King E D, Munger B L
Department of Anatomy, Milton S. Hershey Medical Center, Pennsylvania State University, Hershey 17033.
Anat Rec. 1990 Oct;228(2):191-210. doi: 10.1002/ar.1092280211.
The present study was undertaken in order to verify the identification of profiles of presumptive growth cones in vivo. The developing spinal nerves of chick embryos were studied by light and electron microscopy. We traced the onset of efferent and afferent innervation of the myotome in 2- to 4-day-old chick embryos in order to be sure that we were examining the growing tips of axons. In the process of studying these growing axons, we were able to observe some unique relationships of neural tube, myotome, and differentiating spinal nerves. The neural tube tightly abuts the myotome in Hamburger and Hamilton's (HH) stage 14 chick embryos and cytoplasmic projections from the myotome directly abut the neural tube. The first ventral roots could be identified in HH stage 15 embryos and dorsal roots in HH stage 16 embryos, both under 2 1/2 days of age. The advancing spinal nerve courses toward the anterior or cranial half of the myotome, and growth cones directly contact the medial wall of the myotome. The spinal nerves continue to abut tightly the myotome during the succeeding day of embryonic life, and growth cones enter the substance of the myotome by 3 days, or HH stage 19 embryos. These dorsolaterally directed axons will form the dorsal ramus of the spinal nerves and the ventral ramus continues to be contiguous with the myotome. Invasion of the myotome by axons (putative innervation), and thus innervation of myotomal cells in the 3-day chick embryos, was a totally unexpected finding. The myotome and its potential derivatives thus have extensive neural contact by 3 days of embryonic life in the chick. These findings document a parallel differentiation of afferent and efferent elements of the nervous system and confirm previous accounts identifying growth cones in an intact organism. These findings suggest that afferent as well as efferent nerves may have critical roles in the differentiation of the mesodermal as well as ectodermal derivatives.
本研究旨在验证体内假定生长锥轮廓的识别。通过光学显微镜和电子显微镜研究了鸡胚发育中的脊神经。我们追踪了2至4日龄鸡胚中肌节传出和传入神经支配的起始，以确保我们检查的是轴突的生长尖端。在研究这些生长轴突的过程中，我们能够观察到神经管、肌节和分化中的脊神经之间的一些独特关系。在汉堡和汉密尔顿（HH）14期鸡胚中，神经管紧密邻接肌节，肌节的细胞质突起直接邻接神经管。在HH15期胚胎中可识别出第一对腹根，在HH16期胚胎中可识别出背根，两者均在2.5日龄以下。前进的脊神经向肌节的前半部分或颅侧延伸，生长锥直接接触肌节的内侧壁。在胚胎生命的后续日子里，脊神经继续紧密邻接肌节，到3日龄或HH19期胚胎时，生长锥进入肌节实质。这些向背外侧方向的轴突将形成脊神经的背支，腹支继续与肌节相邻。轴突侵入肌节（假定的神经支配），从而在3日龄鸡胚中支配肌节细胞，这是一个完全出乎意料的发现。因此，在鸡胚胚胎发育3天时，肌节及其潜在衍生物已与神经有广泛接触。这些发现记录了神经系统传入和传出元件的平行分化，并证实了之前在完整生物体中识别生长锥的报道。这些发现表明，传入神经和传出神经可能在中胚层和外胚层衍生物的分化中都起关键作用。
