Sardet Christian, Dru Philippe, Prodon François
BioMarCell, UMR 7009, CNRS/UPMC, Station Zoologique, Observatoire, Villefranche sur Mer, 06230, France.
Biol Cell. 2005 Jan;97(1):35-49. doi: 10.1042/BC20040126.
The peripheral region of ascidian oocytes and zygotes contains five determinants for morphogenesis and differentiation of the embryo. The determinant for the 24 primary muscle cells of the tadpole, macho1, is one of several cortical mRNAs localized in a gradient along the animal-vegetal axis in the oocyte. After fertilization these mRNAs, together with cortical endoplasmic reticulum (cER) and a subcortical mitochondria-rich domain (myoplasm), relocate in two major reorganization phases forming the posterior plasm (postplasm) of the zygote. At the 8-cell stage cortical mRNAs concentrate in a macroscopic cortical structure called the centrosome-attracting body (CAB), forming a characteristic posterior end mark (PEM) in the two posterior vegetal blastomeres. We propose to call the numerous mRNAs showing this particular cortical localization in the posterior region of the embryo postplasmic/PEM RNAs and suggest a nomemclature. We do not know how postplasmic/PEM RNAs reach their polarized distribution in the oocyte cortex but at least PEM1 and macho1 (and probably others) bind to the network of cER retained in isolated cortical fragments. We propose that after fertilization, these postplasmic/PEM mRNAs move in the zygote cortex together with the cER network (cER/mRNA domain) via microfilament- and microtubule-driven translocations. The cER/mRNA domain is localized posteriorly at the time of first cleavage and distributed equally between the first two blastomeres. After the third cleavage, the cER/mRNA domain and dense particles compact to form the CAB in posterior vegetal blastomeres of the 8-cell stage. We discuss the identity of postplasmic/PEM RNAs, how they localize, anchor, relocate and may be translated. We also examine their roles in unequal cleavage and as a source of posterior morphogenetic and differentiation factors.
海鞘卵母细胞和受精卵的周边区域含有五个决定胚胎形态发生和分化的因素。蝌蚪24个初级肌细胞的决定因素，即macho1，是卵母细胞中沿动物-植物轴呈梯度分布的几种皮质mRNA之一。受精后，这些mRNA与皮质内质网（cER）和富含线粒体的皮质下区域（肌质）一起，在两个主要的重组阶段重新定位，形成受精卵的后部细胞质（后质）。在8细胞阶段，皮质mRNA集中在一个称为中心体吸引体（CAB）的宏观皮质结构中，在两个后部植物性卵裂球中形成一个特征性的后端标记（PEM）。我们建议将在胚胎后部区域显示这种特殊皮质定位的众多mRNA称为后质/PEM RNA，并提出一种命名法。我们不知道后质/PEM RNA如何在卵母细胞皮质中实现其极化分布，但至少PEM1和macho1（可能还有其他的）与保留在分离的皮质片段中的cER网络结合。我们提出，受精后，这些后质/PEM mRNA通过微丝和微管驱动的易位，与cER网络（cER/mRNA结构域）一起在受精卵皮质中移动。cER/mRNA结构域在第一次卵裂时位于后部，并在前两个卵裂球之间平均分配。第三次卵裂后，cER/mRNA结构域和致密颗粒紧密结合，在8细胞阶段的后部植物性卵裂球中形成CAB。我们讨论了后质/PEM RNA的身份、它们如何定位、锚定、重新定位以及可能如何被翻译。我们还研究了它们在不等卵裂中的作用以及作为后部形态发生和分化因子来源的作用。
