Stereological Study Of The Witte Matter Changes Of Normal Aged Rats And AD Model Mice

With the emergence of an aging population, the age-related diseases such as senile dementia, for example Alzheimer's disease (AD), are increasingly important. These diseases have caused a national concern and this research area has become a research hotspot. AD that mostly occurs in old age is the neurodegenerative disease with the sharp decline of the brain functions. It has been generally believed for several decades that there is significant death of the neurons in the cortex of aged brain, and people considered the death of neurons in the aged brain as the neurobiological basis of brain function decline. However, with the increase of researches and the improvement of the research methods, most researchers consider the white matter changes as the main biological basis of the brain function changes. Most of the previous studies used qualitative methods that had some defects. The present study combined the new stereological methods and electron microscopic technique to investigate the ultrastructure changes of the myelin sheaths of the myelinated fibers in the frontal region of the corpus callosum in normal aged rats and the change of the white matter volume, the concentration of the Aβin the white matter and the behavior of the transgenic AD mice. The purpose of the current study is try to find the neuropathological mechanisms of normal aging and AD.PARTⅠ1 Materials and methods1.1 Female Long-Evans rats, five young rats (6-month-old), five middle-aged rats (18-month-old), and five old-aged rats (27 month-old). 1.2 One hemisphere was sampled from each rat brain. From the sampled hemisphere, 3 tissue blocks were sampled from the frontal region of corpus callosum in a systematic random fashion. One ultra-thin section was cut from each sampled block. From each cut section, 8 fields of view were randomly photographed under TEM with a magnification of 25000.1.3 The mean diameter of the myelinated fibers and the mean diameter of the axons, the mean outer perimeter and inner perimeter of the myelin sheaths, the mean thickness of the myelin sheaths, and the volume of the myelin sheathes of the myelinaed nerve fibers in the frontal region of the corpus callosum were investigated with the new stereological methods.2 Results2.1 The corpus callosum volume of the young, middle-aged and old-aged female Long-Evans rats was 15.3±1.62 mm~3,12.2±2.11 mm~3 and 10.9±1.74 mm~3,respectively. When compared to young rats, the corpus callosum volume of the middle-aged and the old-aged rats were significantly decreased by 20.4 % and 28.6 % (p<0.05), respectively. However, there was no significant difference of the corpus callosum volume between the middle-aged rats and the old-aged rats.2.2 There were no significant changes in the mean diameter of myelinated fibers and the mean diameter of axons, the mean outer perimeter and inner perimeter of the myelin sheaths,the mean thickness of the myelin sheathes, and the volume of the myelin sheathes of the myelinaed nerve fibers in the frontal region of the corpus callosum, among young, middle-aged and old aged rats.2.3 The mean myelin sheath thickness of the myelinated fibers in the frontal region of corpus callosum was significantly different among three groups. The mean thickness of the myelin sheaths in the myelinated fibers with the diameter from 0.2 to 0.4μm of middle-aged and old-aged rats was significantly decreased by 11.8 % and 16.0 %, respectively, when compared to young rats (p<0.05). The mean thickness of the myelin sheaths in the myelinated fibers with the diameter from 0.4 to 0.6μm in old-aged rats was significantly decreased by 16.0 % when compared to young rats (p<0.05). The mean thickness of the myelin sheaths in the myelinated fibers with the diameter from 0.6 to 0.8μm in old-aged rats was significantly decreased by 14.6 % when compared to middle-aged rats (p<0.05). PARTⅡ1 Materials and methods1.1 Transgenic mouse (tg2576), 6 month-old group and 10 month-old group with age-matched groups (non-transgenic mouse), eight mice in each group, were need.1.2 All mice were examined in a Morris water maze for consecutive seven days with spatial learning and memory trials in first six days and spatial exploratory trials on the seventh day, searching platform latency (SPL) was recorded.1.3 3 mice were randomly sampled from each group after the Morris water maze trials. The concentration of Aβin white matter of each rat was calculated using Elisa.1.4 The other five mice of each group after the Morris water maze trials were used for the quantitative studied with stereological methods.2 Results2.1 In the 6 month-old mice, there were no significant differences in the SPL in the spatial learning and memory trials between the TG group and the control group, and so were in the spatial exploratory trials. In the 10 month-old mice, there were significant differences in the SPL in the spatial learning and memory trials between the TG group and the control group (p<0.05), while in the spatial exploratory trials, the two groups was non-significantly different.2.2 Aβ40:In the 6 month-old mice, the concentration of Aβin white matter of the TG group were significantly increased compared to the control group (p<0.05). In the 10 month-old mice, the concentration of Aβin white matter of the TG group were significantly increased compared to the control group (p<0.05).Aβ42:In the 6 month-old mice, the concentration of Aβin white matter of the TG group were significantly increased compared to the control group (p<0.05). In the 10 month-old mice, the concentration of Aβin white matter of the TG group were significantly increased compared to the control group (p<0.05).2.3 In the 6 month-old mice, there were no significant differences of the white matter volume between the TG group and the control group. In the 10 month-old mice, the white matter volume of the TG group were significantly decreased compared to the control group (p<0.05). The white matter volume of the 10 month-old AD mice were significantly decreased compared to the 6 month-old AD mice (p<0.05).General Conclusions1. The present study for the first time combined the new stereological techniques and transmission electron microscope technique to quantitatively study the myelin sheathes of the myelinaed nerve fibers in the frontal region of the corpus callosum of normal rats and the whiter matter of the transgenic AD mice.2. The present study provided the normal data of the mean diameter of the myelinated fibers and the mean diameter of the axons, the mean outer perimeter and inner perimeter of the myelin sheaths, the mean thickness of the myelin sheathes, and the volume of the myelin sheathes of the myelinaed nerve fibers in the frontal region of the rat corpus callosum. These values might provide valuable reference data for the future study.3. The present results indicated that there were no significant changes in the mean diameter of myelinated fibers (external diameter) and the mean diameter of axons (inner diameter),the mean outer perimeter and inner perimeter of the myelin sheaths,the mean thickness of the myelin sheathes, and the volume of the myelin sheathes of the myelinaed nerve fibers in the frontal region of the corpus callosum among young, middle-aged and old aged rats. The mean thickness of the myelin sheaths in the myelinated fibers with the diameter from 0.2 to 0.4μm of middle-aged and old-aged rats was significantly decreased when compared to young rats.4. The present study obtained the data of the behavior, white matter volume and Aβconcentration in the white matter of the 6-month-old and 10-month-old transgenic AD mice, these data will provide important baseline value for future studies.5. The present study also found that in transgenic AD mice, there were significant changes in the white matter volume and the Aβconcentration in the white matter.