Prodon François, Dru Philippe, Roegiers Fabrice, Sardet Christian
BioMarCell, UMR7009 Biologie du Développement, CNRS/Université Pierre et Marie Curie, Station Zoologique, Observatoire, Villefranche sur Mer 06230, France.
J Cell Sci. 2005 Jun 1;118(Pt 11):2393-404. doi: 10.1242/jcs.02366.
The mature ascidian oocyte is a large cell containing cytoplasmic and cortical domains polarized along a primary animal-vegetal (a-v) axis. The oocyte cortex is characterized by a gradient distribution of a submembrane monolayer of cortical rough endoplasmic reticulum (cER) and associated maternal postplasmic/PEM mRNAs (cER-mRNA domain). Between fertilization and first cleavage, this cER-mRNA domain is first concentrated vegetally and then relocated towards the posterior pole via microfilament-driven cortical contractions and spermaster-microtubule-driven translocations. The cER-mRNA domain further concentrates in a macroscopic cortical structure called the centrosome attracting body (CAB), which mediates a series of asymmetric divisions starting at the eight-cell stage. This results in the segregation of determinant mRNAs and their products in posterior cells of the embryo precursors of the muscle and germ line. Using two species of ascidians (Ciona intestinalis and Phallusia mammillata), we have pursued and amplified the work initiated in Halocynthia roretzi. We have analysed the cortical reorganizations in whole cells and in cortical fragments isolated from oocytes and from synchronously developing zygotes and embryos. After fertilization, we observe that a cortical patch rich in microfilaments encircles the cER-mRNA domain, concentrated into a cortical cap at the vegetal/contraction pole (indicating the future dorsal pole). Isolated cortices also retain microtubule asters rich in cER (indicating the future posterior pole). Before mitosis, parts of the cER-mRNA domain are detected, together with short microtubules, in isolated posterior (but not anterior) cortices. At the eight-cell stage, the posteriorly located cER-mRNA domain undergoes a cell-cycle-dependant compaction into the CAB. The CAB with embedded centrosomal microtubules can be isolated with cortical fragments from eight-cell-stage embryos. These and previous observations indicate that cytoskeleton-driven repositioning and compaction of a polarized cortical domain made of rough ER is a conserved mechanism used for polarization and segregation of cortical maternal mRNAs in embryos of evolutionarily distant species of ascidians.
成熟的海鞘卵母细胞是一个大细胞，其细胞质和皮质区域沿动物-植物主轴（a-v轴）呈极化分布。卵母细胞皮质的特征是皮质粗面内质网（cER）的亚膜单层和相关的母体胞质后/ PEM mRNA呈梯度分布（cER-mRNA区域）。在受精和第一次卵裂之间，这个cER-mRNA区域首先在植物极集中，然后通过微丝驱动的皮质收缩和精子星体微管驱动的转运向后极重新定位。cER-mRNA区域进一步集中在一个称为中心体吸引体（CAB）的宏观皮质结构中，该结构介导从八细胞阶段开始的一系列不对称分裂。这导致决定性mRNA及其产物在肌肉和生殖系胚胎前体的后部细胞中分离。我们使用两种海鞘（玻璃海鞘和乳头海鞘），继续并扩展了在柄海鞘中开始的工作。我们分析了全细胞以及从卵母细胞、同步发育的受精卵和胚胎中分离出的皮质片段中的皮质重组。受精后，我们观察到富含微丝的皮质斑块环绕着cER-mRNA区域，在植物极/收缩极集中形成皮质帽（指示未来的背极）。分离出的皮质也保留了富含cER的微管星体（指示未来的后极）。在有丝分裂之前，在分离出的后部（但不是前部）皮质中检测到部分cER-mRNA区域以及短微管。在八细胞阶段，位于后部的cER-mRNA区域经历细胞周期依赖性压缩形成CAB。带有嵌入式中心体微管的CAB可以与八细胞阶段胚胎的皮质片段一起分离出来。这些以及之前的观察结果表明，由粗面内质网构成的极化皮质区域通过细胞骨架驱动的重新定位和压缩是一种保守机制，用于在进化上距离遥远的海鞘物种的胚胎中极化和分离皮质母体mRNA。
