Yoshida Noriyuki, Arai Asuna, Aoki Momoe, Moriya Miho, Sekiguchi Kaho, Shimizu Takashi
Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Division of Life Sciences, Graduate School of Life Science, Hokkaido University, Sapporo, Japan.
J Morphol. 2019 Apr;280(4):568-586. doi: 10.1002/jmor.20966. Epub 2019 Feb 14.
Early development in clitellate annelids is characterized by a highly stereotyped sequence of unequal, spiral cleavages. Cell 2d (i.e., the second micromere of the D quadrant) in the oligochaete Tubifex tubifex also undergoes an evolutionarily conserved sequence of cell division to produce four bilateral pairs of ectodermal teloblasts that act as embryonic stem cells. This study was conducted to characterize each of the 15 rounds of cell division that occur in the 2d cell lineage in this clitellate. After its occurrence, cell 2d undergoes three rounds of highly unequal divisions, giving off the first smaller daughter cell toward the posterior right of the larger daughter cell, the second cell toward the posterior left, and the third cell toward the anterior side of the cell; the larger daughter cell that results from the third division (i.e., the great-granddaughter cell of 2d) then divides equally into a bilateral pair of NOPQ proteloblasts. Cell NOPQ on either side of the embryo undergoes 11 rounds of cell division, during which ectoteloblasts N, Q, and O/P are produced in this order. After its appearance, NOPQ undergoes highly unequal divisions twice cutting off the smaller cells toward the anterior end of the embryo and then divides almost equally into ectoteloblast N and proteloblast OPQ. After its appearance, OPQ undergoes highly unequal divisions twice giving off the first smaller cell toward the anterior and the second smaller cell toward the posterior of the embryo and then divides almost equally into ectoteloblast Q and proteloblast OP. Finally, OP undergoes highly unequal division four times after its birth budding off the smaller cells toward the anterior and then cleaves equally into ectoteloblasts O and P. In the unequally dividing cells of the 2d cell lineage, the mitotic apparatus (MA), which forms at the cell's center, moves eccentrically toward the cortical site where the smaller cell will be given off. The moving MA is oriented perpendicular to the surface it approaches, and its peripheral pole becomes closely associated with the cell cortex. In contrast, the MA involved in the equal divisions remains in the cell center throughout mitosis. The key features of the cleavage program in the 2d cell lineage are discussed in light of the present observations. The mechanical aspects of unequal cleavage in the 2d cell lineage and the modes of specification of MA orientation are discussed. A comparison of the cleavage mode in the 2d cell lineage is also performed among six selected clitellate annelid species.
寡毛纲环节动物的早期发育特征是一系列高度模式化的不等螺旋卵裂。在寡毛类颤蚓的胚胎发育中，2d细胞（即D象限的第二个小分裂球）也经历了一系列进化上保守的细胞分裂，产生四对双侧排列的外胚层端细胞，这些细胞起着胚胎干细胞的作用。本研究旨在描述在这种寡毛纲动物2d细胞谱系中发生的15轮细胞分裂。2d细胞出现后，经历三轮高度不等的分裂，第一个较小的子细胞从较大子细胞的右后方产生，第二个细胞从左后方产生，第三个细胞从细胞的前侧产生；第三次分裂产生的较大子细胞（即2d的曾孙细胞）然后平均分裂为一对双侧排列的NOPQ原端细胞。胚胎两侧的NOPQ细胞进行11轮细胞分裂，在此期间依次产生外胚层端细胞N、Q和O/P。NOPQ出现后，进行两次高度不等的分裂，向胚胎前端切下较小的细胞，然后几乎平均分裂为外胚层端细胞N和原端细胞OPQ。OPQ出现后，进行两次高度不等的分裂，第一个较小的细胞从胚胎前端产生，第二个较小的细胞从胚胎后端产生，然后几乎平均分裂为外胚层端细胞Q和原端细胞OP。最后，OP出生后进行四次高度不等的分裂，向胚胎前端产生较小的细胞，然后平均分裂为外胚层端细胞O和P。在2d细胞谱系的不等分裂细胞中，在细胞中心形成的有丝分裂器（MA）向较小细胞将产生的皮质部位偏心移动。移动的MA垂直于它接近的表面定向，其外周极与细胞皮质紧密相连。相比之下，参与均等分裂的MA在整个有丝分裂过程中都留在细胞中心。根据目前的观察结果，讨论了2d细胞谱系中卵裂程序的关键特征。讨论了2d细胞谱系中不等卵裂的力学方面以及MA定向的指定模式。还对六种选定的寡毛纲环节动物的2d细胞谱系中的卵裂模式进行了比较。
