Oakley R A, Tosney K W
Neuroscience Program, University of Michigan, Ann Arbor 48109.
Dev Biol. 1991 Sep;147(1):187-206. doi: 10.1016/s0012-1606(05)80017-x.
Axon outgrowth between the spinal cord and the hindlimb of the chick embryo is constrained by three tissues that border axon pathways. Growth cones turn to avoid the posterior sclerotome, perinotochordal mesenchyme, and pelvic girdle precursor during normal development and after experimental manipulation. We wanted to know if these functionally similar barriers to axon advance also share a common molecular composition. Since the posterior sclerotome differentially binds peanut agglutinin (PNA) and since PNA binding is also typical of prechondrogenic differentiation, we examined the pattern of expression of PNA binding sites and cartilage proteoglycan epitopes in relation to axon outgrowth. We found that all three barrier tissues preferentially express both PNA binding sites and chondroitin-6-sulfate (C-6-S) immunoreactivity at the time when growth cones avoid these tissues. Moreover, both epitopes are expressed in the roof plate of the spinal cord and in the early limb bud, two additional putative barriers to axon advance. In contrast, neither epitope is detected in peripheral axon pathways. In the somites, this dichotomous pattern of expression clearly preceded the invasion of the anterior sclerotome by either motor growth cones or neural crest cells. However, in the limb, barrier markers disappeared from presumptive axon pathways in concert with the invasion of axons. Since this coordinate pattern suggested that the absence of barrier markers in these axon pathways requires an interaction with growth cones, we analyzed the pattern of barrier marker expression following unilateral neural tube deletions. We found that PNA-negative axon pathways developed normally even in the virtual absence of axon outgrowth. We conclude that the absence of staining with carbohydrate-specific barrier markers is an independent characteristic of the cells that comprise axon pathways. These results identify two molecular markers that characterize known functional barriers to axon advance and suggest that barrier tissues may impose patterns on peripheral nerve outgrowth by virtue of their distinct molecular composition.
鸡胚脊髓与后肢之间的轴突生长受到三种与轴突通路相邻的组织的限制。在正常发育过程中以及实验操作后，生长锥会转向避开后体节、脊索周围间充质和骨盆带前体。我们想知道这些在功能上类似的轴突生长障碍是否也具有共同的分子组成。由于后体节与花生凝集素（PNA）有差异结合，且PNA结合也是软骨形成前分化的典型特征，我们研究了PNA结合位点和软骨蛋白聚糖表位的表达模式与轴突生长的关系。我们发现，在生长锥避开这些组织时，所有三种障碍组织都优先表达PNA结合位点和硫酸软骨素-6-硫酸酯（C-6-S）免疫反应性。此外，这两种表位都在脊髓顶板和早期肢芽中表达，这是另外两个假定的轴突生长障碍。相比之下，在外周轴突通路中未检测到这两种表位。在体节中，这种二分的表达模式明显先于运动生长锥或神经嵴细胞对前体节的侵入。然而，在肢体中，随着轴突的侵入，障碍标记物从假定的轴突通路中消失。由于这种协调的模式表明这些轴突通路中障碍标记物的缺失需要与生长锥相互作用，我们分析了单侧神经管缺失后障碍标记物的表达模式。我们发现，即使在几乎没有轴突生长的情况下，PNA阴性的轴突通路也能正常发育。我们得出结论，碳水化合物特异性障碍标记物染色的缺失是构成轴突通路的细胞的一个独立特征。这些结果确定了两种分子标记物，它们表征了已知的轴突生长功能障碍，并表明障碍组织可能凭借其独特的分子组成对外周神经生长施加模式。
