Small R K
J Neurosci Res. 1985;13(1-2):39-53. doi: 10.1002/jnr.490130104.
The internal structure of the membranes of axonal and Schwann cell growth processes was examined by freeze-fracture in the growing olfactory nerve, a simple in vivo system consisting of a homogeneous neuronal population. Excision of the mature nerve of adult bullfrogs provides well-synchronized primary neuritic outgrowth that is highly enriched in growth cones at its distalmost segment. The extreme uniformity of olfactory axons in terms of their diameter and their intramembrane particle (IMP) composition permits clear identification of the cellular origin of the growth cone structures observed in replicas. In vivo, growth cones of the olfactory nerve appear as irregularly shaped enlargements of the distal tip; filopodia are only infrequently exposed by the fracture plane. Axonal and Schwann cell growth cones are distinguished by 1) the larger size of the Schwann cell growth cone and the smaller diameter of its attached processes, and 2) the distinct differences in IMP composition of Schwann cell and axonal growth cones and cell processes. Schwann cell growth cones display a uniformly high IMP density on their P-face leaflet, with the exception of circumscribed moundlike protrusions that are relatively free of IMPs. In contrast, axonal growth cones display sharp regional variation in IMP density on their P-face: broad regions almost devoid of IMPs are interspersed with zones of high IMP density. Cytotic profiles occur within high IMP density zones located, most often, at the base of the axonal growth cone. A comparison of IMP size histograms of both high and low-density regions of axonal growth cones and that of the neighboring distal shaft of the axon indicates a strict partitioning of membrane components between these two regions. The IMP profile of the axonal growth cone, notable for its relative enrichment in large-diameter particles, suggests that IMP components of the growth cone are delivered to the distal tip by a mechanism that is distinct from the lateral diffusion process described for particles of the growing axon's shaft [cf. Small and Pfenninger, 1984]. The IMP profile of the concave P-face leaflet of the internal vesicles found clustered at the base of the growth cone is more similar in composition to the profile of the neuronal shaft than that of the growth cone.
在生长中的嗅神经（一个由同质神经元群体组成的简单体内系统）中，通过冷冻断裂技术检查轴突和施万细胞生长过程中膜的内部结构。切除成年牛蛙的成熟神经，可提供同步良好的初级神经突生长，其最远端部分富含生长锥。嗅轴突在直径和膜内颗粒（IMP）组成方面的极端均匀性，使得在复制品中观察到的生长锥结构的细胞起源得以清晰识别。在体内，嗅神经的生长锥表现为远端尖端不规则形状的膨大；丝状伪足很少被断裂面暴露。轴突和施万细胞生长锥的区别在于：1）施万细胞生长锥尺寸较大，其附着突起的直径较小；2）施万细胞和轴突生长锥及其细胞突起的IMP组成存在明显差异。施万细胞生长锥在其P面小叶上显示出均匀的高IMP密度，但有一些相对不含IMP的局限性丘状突起除外。相比之下，轴突生长锥在其P面上的IMP密度呈现出明显的区域差异：几乎不含IMP的宽阔区域与高IMP密度区域相间分布。胞吞轮廓出现在高IMP密度区域内，这些区域最常位于轴突生长锥的基部。对轴突生长锥高密度和低密度区域以及相邻轴突远端轴杆的IMP大小直方图进行比较，表明这两个区域之间膜成分存在严格的划分。轴突生长锥的IMP轮廓以其在大直径颗粒中的相对富集为显著特征，这表明生长锥的IMP成分是通过一种不同于生长轴突杆颗粒的侧向扩散过程的机制传递到远端尖端的[参见Small和Pfenninger，1984]。在生长锥基部聚集的内部囊泡的凹面P面小叶的IMP轮廓，其组成与神经元轴杆的轮廓比与生长锥的轮廓更相似。
