中文摘要：

目的探讨老年大鼠大脑白质及白质内有髓神经纤维的改变。方法运用透射电子显微镜和体视学方法分别对年轻组和老年组Long-Evans大鼠大脑白质及其内有髓神经纤维进行定量研究。结果老年组大鼠大脑白质总体积，有髓神经纤维总长度分别下降了24．1％和41％；老年组大鼠有髓神经纤维体积密度、髓鞘体积密度和纤维直径分别增加了30％、23．9％和31％，具有统计学意义。但是有髓神经纤维长度密度、有髓神经纤维总体积和髓鞘的总体积没有显著老年性改变。结论正常老年大脑的萎缩主要是由白质体积的下降引起的。正常老年大脑白质的有髓神经纤维总长度显著性降低，并且主要是由于白质内细小直径的有髓神经纤维丢失所造成的。

英文摘要：

Objective To explore the changes of the white matter and the myelinated fibers in the white matter of aged rat brain. Methods The white matter and the myelinated fibers in the white matter of 10 young Long-Evans rats （5 female and 5 male rats, 6-8-month-old） and 9 aged Long-Evans rats （5 female and 4 male rats, 18 month old） were quantitatively investigated with stereological techniques and transmission electronic microscopy. Results When compared with those of young rats, the total volume of white matter and the total length of myelinated fibers in the white matter of aged rats were decreased by 24.1% and 41%, respectively. In the white matter of aged rats, the volume density of myelinated fibers, the volume density of myelin sheathes and the mean diameter of myelinated fibers were increased by 30% , 23.9% and 31% , respectively, when compared with those of young rats. However, there were no statistically significant changes of the length density and the total volume of myelinated fibers and the total volume of myelin sheathes in the white matter of aged rats when compared with those of young rats. Conclusion The atrophy of normal aged brain is mainly due to the reduction of the white matter volume. The total length of myelinated fibers in the white matter of aged rats is signigicantly decreased when compared with young rats, which is primarily caused by the loss of myelinated nerve fibers with thinner diameters in the white matter.