell-like cell divisions, whereby large mother stem cells generate a chain of small daughter cells, were involved in formation of the Nasse region and ventral sensory organ. These are the first examples of this kind of stem-cell-like cell division in deuterostomes. Furthermore, labeling of the left or right blastomere of the two-cell-stage embryo, which roughly gives rise to the left or right side of the body, respectively, revealed that the boundary of the descendant cells does not match with the midline of the trunk epidermis. Left and right descendants largely invade into the opposite side in an invariant way, suggesting the possibility that specification of the OE cell identities may occur later in development, most probably around hatching, and depending on cell position in the OE epithelial sheet. These detailed descriptions of OE patterning processes provide basic and essential information to analyze further cell behaviors in the generation of elaborate and intricate but stereotyped 2D cellular patterns in this advantageous model system for developmental and cell biological studies in chordates.
以可重复方式产生复杂模式的形态发生过程的机制仍然难以捉摸。实时成像提供了一个记录细胞行为的强大工具。尾海鞘，即双鳃樽海鞘，是一种浮游被囊动物，发育速度快，细胞数量少（幼体中少于3500个细胞），身体透明。躯干表皮，称为尾海鞘质上皮（OE），具有精细的细胞排列，呈现出一种复杂的模式，用于分泌由细胞外成分构成的所谓“房子”。OE的特征在于单层上皮细胞的数量、大小和形状不变。在幼虫发育的5小时内，身体没有生长，图案形成得以实现，这使得它成为一个用于二维（2D）薄片实时成像的合适系统。首先，我们将OE细分，并通过细胞分辨率定义了几个区域，由于个体之间没有差异，我们系统地为组成细胞命名。表皮细胞的延时成像揭示了区域特异性的图案形成过程。每个确定的区域作为一个隔室，奠基细胞的后代细胞分布受限于此。对细胞分裂的方向、时间和轮数的调控，而非细胞死亡和迁移，是决定最终细胞排列和大小的关键机制。此外，在艾森区域观察到上皮薄片板的位移。干细胞样细胞分裂，即大的母干细胞产生一连串小的子细胞，参与了纳塞区域和腹侧感觉器官的形成。这些是在后口动物中这种干细胞样细胞分裂的首个例子。此外，对两细胞期胚胎的左或右卵裂球进行标记，它们大致分别产生身体的左侧或右侧，结果显示后代细胞的边界与躯干表皮的中线不匹配。左右后代细胞以不变的方式大量侵入到对侧，这表明OE细胞身份的特化可能在发育后期发生，很可能在孵化前后，并且取决于OE上皮薄片中的细胞位置。这些对OE图案形成过程的详细描述为进一步分析这个在脊索动物发育和细胞生物学研究中具有优势的模型系统中精细、复杂但刻板的2D细胞图案生成过程中的细胞行为提供了基础且关键的信息。
