Developmental Mechanisms of Neurogenesis

我们对神经系统胚胎发育的了解大多来自于对苍蝇或老鼠的研究。由于这些研究揭示了类似的信号系统和分子途径，因此有人提出，神经系统在多细胞动物的早期历史中只出现过一次。随后对几种无脊椎动物的研究支持了这一观点，但神经系统发育方式的惊人多样性使得很难将单一起源合理化。一种更精确的解释是，在许多群体中存在着细胞组织的独立进化，随着动物的多样化，神经发生以各种方式利用了保守的信号系统和分子途径。我们缺乏必要的模型来验证这一假设，因为神经发生的分子和细胞机制需要仔细分析，以便在相关动物群体中进行有用的比较。

Most of what we know about the embryonic development of nervous systems comes from studies of flies or mice. As those studies revealed similar signaling systems and molecular pathways, it was proposed that nervous systems arose once, early in the history of multicellular animals. Subsequent studies in several types of invertebrates support this idea, but the striking diversity in how nervous systems develop makes it difficult to rationalize a single origin. A more refined explanation is that there has been independent evolution of the cellular organization in many groups and neurogenesis has drawn upon conserved signaling systems and molecular pathways in various ways as animals diversified. We lack the models necessary to test this hypothesis, as the molecular and cellular mechanisms underlying neurogenesis need to be carefully analyzed to make useful comparisons in groups of related animals.