Tsui H T, Lankford K L, Ris H, Klein W L
J Neurosci. 1984 Dec;4(12):3002-13. doi: 10.1523/JNEUROSCI.04-12-03002.1984.
Using the high voltage electron microscope, we have examined cultured embryonic neurons in order to understand better the organization of microtubules in developing neurites. We found that, in embryonic chick retina neurons, microtubules were abundant in the ends of neurites and showed an unusual pattern of organization. Most striking was the presence of microtubule loops; after entering the flattened region of a growth cone, microtubules frequently made tight 180 degrees turns. Occasionally these looping microtubules re-entered the neurite and returned in the direction of the cell body. Positive identification of the loop structures as microtubules was made by specific immunocytochemical labeling. Quantitative analysis showed that more than half of the retina neurons that were dissociated on embryonic day 8 and kept in culture for 4 to 6 days (E8C4 and E8C6) contained at least one microtubule that made a 180 degrees turn at flat regions along or at the tips of neurites. The area within the loops typically contained larger membranous organelles, whereas only small vesicles were seen outside the loops. Fine filaments were seen to interconnect the loops at various places, suggesting the possibility that they played a role in maintaining the shape of microtubule loops. Examination of other neurons showed that tight microtubule loops were prominent in chick spinal cord neurons, but they were rarely seen in neurons of the sympathetic ganglia or dorsal root ganglia or in NG108-15 cloned cells. Developmentally, no loops were observed in E8C1 retina neurons, but retina neurons dissociated from older embryos (12 days) did show loops after 1 day in culture; these data suggest that microtubule loops may be abundant around embryonic day 12 to 13 in the chick retina. The possible significance of this unusual microtubule organization to the control of neurite growth and bidirectional transport is discussed.
我们使用高压电子显微镜对培养的胚胎神经元进行了检查，以便更好地了解发育中的神经突中微管的组织情况。我们发现，在胚胎鸡视网膜神经元中，微管在神经突末端丰富，并呈现出一种不寻常的组织模式。最引人注目的是微管环的存在；进入生长锥的扁平区域后，微管经常进行紧密的180度转弯。这些成环的微管偶尔会重新进入神经突并朝着细胞体的方向返回。通过特异性免疫细胞化学标记对环结构作为微管进行了阳性鉴定。定量分析表明，在胚胎第8天解离并在培养中保持4至6天（E8C4和E8C6）的视网膜神经元中，超过一半至少含有一根微管，该微管在神经突沿线或末端的扁平区域进行了180度转弯。环内区域通常含有较大的膜性细胞器，而环外仅可见小泡。可见细丝在各个位置将环相互连接，这表明它们可能在维持微管环的形状方面发挥作用。对其他神经元的检查表明，紧密的微管环在鸡脊髓神经元中很突出，但在交感神经节或背根神经节的神经元或NG108 - 15克隆细胞中很少见到。在发育过程中，E8C1视网膜神经元中未观察到环，但从较老胚胎（12天）解离的视网膜神经元在培养1天后确实显示出环；这些数据表明，微管环在鸡视网膜中可能在胚胎第12至13天左右丰富。本文讨论了这种不寻常的微管组织对神经突生长和双向运输控制的可能意义。
