Makabe Kazuhiro W, Nishida Hiroki
Institute of Socio-Arts and Sciences, University of Tokushima, Tokushima, Japan.
Wiley Interdiscip Rev Dev Biol. 2012 Jul-Aug;1(4):501-18. doi: 10.1002/wdev.54. Epub 2012 Apr 9.
Localization of maternal molecules in eggs and embryos and cytoplasmic movements to relocalize them are fundamental for the orderly cellular and genetic processes during early embryogenesis. Ascidian embryos have been known as 'mosaic eggs' because of their autonomous differentiation abilities based on localized cell fate determinants. This review gives a historical overview of the concept of cytoplasmic localization, and then explains the key features such as ooplasmic movements and cell lineages that are essential to grasp the process of ascidian development mediated by localized determinant activities. These activities are partly executed by localized molecules named postplasmic/PEM RNAs, originating from approximately 50 genes, of which the muscle determinant, macho-1, is an example. The cortical domain containing these RNAs is relocalized to the posterior-vegetal region of the egg by cytoskeletal movements after fertilization, and plays crucial roles in axis formation and cell fate determination. The cortical domain contains endoplasmic reticulum and characteristic granules, and gives rise to a subcellular structure called the centrosome-attracting body (CAB), in which postplasmic/PEM RNAs are highly concentrated. The CAB is responsible for a series of unequal partitionings of the posterior-vegetal cytoplasmic domain and the postplasmic/PEM RNAs at the posterior pole during cleavage. Some components of this domain, which is rich in granules, are eventually inherited by prospective germline cells with particular postplasmic/PEM RNAs such as vasa. The postplasmic/PEM RNAs are classified into two groups according to their final cellular destinations and localization pathways. Localization of these RNAs is regulated by specific nucleotide sequences in the 3' untranslated regions (3'UTRs).
母体分子在卵子和胚胎中的定位以及使其重新定位的细胞质运动，对于早期胚胎发育过程中有序的细胞和遗传过程至关重要。由于海鞘胚胎基于局部细胞命运决定因素具有自主分化能力，因此一直被称为“镶嵌卵”。本综述对细胞质定位概念进行了历史概述，然后解释了诸如卵质运动和细胞谱系等关键特征，这些特征对于理解由局部决定因素活动介导的海鞘发育过程至关重要。这些活动部分由名为后质/ PEM RNA的局部分子执行，这些分子源自大约50个基因，其中肌肉决定因素macho-1就是一个例子。受精后，含有这些RNA的皮质区域通过细胞骨架运动重新定位到卵子的后植物区域，并在轴形成和细胞命运决定中发挥关键作用。皮质区域包含内质网和特征性颗粒，并产生一种称为中心体吸引体（CAB）的亚细胞结构，后质/ PEM RNA在其中高度浓缩。CAB负责在卵裂期间后植物细胞质区域和后质/ PEM RNA在后端的一系列不均等分配。这个富含颗粒的区域的一些成分最终由具有特定后质/ PEM RNA（如vasa）的潜在生殖系细胞继承。后质/ PEM RNA根据其最终细胞目的地和定位途径分为两组。这些RNA的定位由3'非翻译区（3'UTR）中的特定核苷酸序列调节。
