Holt C E
Department of Biology, University of California, San Diego, La Jolla 92093.
J Neurosci. 1989 Sep;9(9):3123-45. doi: 10.1523/JNEUROSCI.09-09-03123.1989.
Intracellular injections of Lucifer yellow (LY) were made into the cell bodies of Xenopus retinal ganglion cells from the earliest stages of axonogenesis to the beginning of target innervation. Embryos were intact during the injection so that the entire cell (cell body, dendrites, axon, and growth cone) could be visualized. The purpose of the study was 3-fold: (1) to characterize the early steps in retinal ganglion cell differentiation before the axon reaches its target; (2) to determine whether guidepost cells exist as possible navigation cues in the vertebrate optic pathway; and (3) to investigate whether the morphology of early retinal ganglion cell growth cones varies in a position-dependent manner along the primordial optic pathway. Axons were generally initiated before dendrites and followed a well-defined course along the primordial optic pathway without branching. Surprisingly, at least 5% of the retinal ganglion cells sent more than one axon into the optic pathway. Sister axons from the same parent cell traveled separately in the pathway, indicating that their growth cones navigated independently. Examination of dendrite genesis showed that dendrites usually begin to emerge from the cell body well before the axon tip reaches the target. This observation argues against the possibility that target contact influences dendrite initiation. Nascent dendrites were commonly tipped with pronounced varicosities that did not resemble axon growth cones. Their number and branching correlated well with axon length, indicating that the age of the retinal ganglion cell itself, rather than the age of its presynaptic cells or local environment, is the strongest influence on dendrite genesis. Examination of LY-filled growth cones at varying points in the pathway showed no evidence of dye transfer to adjacent cells. This indicates that gap junctional contacts probably do not form during axonal pathfinding and suggests that direct intercellular communication between growing axons and other cells in the pathway does not play a major role in axon guidance. Growth cone morphology was analyzed quantitatively and found to vary at different positions along the pathway. Growth cones entering the optic nerve head were the largest and most complex; those on the retinal surface were the smallest and showed a simple morphology. Growth cones in the chiasm and optic tract showed a degree of complexity similar to those in the optic nerve head but were smaller.(ABSTRACT TRUNCATED AT 400 WORDS)
从轴突发生的最早阶段到靶标神经支配开始，将荧光黄（LY）细胞内注射到非洲爪蟾视网膜神经节细胞的胞体中。注射过程中胚胎保持完整，以便能够观察到整个细胞（胞体、树突、轴突和生长锥）。本研究的目的有三个：（1）描述轴突到达靶标之前视网膜神经节细胞分化的早期步骤；（2）确定在脊椎动物视觉通路中是否存在作为可能导航线索的引导细胞；（3）研究早期视网膜神经节细胞生长锥的形态是否沿原始视觉通路以位置依赖的方式变化。轴突通常在树突之前起始，并沿着原始视觉通路遵循明确的路径，且无分支。令人惊讶的是，至少5%的视网膜神经节细胞向视觉通路发出不止一条轴突。来自同一母细胞的姐妹轴突在通路中单独行进，这表明它们的生长锥独立导航。对树突发生的检查表明，在轴突尖端到达靶标之前，树突通常就已开始从胞体中伸出。这一观察结果排除了靶标接触影响树突起始的可能性。新生树突通常末端有明显的膨体，这与轴突生长锥不同。它们的数量和分支与轴突长度密切相关，表明视网膜神经节细胞自身的年龄，而非其突触前细胞的年龄或局部环境，对树突发生的影响最大。在通路的不同点对充满LY的生长锥进行检查，未发现染料转移至相邻细胞的证据。这表明在轴突寻路过程中可能不会形成间隙连接接触，并提示生长中的轴突与通路中的其他细胞之间的直接细胞间通讯在轴突导向中不发挥主要作用。对生长锥形态进行了定量分析，发现其在通路的不同位置有所不同。进入视神经乳头的生长锥最大且最复杂；视网膜表面的生长锥最小，形态简单。视交叉和视束中的生长锥显示出与视神经乳头中的生长锥相似的复杂程度，但较小。（摘要截取自400字）
