Yasui K, Tabata S, Ueki T, Uemura M, Zhang S C
Department of Oral Anatomy 1, Kagoshima University Dental School, Japan.
J Comp Neurol. 1998 Apr 20;393(4):415-25.
The developmental pattern of the lancelet (amphioxus) peripheral nervous system from embryos to larvae has been studied by using wholemount immunostaining and transmission electron microscopy. The peripheral nerves first appeared on the anterior dorsal surface of the medulla at the middle neurula stage, when the anterior nerve cord was just closing. A single axon with a large growth cone was the progenitor of each nerve. The nerve roots adopted an asymmetric arrangement soon after. The first nerve, likely a pair of pure sensory nerves, sprouted from the anterior tip of the nerve cord. This nerve may be comparable topographically to the preoptic nerve (the posterior branch of the terminal nerve) in lungfishes. However, the neuron that first extends its axon was located in the medulla, as in the other posterior nerves. One of the extramedullary primary sensory neurons, the corpuscles of de Quatrefages, appeared in larvae with the mouth and two anterior gill pores. Their axons were seemingly fasciculated with the efferent axon of the first nerve. The second nerve, the most complex one to appear during embryonic and early larval development, innervated the preoral pit and the buccal region. The third and fourth nerves on the left side also innervated the buccal region. The larval innervation patterns in the anterior region differed from the adult organization, suggesting a segmental rearrangement of the nerve supply during development. There was no evidence to dichotomize the peripheral nerves into cranial and spinal nerves, as exist in vertebrates. These characteristics of the peripheral nervous system in the lancelet indicate that this animal has a rather derived or primitive developmental system of peripheral nerves, making the analysis of homology with vertebrates difficult.
通过整体免疫染色和透射电子显微镜技术，对文昌鱼（ lancelet ）从胚胎到幼体的外周神经系统发育模式进行了研究。外周神经最早出现在神经胚中期延髓的前背表面，此时前神经索刚刚闭合。每根神经的祖先是一条带有大生长锥的单轴突。神经根很快就呈现出不对称排列。第一根神经，可能是一对纯感觉神经，从神经索的前端发出。这条神经在拓扑学上可能与肺鱼的视前神经（终末神经的后支）相当。然而，第一个伸出轴突的神经元位于延髓内，与其他后神经中的情况相同。一种髓外初级感觉神经元，即德卡特勒法热小体，在幼体中随着口和两个前鳃裂出现。它们的轴突似乎与第一根神经的传出轴突成束状。第二根神经是胚胎和幼体早期发育过程中出现的最复杂的神经，支配口前凹和颊区。左侧的第三和第四根神经也支配颊区。前部区域的幼体神经支配模式与成体结构不同，这表明在发育过程中神经供应发生了节段性重排。没有证据将外周神经分为脊椎动物中存在的脑神经和脊神经。文昌鱼外周神经系统的这些特征表明，这种动物具有相当特化或原始的外周神经发育系统，这使得与脊椎动物的同源性分析变得困难。
