Okada T, Hirano H, Takahashi K, Okamura Y
National Institute of Bioscience and Human-technology, AIST, Ibaraki, Higashi 1-1, Tsukuba, 305, Japan.
Dev Biol. 1997 Oct 15;190(2):257-72. doi: 10.1006/dbio.1997.8708.
The ascidian larva contains tubular neural tissue, one of the prominent anatomical features of the chordates. The cell-cleavage pattern and cell maps of the nervous system have been described in the ascidian larva in great detail. Cell types in the neural tube, however, have not yet been defined due to the lack of a suitable molecular marker. In the present work, we identified neuronal cells in the caudal neural tube of the Halocynthia embryo by utilizing a voltage-gated Na+ channel gene, TuNa I, as a molecular marker. Microinjection of a lineage tracer revealed that TuNa I-positive neurons in the brain and in the trunk epidermis are derived from the a-line of the eight-cell embryo, which includes cell fates to epidermal and neural tissue. On the other hand, TuNa I-positive cells in the more caudal part of the neural tissue were not stained by microinjection into the a-line. These neurons are derived from the A-line, which contains fates of notochord and muscle, but not of epidermis. Electron microscopic observation confirmed that A-line-derived neurons consist of motor neurons innervating the dorsal and ventral muscle cells. Isolated A-line blastomeres have active membrane excitability distinct from those of the a-line-derived neuronal cells after culture under cleavage arrest, suggesting that the A-line gives rise to a neuronal cell distinct from that of the a-lineage. TuNa I expression in the a-line requires signals from another cell lineage, whereas that in the A-line occurs without tight cell contact. Thus, there are at least two distinct neuronal lineages with distinct cellular behaviors in the ascidian larva: the a-line gives rise to numerous neuronal cells, including sensory cells, controlled by a mechanism similar to vertebrate neural induction, whereas A-line cells give rise to motor neurons and ependymal cells in the caudal neural tube that develop in close association with the notochord or muscle lineage, but not with the epidermal lineage.
海鞘幼虫含有管状神经组织，这是脊索动物突出的解剖学特征之一。海鞘幼虫的细胞分裂模式和神经系统的细胞图谱已被详细描述。然而，由于缺乏合适的分子标记，神经管中的细胞类型尚未确定。在本研究中，我们利用电压门控Na+通道基因TuNa I作为分子标记，鉴定了柄海鞘胚胎尾神经管中的神经元细胞。谱系示踪剂的显微注射显示，脑和躯干表皮中的TuNa I阳性神经元来源于八细胞胚胎的a系，该系包括表皮和神经组织的细胞命运。另一方面，神经组织更尾部的TuNa I阳性细胞在显微注射到a系后未被染色。这些神经元来源于A系，该系包含脊索和肌肉的命运，但不包含表皮的命运。电子显微镜观察证实，A系来源的神经元由支配背侧和腹侧肌肉细胞的运动神经元组成。在卵裂阻滞培养后，分离的A系卵裂球具有与a系来源的神经元细胞不同的活跃膜兴奋性，这表明A系产生了与a系不同的神经元细胞。TuNa I在a系中的表达需要来自另一个细胞谱系的信号，而在A系中的表达则不需要紧密的细胞接触。因此，海鞘幼虫中至少有两个具有不同细胞行为的不同神经元谱系：a系产生许多神经元细胞，包括感觉细胞，其受类似于脊椎动物神经诱导的机制控制，而A系细胞产生尾神经管中的运动神经元和室管膜细胞，这些细胞与脊索或肌肉谱系密切相关，但与表皮谱系无关。
