de Cuevas M, Spradling A C
Howard Hughes Medical Institute, Department of Embryology, Carnegie Institution of Washington, Baltimore, MD 21210, USA.
Development. 1998 Aug;125(15):2781-9. doi: 10.1242/dev.125.15.2781.
The Drosophila oocyte develops within a cyst of 16 germline cells interconnected by ring canals. Polarized, microtubule-based transport of unknown determinants is required for oocyte formation, but whether polarity is established during or after cyst formation is not clear. We have analyzed how polarity develops in stem cells and dividing cysts by following the growth of the fusome, a vesiculated cytoplasmic organelle. Our studies show that the fusome grows by a regular, polarized process throughout the stem cell and cyst cell cycles. Each polarization cycle begins in mitosis, when the fusome segregates to a single daughter cell of each pair. Following mitosis, a 'plug' of fusomal material forms in each nascent ring canal and gradually fuses with the pre-existing fusome. In stem cells, the ring canal is transient and closes down after the fusome is partitioned through it. In dividing cysts, as the fusome plugs move toward the pre-existing fusome, their associated ring canals also move, changing the geometry of the cyst. At the end of each cycle of cyst growth, the fusome remains asymmetrically distributed within the cyst; one of the two cells with four ring canals retains a bigger piece of fusome than any other cell, including the other cell with four ring canals. Based on these observations, we argue that the oocyte is specified at the first cyst division.
果蝇卵母细胞在由环管相互连接的16个生殖系细胞的囊中发育。卵母细胞形成需要基于微管的未知决定因素的极化运输，但极性是在囊形成期间还是之后建立尚不清楚。我们通过追踪纺锤体（一种泡状细胞质细胞器）的生长，分析了干细胞和分裂囊中的极性是如何发展的。我们的研究表明，在整个干细胞和囊细胞周期中，纺锤体通过一个规则的、极化的过程生长。每个极化周期始于有丝分裂，此时纺锤体分离到每对中的一个子细胞中。有丝分裂后，每个新生环管中形成一个纺锤体物质的“塞子”，并逐渐与预先存在的纺锤体融合。在干细胞中，环管是短暂的，在纺锤体通过它进行分配后关闭。在分裂的囊中，随着纺锤体塞子向预先存在的纺锤体移动，它们相关的环管也移动，改变了囊的几何形状。在囊生长的每个周期结束时，纺锤体在囊内仍呈不对称分布；两个有四个环管的细胞中，其中一个保留的纺锤体片段比任何其他细胞都大，包括另一个有四个环管的细胞。基于这些观察结果，我们认为卵母细胞在第一次囊分裂时就已确定。
