Roegiers F, McDougall A, Sardet C
URA 671 Biologie Cellulaire Marine, CNRS, Université P. et M. Curie, Villefranche-sur-Mer, France.
Development. 1995 Oct;121(10):3457-66. doi: 10.1242/dev.121.10.3457.
Ascidians eggs are spawned with their cytoskeleton and organelles organized along a preexisting animal-vegetal axis. Fertilization triggers a spectacular microfilament-dependant cortical contraction that causes the relocalization of preexisting cytoplasmic domains and the creation of new domains in the lower part of the vegetal hemisphere. We have investigated the relationship between fertilization, the cortical contraction and the localization of cytoplasmic domains in eggs of the ascidian Phallusia mammillata. We have also examined the link between this first phase of ooplasmic segregation and the site of gastrulation. The cortical contraction was found to be initiated on the side of the egg where intracellular calcium is first released either by the entering sperm or by photolysis of caged InsP3. The cortical contraction carries the sperm nucleus towards the vegetal hemisphere along with a subcortical mitochondria-rich domain (the myoplasm). If the sperm enters close to the animal or vegetal poles the cortical contraction is symmetrical, travelling along the animal-vegetal axis. If the sperm enters closer to the equator, the contraction is asymmetrical and its direction does not coincide with the animal-vegetal axis. The direction of contraction defines an axis along which preexisting (such as the myoplasm) or newly created cytoplasmic domains are relocalized. Two microfilament-rich surface constrictions, the 'contraction pole' and the 'vegetal button' (which forms 20 minutes later), appear along that axis approximately opposite the site where the contraction is initiated. The contraction pole can be situated as much as 55 degrees from the vegetal pole, and its location predicts the site of gastrulation. It thus appears that in ascidian eggs, the organization of the egg before fertilization defines a 110 degrees cone centered around the vegetal pole in which the future site of gastrulation of the embryo will lie. The calcium wave and cortical contraction triggered by the entering sperm adjust the location of cytoplasmic domains along an axis within that permissive zone. We discuss the relation between that axis and the establishment of the dorsoventral axis in the ascidian embryo.
海鞘卵产出时，其细胞骨架和细胞器沿预先存在的动物 - 植物轴排列。受精引发了一场壮观的依赖微丝的皮层收缩，这导致预先存在的细胞质区域重新定位，并在植物半球下部形成新的区域。我们研究了海鞘乳突海鞘卵中受精、皮层收缩与细胞质区域定位之间的关系。我们还研究了卵质分离的第一阶段与原肠胚形成部位之间的联系。发现皮层收缩在卵的一侧开始，在该侧，细胞内钙首先由进入的精子或通过笼锁肌醇三磷酸（InsP3）的光解而释放。皮层收缩将精子细胞核连同富含线粒体的皮层下区域（肌质）带向植物半球。如果精子在靠近动物极或植物极处进入，皮层收缩是对称的，沿动物 - 植物轴行进。如果精子在更靠近赤道处进入，收缩是不对称的，其方向与动物 - 植物轴不一致。收缩方向定义了一个轴，沿着该轴，预先存在的（如肌质）或新形成的细胞质区域重新定位。沿着该轴，大约在收缩起始部位的相对位置出现两个富含微丝的表面收缩，即“收缩极”和“植物钮”（20分钟后形成）。收缩极与植物极的距离可达55度，其位置可预测原肠胚形成的部位。因此，在海鞘卵中，受精前卵的组织定义了一个以植物极为中心的110度圆锥体，胚胎未来的原肠胚形成部位将位于该圆锥体内。进入的精子引发的钙波和皮层收缩沿着该允许区域内的一个轴调整细胞质区域的位置。我们讨论了该轴与海鞘胚胎背腹轴建立之间的关系。
