Shain D H, Stuart D K, Huang F Z, Weisblat D A
Department of Molecular and Cell Biology, University of California, Berkeley, CA, 94720-3200, USA.
Development. 2000 Feb;127(4):735-44. doi: 10.1242/dev.127.4.735.
Central nervous system (CNS) in leech comprises segmentally iterated progeny derived from five embryonic lineages (M, N, O, P and Q). Segmentation of the leech CNS is characterized by the formation of a series of transverse fissures that subdivide initially continuous columns of segmental founder cells in the N lineage into distinct ganglionic primordia. We have examined the relationship between the N lineage cells that separate to form the fissures and lateral ectodermal and mesodermal derivatives by differentially labeling cells with intracellular lineage tracers and antibodies. Although subsets of both lateral ectoderm and muscle fibers contact N lineage cells at or near the time of fissure formation, ablation experiments suggest that these contacts are not required for initiating fissure formation. It appears, therefore, that this aspect of segmentation occurs autonomously within the N lineage. To support this idea, we present evidence that fundamental differences exist between alternating ganglionic precursor cells (nf and ns primary blast cells) within the N lineage. Specifically, ablation of an nf primary blast cell sometimes resulted in the fusion of ipsilateral hemi-ganglia, while ablation of an ns primary blast cell often caused a 'slippage' of blast cells posterior to the lesion. Also, differences in cell behavior were observed in biochemically arrested nf and ns primary blast cells. Collectively, these results lead to a model of segmentation in the leech CNS that is based upon differences in cell adhesion and/or cell motility between the alternating nf and ns primary blast cells. We note that the segmentation processes described here occur well prior to the expression of the leech engrailed-class gene in the N lineage.
水蛭的中枢神经系统（CNS）由源自五个胚胎谱系（M、N、O、P和Q）的节段性重复后代组成。水蛭中枢神经系统的节段化特征是形成一系列横向裂缝，这些裂缝将N谱系中最初连续的节段性奠基细胞柱细分为不同的神经节原基。我们通过用细胞内谱系追踪剂和抗体对细胞进行差异标记，研究了分离形成裂缝的N谱系细胞与外侧外胚层和中胚层衍生物之间的关系。尽管外侧外胚层和肌肉纤维的亚群在裂缝形成时或接近裂缝形成时与N谱系细胞接触，但消融实验表明，这些接触对于启动裂缝形成并非必需。因此，节段化的这一方面似乎在N谱系内自主发生。为支持这一观点，我们提供证据表明N谱系内交替的神经节前体细胞（nf和ns初级胚细胞）之间存在根本差异。具体而言，消融一个nf初级胚细胞有时会导致同侧半神经节融合，而消融一个ns初级胚细胞通常会导致损伤后方的胚细胞“滑移”。此外，在生化阻滞的nf和ns初级胚细胞中观察到细胞行为的差异。这些结果共同导致了一个水蛭中枢神经系统节段化模型，该模型基于交替的nf和ns初级胚细胞之间细胞黏附力和/或细胞运动性的差异。我们注意到，这里描述的节段化过程在N谱系中水蛭engrailed类基因表达之前就已经很好地发生了。
