Yu W, Baas P W
Department of Anatomy, University of Wisconsin Medical School, Madison 53706.
J Neurosci. 1994 May;14(5 Pt 1):2818-29. doi: 10.1523/JNEUROSCI.14-05-02818.1994.
Hippocampal neurons in culture initially extend several minor processes that are approximately 20 microns in length. The first minor process to grow approximately 10 microns longer than the others will continue to grow rapidly and become the axon (Goslin and Banker, 1989). We sought to define changes in the microtubule (MT) array that occur during axon differentiation. In theory, axon differentiation could involve an increase in MT number, MT length, or some combination of both. To address this issue, we first serially reconstructed the entire MT array of a minor process from a cell whose axon had not yet differentiated. This minor process contained 182 MTs that ranged in length from 0.14 to 20.09 microns. The average MT length was 3.87 +/- 3.83 microns, and the total MT length was 704 microns. We then reconstructed the MT arrays of a minor process and the 56 microns axon from a cell that had undergone axon differentiation. The minor process contained 157 MTs that ranged in length from 0.24 to 17.95 microns. The average MT length was 3.91 +/- 4.84 microns, and the total MT length was 600 microns. The axon contained 1430 MTs that ranged in length from 0.05 to 40.14 microns. The average MT length was 4.02 +/- 5.28 microns, and the total MT length was 5750 microns. These data indicate that a shift occurs toward shorter as well as longer MTs, but that virtually no change in average MT length occurs during axon differentiation. Thus, elongation of existing MTs cannot account for the major expansion of the MT array that occurs as a minor process becomes an axon. In contrast, the number of MTs increases by approximately 10-fold as a minor process differentiates and grows into an axon of the length we analyzed. Based on these data, we conclude that the MT array of a minor process is substantially expanded as it differentiates into an axon, and that the principal mechanism by which this expansion occurs is the copious addition of new MTs.
培养的海马神经元最初会伸出几个长度约为20微米的小突起。第一个比其他小突起长约10微米的小突起会继续快速生长并成为轴突（戈斯林和班克，1989年）。我们试图确定轴突分化过程中微管（MT）阵列发生的变化。理论上，轴突分化可能涉及微管数量增加、微管长度增加或两者的某种组合。为了解决这个问题，我们首先对一个轴突尚未分化的细胞的一个小突起的整个微管阵列进行了连续重建。这个小突起包含182根微管，长度从0.14微米到20.09微米不等。微管平均长度为3.87±3.83微米，微管总长度为704微米。然后，我们对一个已经经历轴突分化的细胞的一个小突起和56微米长的轴突的微管阵列进行了重建。这个小突起包含157根微管，长度从0.24微米到17.95微米不等。微管平均长度为3.91±4.84微米，微管总长度为600微米。轴突包含1430根微管，长度从0.05微米到40.14微米不等。微管平均长度为4.02±5.28微米，微管总长度为5750微米。这些数据表明，微管向更短和更长的方向都有变化，但在轴突分化过程中微管平均长度几乎没有变化。因此，现有微管的伸长不能解释当一个小突起变成轴突时微管阵列的主要扩展。相反，当一个小突起分化并生长成我们分析长度的轴突时，微管数量增加了约10倍。基于这些数据，我们得出结论，一个小突起在分化成轴突时，其微管阵列会大幅扩展，而这种扩展发生的主要机制是大量添加新的微管。
