| Objective : With the acceleration of aging process,Alzheimer's disease(AD),which is the most common type of dementia worldwide and a kind of high incidence disease,may become a serious medical and social problem.At the same time,AD also brings huge economic burden to the family.Researchers have been exploring the etiology and pathogenesis of early AD,but these issues are not completely understood yet.In recent years,the studies have found that APP/PS1 double transgenic mouse is an ideal animal model of Alzheimer's disease,that can be used to investigate the neurobiology and pathogenesis of AD.Previous neurobiological researches mainly concentrated on the synapses and neurons in APP/PS1 transgenic mouse brain.Diffusion tensor imaging(DTI)found that the fractional anisotropy(FA)value,which reflects the integrity of myelinated nerve fibers and myelin sheaths in neural circuits,was decreased in the hippocampus of AD patients,which indicated that the damaged integrity of the neural circuit and myelin sheath breakdown in the hippocampus of AD.However,until now,there have been no quantitative studies on the myelinated nerve fibers and myelin sheaths in the hippocampus of the early APP/PS1 transgenic mice.So what are the quantitative changes of the myelinated nerve fibers and myelin sheaths in the hippocampus during early AD? In this study,Morris water maze test was used to detect the spatial learning and memory abilities of the mice,and then the myelinated nerve fiber changes in the hippocampus of the early APP/PS1 transgenic AD mice were investigated with the accurate three-dimensional quantitative stereological methods.This study might provide accurate morphological data for exploring the structural basis of the spatial learning and memory impairment in the early stage of AD mice and for further searches for the pathogenesis of AD.Methods : The spatial learning and memory abilities of the 10-month-old wild-type group mice and the 10-month-old APP/PS1 group mice were tested using the Morris water maze for six consecutive days.From day 1 to day 5,the time latency was recorded for the spatial learning capability.On day 6,the target zone frequency and the percentage of time in target quadrant zone were recorded for the spatial exploration and memory capability.After Morris water maze,seven mice were randomly picked up from each group.The brain was fixed and dissected.One of the two hemispheres from each mouse was selected at random for experiment.Each hemisphere was cut into 1mm thick slabs along the coronal direction.Each brain slice was photographed using the dissect microscope.The total volume of the white matter was calculated using the Cavalieri's principle.Then,a 20 ?m thick frozen section was cut using the freezing microtome in the surface of pillow on each slice.The cut sections were stained with hematoxylin,and photographed under light microscope(×40 magnification).Then,the boundary of dentate gyrus(DG)and the boundary of CA1 were drawn using the stereology analysis system.Equidistant test points were put on each brain slice randomly.The total points hitting the CA1 field and the total points hitting the DG were counted,respectively.According to the volume of the hippocampus,the DG volume and CA1 volume were calculated.From each hemisphere,3 to 4 tissue blocks with the size of 1mm3 were sampled in the places where the points hitting CA1 field and the points hitting DG.The tissues were embedded using the isector methods to ensure isotropic uniform random sampling.Then the tissues were embedded using the conventional electrion microscope embedding method.One section about 60 nm-thickness was cut from each epon block using an ultramicrotome and viewed in a transmission electron microscope.Twenty vision fields were randomly chosen and photographed at a magnification of 8000 from each section.The volumes of hippocampus,CA1 and DG,the total length of the myelinated nerve fibers and the volumes of the myelinated nerve fibers and myelin sheathes in the CA1 field and DG were calculated using the unbiased stereological methods.Results:1.Compared with the wild-type group mice,the mean escape latency in the APP/PS1 group mice was significantly increased(P < 0.05).The percentage of the time in the target quadrant in the APP/PS1 group mice was significantly decreased compared to the wild-type group mice(P < 0.01),while there was no significant difference in the times of platform location crosses between the two groups(P > 0.05).2.Compared with the wild-type group mice,the volumes of the hippocampus,field CA1 and DG in the APP/PS1 group mice were significantly decreased(P < 0.01,P < 0.05,P < 0.01,respectively).3.Compared with the wild-type group mice,there were no significant changes in the total length of the myelinated fibers within the CA1 field and the total length of the myelinated fibers within the DG in the APP/PS1 group(P > 0.05,P > 0.05,respectively).However,compared to the wild-type group mice,the total volumes of the myelinated fibers in the CA1 field and DG of the APP/PS1 group mice were significantly decreased(P < 0.05,P < 0.05,respectively).Moreover,the total volumes of the myelin sheathes in the CA1 field and DG of the APP/PS1 group mice were significantly longer than those in the wild-type group mice(P < 0.05,P < 0.01,respectively).Conclusions: The spatial learning and memory abilities in the 10-month-old APP/PS1 transgenic mice were decreased significantly compared to the same old wild-type mice.The volumes of the myelinated nerve fibers and myelin sheathes within the field CA1 and DG in the 10-month-old APP/PS1 transgenic mice were decreased significantly compared to the same old the wild-type mice.These results indicated that there were marked demyelinations of the myelinated fibers within the CA1 field and DG 10-month-old APP/PS1 mice.The demyelination of the myelinated fibers within the CA1 field and DG of 10-month-old APP/PS1 mice might be one of the structural bases for the spatial learning and memory deficits of the early transgenetic AD mice. |
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