Fraher J P, O'Sullivan V R
Department of Anatomy, University College, Cork, Ireland.
J Comp Neurol. 1989 Feb 8;280(2):171-82. doi: 10.1002/cne.902800202.
Axon counts were made at two standardised levels of C7 ventral spinal nerve roots from 46 female rats representing nine ages between birth and 500 days. The objective was to provide a definitive account of proximodistal changes in axon numbers and of age changes in axon numbers both during postnatal development and at several stages during maturity. At each age there is a proximodistal increase in the numbers of axons in all categories examined (myelinated, promyelin, transitional, and fetal) between levels midway along the subarachnoid course of the root and where it is apposed to but separate from the dorsal root ganglion. During maturation and throughout maturity axon totals change similarly at both levels: After a slight increase immediately postnatum, they decline sharply between 4 and 20 days due to a marked loss of unmyelinated axons. A gradual decline in myelinated axon numbers continues to 500 days. While these changes are occurring, axon numbers in all categories show a proximodistal increase throughout. The magnitude of this increase lessens with age for all but the transitional category due to a preferential decrease in numbers distally. Though these observations do not differentiate between axon branching and looping of sensory axons into the ventral root as a cause of the proximodistal increase in numbers, they tend to support the former. At each age during maturation axon proportions at proximal and distal levels correspond well for each animal, indicating that axon segregation proceeds at related rates within each root. Age changes in axon proportions within the transitional and fetal categories indicate that the postnatal stage of axon segregation results from axon loss, rather than Schwann cell proliferation.
对46只雌性大鼠C7腹侧脊髓神经根的两个标准化水平进行轴突计数，这些大鼠代表了出生至500天之间的9个年龄阶段。目的是明确产后发育期间以及成熟阶段几个时期轴突数量的近远侧变化和年龄变化。在每个年龄阶段，在所检查的所有类别（有髓鞘的、前髓鞘的、过渡型的和胎儿型的）中，从神经根蛛网膜下行程的中途水平到与背根神经节相邻但分离的位置，轴突数量均呈现近远侧增加。在成熟过程中和整个成熟阶段，两个水平的轴突总数变化相似：出生后立即略有增加，随后在4至20天之间急剧下降，这是由于无髓鞘轴突明显减少所致。有髓鞘轴突数量持续逐渐下降至500天。在这些变化发生的同时，所有类别的轴突数量在整个过程中均呈现近远侧增加。除过渡型类别外，所有类别的这种增加幅度随年龄减小，这是由于远侧数量优先减少所致。尽管这些观察结果无法区分轴突分支和感觉轴突向腹侧神经根的环行是近远侧数量增加的原因，但它们倾向于支持前者。在成熟过程中的每个年龄阶段，每只动物近端和远端水平的轴突比例都非常吻合，这表明轴突分离在每个神经根内以相关速率进行。过渡型和胎儿型类别内轴突比例的年龄变化表明，轴突分离的产后阶段是由于轴突丢失，而非施万细胞增殖。
