Friede R L, Meier T, Diem M
J Neurol Sci. 1981 May;50(2):217-28. doi: 10.1016/0022-510x(81)90168-4.
(1) The length of the human ventral spinal roots in the newborn and in the adult were compared with the length of the internodes in these roots. Internodes of fibers of a given caliber were more than twice as long in the adult sacral roots than in the cervical roots. The factors of root elongation corresponded closely to the factors of internode elongation. (2) Internode length was in a statistically significant linear relation with fiber caliber in individual roots. The slope of the regression lines differed among roots, and were steeper caudally. If data from roots having different elongation factors were pooled there was no longer a linear relationship between fiber caliber and internode length. Internode length, therefore, correlated directly and quantitatively with root elongation; it was not directly linked to fiber caliber. (3) The total Schwann cell population per fiber was nearly stable from the newborn to the adult, but the sacral roots had 5 times as many Schwann cells as the cervical roots. Calculations based on the number of internodes and on the length of fetal roots at the onset of myelination showed that the mean "initial length" of the Schwann cell at the onset of myelination was 187 micrometer, the same for all roots. (4) The Schwann cell population per fiber is determined by 2 reciprocal growth phases: elongation of the fetal fiber up to the onset of myelination corresponds to a multiplication of Schwann cells each having a given initial length. Schwann cell populations stabilize after the onset of myelination; from then on the geometry of each internode is determined by its passive elongation. The timing of myelination is critical for determining the ultimate length of the Schwann cell. If myelination does not begin before a fiber system has attained approximately one eighth of its definitive size, the longest internodes of that system will average 1.5 mm and most will remain below 2 mm, regardless of body size.
(1) 将新生儿和成年人的人类腹侧脊髓神经根长度与这些神经根中节间的长度进行了比较。给定管径纤维的节间在成人骶神经根中的长度是颈神经根中的两倍多。神经根伸长的因素与节间伸长的因素密切对应。(2) 节间长度与单个神经根中的纤维管径呈统计学上显著的线性关系。回归线的斜率在不同神经根之间有所不同，且在尾部更陡。如果将具有不同伸长因素的神经根数据合并，则纤维管径与节间长度之间不再存在线性关系。因此，节间长度与神经根伸长直接且定量相关；它与纤维管径没有直接联系。(3) 从新生儿到成年人，每根纤维的施万细胞总数几乎稳定，但骶神经根中的施万细胞数量是颈神经根的5倍。根据节间数量和髓鞘形成开始时胎儿神经根的长度进行的计算表明，髓鞘形成开始时施万细胞的平均“初始长度”为187微米，所有神经根均相同。(4) 每根纤维的施万细胞数量由两个相互的生长阶段决定：胎儿纤维在髓鞘形成开始前的伸长对应于每个具有给定初始长度的施万细胞的增殖。髓鞘形成开始后施万细胞数量稳定；从那时起，每个节间的几何形状由其被动伸长决定。髓鞘形成的时间对于确定施万细胞的最终长度至关重要。如果在一个纤维系统达到其最终大小的大约八分之一之前髓鞘形成没有开始，那么该系统最长的节间平均将为1.5毫米，并且大多数将保持在2毫米以下，无论身体大小如何。
