Kaltschmidt J A, Davidson C M, Brown N H, Brand A H
Wellcome/CRC Institute and Department of Genetics, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, UK.
Nat Cell Biol. 2000 Jan;2(1):7-12. doi: 10.1038/71323.
The asymmetric segregation of cell-fate determinants and the generation of daughter cells of different sizes rely on the correct orientation and position of the mitotic spindle. In the Drosophila embryo, the determinant Prospero is localized basally and is segregated equally to daughters of similar cell size during epidermal cell division. In contrast, during neuroblast division Prospero is segregated asymmetrically to the smaller daughter cell. This simple switch between symmetric and asymmetric segregation is achieved by changing the orientation of cell division: neural cells divide in a plane perpendicular to that of epidermoblast division. Here, by labelling mitotic spindles in living Drosophila embryos, we show that neuroblast spindles are initially formed in the same axis as epidermal cells, but rotate before cell division. We find that daughter cells of different sizes arise because the spindle itself becomes asymmetric at anaphase: apical microtubules elongate, basal microtubules shorten, and the midbody moves basally until it is positioned asymmetrically between the two spindle poles. This observation contradicts the widely held hypothesis that the cleavage furrow is always placed midway between the two centrosomes.
细胞命运决定因子的不对称分离以及不同大小子细胞的产生依赖于有丝分裂纺锤体的正确定向和位置。在果蝇胚胎中，决定因子Prospero定位于基部，在表皮细胞分裂期间会平均分配到大小相似的子细胞中。相比之下，在神经母细胞分裂期间，Prospero会不对称地分配到较小的子细胞中。这种对称和不对称分离之间的简单转换是通过改变细胞分裂的方向实现的：神经细胞在与表皮母细胞分裂平面垂直的平面上分裂。在这里，通过标记活的果蝇胚胎中的有丝分裂纺锤体，我们发现神经母细胞纺锤体最初与表皮细胞在同一轴上形成，但在细胞分裂前会发生旋转。我们发现不同大小的子细胞的产生是因为纺锤体本身在后期变得不对称：顶端微管伸长，基部微管缩短，并且中间体向基部移动，直到它不对称地位于两个纺锤体极之间。这一观察结果与广泛持有的假设相矛盾，即分裂沟总是位于两个中心体之间的中间位置。
