Sims T J, Gilmore S A
Department of Anatomy, University of Arkansas for Medical Sciences, Little Rock 72205.
Exp Brain Res. 1989;75(3):513-22. doi: 10.1007/BF00249902.
Irradiation of the rat spinal cord during the first postnatal week results in a profound reduction of oligodendrocyte myelin formation in the dorsal funiculi (DF). Despite this absence of myelin, however, axons in the irradiated region in the DF increase in diameter and approximate the size distribution seen in the control spinal cord. By 25 days of age Schwann cells are present in the irradiated DF where they undergo cell division and myelinate the axons. During the early stages of this myelin formation, these intraspinal Schwann cells exhibit a relationship to axons that is somewhat different from that seen in the normal developing peripheral nervous system (PNS). For example, within a given region, intraspinal Schwann cells myelinate axons of large diameter prior to ensheathing bundles of small diameter axons. Additionally, during myelination a Schwann cell will surround a single axon with multiple processes which appear to compete for contact with the axolemma. On axons of larger diameter, the elaboration of these processes is so excessive that it is often difficult to trace them back to the parent Schwann cell. Later, when a single process establishes several spirals about an axon, additional processes are no longer elaborated, and the "extra" processes disappear as myelin formation advances to the stage of compact lamellae. Thereafter, the myelin sheath continues to form in a normal manner. Excess processes have been observed during myelinogenesis in the normal developing PNS, but their frequency in that environment is much less than in the irradiated cord. These observations support the hypothesis that the signal(s) to initiate myelin formation are expressed on the axolemmal surface and are controlled by the neuron. In addition, these observations suggest that the delay in myelination results in an affinity or tropism between axons and Schwann cells which exceeds the level existing at the normal time of myelin formation.
在出生后的第一周对大鼠脊髓进行照射，会导致背索（DF）中少突胶质细胞髓鞘形成显著减少。然而，尽管缺乏髓鞘，DF中受照射区域的轴突直径却增大了，并且接近对照脊髓中观察到的大小分布。到25日龄时，受照射的DF中出现了施万细胞，它们进行细胞分裂并使轴突髓鞘化。在这种髓鞘形成的早期阶段，这些脊髓内的施万细胞与轴突的关系，与正常发育的周围神经系统（PNS）中所见的有所不同。例如，在给定区域内，脊髓内的施万细胞在包裹小直径轴突束之前，先使大直径轴突髓鞘化。此外，在髓鞘形成过程中，一个施万细胞会用多个突起围绕单个轴突，这些突起似乎在竞争与轴膜的接触。在较大直径的轴突上，这些突起的形成过于繁多，以至于常常难以追溯到它们的母施万细胞。后来，当单个突起围绕轴突形成几个螺旋时，就不再形成额外的突起，并且随着髓鞘形成进展到紧密板层阶段，“多余”的突起会消失。此后，髓鞘以正常方式继续形成。在正常发育的PNS的髓鞘形成过程中也观察到了多余的突起，但在那种环境中它们出现的频率远低于受照射的脊髓。这些观察结果支持了这样的假设，即启动髓鞘形成的信号表达在轴膜表面并受神经元控制。此外，这些观察结果表明，髓鞘形成的延迟导致轴突与施万细胞之间的亲和力或趋向性超过了正常髓鞘形成时的水平。
