What does anisotropy measure? Insights from increased and decreased anisotropy in selective fiber tracts in schizophrenia.
What does anisotropy measure? Insights from increased and decreased anisotropy in selective fiber tracts in schizophrenia.
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DOI：
10.3389/fnint.2013.00009
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发表时间：
2013
影响因子：
3.5
通讯作者：
de Erausquin GA
中科院分区：
文献类型：
--
作者：
Alba-Ferrara LM;de Erausquin GA
精神分裂症是一种常见的、严重的、慢性致残的精神疾病，原因不明。近年来的MRI研究主要集中在利用弥散张量成像（DTI）研究精神分裂症患者的白质异常。通常由DTI导出的指数包括(1)独立于方向的平均扩散系数，(2)分数各向异性（FA）或相对各向异性（RA），(3)轴向扩散系数，(4)径向扩散系数。在脑白质中，对这些指标的贡献来自纤维排列、髓鞘形成程度和轴突完整性。相对纯粹的髓磷脂缺陷导致径向扩散率适度增加，不影响轴向扩散率，并保留各向异性。虽然精神分裂症不以白质的明显异常为特征，但它确实涉及髓磷脂相关基因表达的严重失调、少突胶质细胞数量的减少和髓鞘超微结构的明显异常。由于每个少突胶质细胞形成多达40个轴突节段，因此少突胶质细胞（OLG）数量和/或髓鞘完整性的变化和/或轴胶质接触的变化可能对信号传播和神经元回路的完整性产生深远的影响。尽管许多研究显示精神分裂症各向异性的减少不一致，但我们和其他人发现，与精神分裂症症状产生相关的关键皮质下束FA增加。我们回顾了揭示精神分裂症患者及其未受影响亲属中脑多巴胺能束各向异性增加的数据，并讨论了这一发现可能的生物学基础和生理意义。
Schizophrenia is a common, severe, and chronically disabling mental illness of unknown cause. Recent MRI studies have focused attention on white matter abnormalities in schizophrenia using diffusion tensor imaging (DTI). Indices commonly derived from DTI include (1) mean diffusivity, independent of direction, (2) fractional anisotropy (FA) or relative anisotropy (RA), (3) axial diffusivity, and (4) radial diffusivity. In cerebral white matter, contributions to these indices come from fiber arrangements, degree of myelination, and axonal integrity. Relatively pure deficits in myelin result in a modest increase in radial diffusivity, without affecting axial diffusivity and with preservation of anisotropy. Although schizophrenia is not characterized by gross abnormalities of white matter, it does involve a profound dysregulation of myelin-associated gene expression, reductions in oligodendrocyte numbers, and marked abnormalities in the ultrastructure of myelin sheaths. Since each oligodendrocyte myelinates as many as 40 axon segments, changes in the number of oligodendrocytes (OLG), and/or in the integrity of myelin sheaths, and/or axoglial contacts can have a profound impact on signal propagation and the integrity of neuronal circuits. Whereas a number of studies have revealed inconsistent decreases in anisotropy in schizophrenia, we and others have found increased FA in key subcortical tracts associated with the circuits underlying symptom generation in schizophrenia. We review data revealing increased anisotropy in dopaminergic tracts in the mesencephalon of schizophrenics and their unaffected relatives, and discuss the possible biological underpinnings and physiological significance of this finding.