Stereological Study Of Age-related Changes In The Mid-Region And Splenium Of Rat Corpus Callosum And The Myelinated Fibers In This Region

1 BackgroundCorpus callosum is the structure only appearing in the placentai mammals. In previous studies, fibers from the temporo-parieto-occipital junctional region course through the splenium and caudal portion of the body of corpus callosum, and continue to project to the occipital lobe, the lateral dorsal and the bottom of temporal lobe, which are involved with hearing. Researchers found significant correlations between ear asymmetry and callosal size, specifically in the anterior and posterior callosal subregions. In another research, the researchers found that there was an age-related hearing loss. To determine the age-related changes in the postmedian of corpus callosum, we study the structure characteristics of this region. Long-Evans is a common rat strain used for aging research involved memory, learning function and aging brain changes. Therefore, we choosed female Long-Evans rats for our research. In this study, we investigate volumes of regions in corpus callosum and age-related changes in the paremeters of myelinated fibers in these regions by stereological techniques and transmission electron microscopy. 2 ObjectThis research investigated the volume of the mid-region and splenium of rat corpus callosum, the length density, the total length and the diameter of myelinated fibers, the volume of myelinated fibers, the volume of myelin sheaths, the myelin sheath thickness, and the outer and inner perimeters of myelin sheaths. These parameters of young, middle-aged and old-aged female Long-Evans rats were quantitatively studied using stereological techniques.3 methodsFive young (7 month old, 250±20 g), five middle-aged (18 month old, 350±25 g) and five old-aged female Long-Evans rats (27 month old, 380±40 g) were used in this study. The Long-Evans rats were anaesthetized and perfusion-fixed. After perfusion, the cerebral hemispheres were taken out and cut midsagittally into two hemispheres. The right or left hemisphere was selected at random for analysis. The corpus callosum was divided into three parts using a method based on the previous work by Witelson (1989). The anterior region represents the anterior five-fifteenths of the total corpus callosum length, which includes rostrum, genu and rostral body. Splenium represents the posterior three-fifteenths of the total corpus callosum length. The mid-region represents the middle remainder of the total corpus callosum length, which includes anterior midbody, posterior midbody and isthmus. The hemisphere was seriately cut into 1 mm-thick slabs. Transmission electron microscopic sections were prepared by general methods. From each section, 5 to 6 fields of vision were randomly photographed at a magnification of 14400x. 10 to 12 pictures were obtained per rat in the mid-region and splenium, respectively. In this study, we investigated the volume of the mid-region and splenium of rat corpus callosum, the length density, the total length and the diameter of myelinated fibers, the volume of myelinated fibers, the volume of myelin sheaths, the myelin sheath thickness, and the outer and inner perimeters of myelin sheaths. These parameters of young, middle-aged and old-aged female Long-Evans rats were quantitatively studied using stereological techniques.4 Results4.1 The mid-region volume in old-aged rats was 21.5 % (p<0.05) and 18.1 % (p<0.01) smaller than those in young rats and middle-aged rats, respectively. The splenium volume did not change with age (p>0.05).4.2 The total length of the myelinated fibers in the mid-region of old-aged rats was 28.8% and 27.7% shorter than those in young rats and middle-aged rats, but the changes were not significant(p>0.05). The total length of the myelinated fibers in the splenium did not change with age (p>0.05).4.3 In the mid-region and splenium of corpus callosum, the mean diameters of myelinated fibers did not change with age.4.4 In the mid-region of the corpus callosum, the total length of the myelinated fibers with the diameter of 0.3~0.9μm was non-significantly decreased in old-aged rats. The total length of the myelinated fibers with the diameter of 0.7~0.8μm in old-aged rats (3.152km) was significantly decreased by 52.3 %(p<0.05), when compared to young rats (6.602km). The total length of the myelinated fibers with the diameter of 1.5~1.6μm in old-aged rats (0.614 km) was significantly increased by 4.55 times(p<0.05), when compared to young rats (0.114 km). In the splenium of corpus callosum, the total length of the myelinated fibers with the diameters of 0.4～0.7μm in old-aged rats increased, when compared to young rats and mid-aged rats. However the changes were not statistically significant. The t otal length of the myelinated fibers with the diameters of 1.2～1.3μm in old-aged rats (0.189 km) was significantly increased by 1.86 times (p<0.05) and 1.55 times (p<0.05), when compared to young rats (0.066 km) and mid-aged rats (0.074 km). The total length of the myelinated fibers with the diameters of 1.4～1.5μm in old-aged rats (0.113 km) was significantly increased by 21.6 times (p<0.05), when compared to young rats (0.005 km).4.5 The mean myelinated fiber volumes in the mid-region of corpus callosum were 27.84±1.29 mm~3 in young rats, 29.09±1.39 mm~3 in middle-aged rats and 20.12±1.41 mm~3 in old-aged rats. The myelinated fiber volumes in old-aged rats were significantly smaller than in young rats and middle-aged rats. The mean myelinated fiber volumes did not change with age in splenium.4.6 When compared young rats and mid-aged rats, the myelin sheath volume was decreased in the mid-region of old rats (p<0.01), but not in the splenium of old rats.4.7 There were not significant changes in the mean diameters of myelinated fibers, the mean diameters of axons and mean myelin thickness in the mid-region and splenium of rat corpus callosum. In the mid-region of corpus callosum, the myelin sheath thickness of the myelinated fibers with the diameters of 0.9~1.0μm and 1.2~1.3μm in middle-aged rats was increased (p<0.01 and p<0.05, respectively), when compared to young rats. The myelin sheath thickness of the myelinated fibers with diameters of 1.5~1.6μm in middle-aged rats was smaller than that in young rats, and was larger than that in old-aged rats (p<0.05). In the splenium of corpus callosum, the myelin sheath thickness of the myelinated fibers with diameters of 0.9~1.0μm and 1.1~1.2μm was increased in old-aged rats when compared to young rats.4.8 The mean difference of the outer and inner perimeters of myelin sheaths did not change with age in the mid-region and splenium of corpus callosum. The mean quotient of the outer and inner perimeters of myelin sheaths did not change with age in the mid-region and splenium of rat corpus callosum. The difference of the outer and inner perimeters of myelin sheaths in the myelinated fibers with diameters larger than 0.9μm was increased in both middle-aged and old-aged rats, when compared to those in young rats. The quotient of the outer and inner perimeters of myelin sheaths in the myelinated fibers with diameters larger than 0.9μm was increased in both middle-aged and old-aged rats, when compared to those in young rats. The difference and quotient of the outer and inner perimeters of myelin sheaths indicated that the myelin sheaths of larger-diameter myelinated fibers become thicker with age in the mid-region of corpus callosum. In splenium, both the difference and quotient of the outer and inner perimeters of myelin sheaths with larger diameters were increased in middle-aged and old-aged rats, when compared to those in young rats.5 Conclusion5.1 In rat corpus callosum, the mean mid-region volume was decreased significantly. The mean splenuim volume did not change with age.5.2 The significant decrease of the total length of the myelinated fibers with smaller diameters in old rats may be one of the important reasons for the decrease of the mid-region volume in old rat corpus callosum.5.3 In splenium, the total length of the myelinated fibers with larger diameters was increased with age.5.4 In the mid-region and splenium of corpus callosum, the myelin sheath thickness of thicker fibers was increased with age, but the myelin sheath thickness of thinner fibers did not change with age.