Clinical and Behavioural Neurology, IRCCS Santa Lucia Foundation, Rome, Italy.
J Alzheimers Dis. 2010;20(1):67-95. doi: 10.3233/JAD-2010-1370.
The corpus callosum (CC), which connects the two cerebral hemispheres, is the largest white matter fiber bundle in the human brain. This structure presents a peculiar myelination pattern: it has small diameter fibers, located in the genu, which myelinate much later in normal development, and large diameter fibers of the splenium, which myelinate early in development. Although the pathology of AD mainly involves the cerebral gray matter structure, there is evidence that white matter may also be involved. To illustrate callosal white matter changes in AD pathology, in this review we summarize in vivo imaging studies in humans, focusing on region of interest, voxel-based morphometry, diffusion-weighted imaging, and diffusion tensor imaging techniques. Our aims were to identify where in the CC, when in the different stages of AD, and how callosal changes can be detected with different MRI techniques. Results showed that changes in the anterior (genu and anterior body) as well as in the posterior (isthmus and splenum) portions of the CC might already be present in the early stages of AD. These findings support the hypothesis that two mechanisms, Wallerian degeneration and myelin breakdown, might be responsible for the region-specific changes detected in AD patients. Wallerian degeneration affects the posterior CC subregion, which receives axons directly from those brain areas (temporo-parietal lobe regions) primarily affected by the AD pathology. Instead, the myelin breakdown process affects the later-myelinating CC subregion and explains the earlier involvement of the genu in CC atrophy.
胼胝体（CC）连接两个大脑半球，是人类大脑中最大的白质纤维束。该结构呈现出一种特殊的髓鞘形成模式：小直径纤维位于膝部，在正常发育过程中髓鞘形成较晚，而大直径纤维位于压部，在发育早期髓鞘形成。尽管 AD 的病理学主要涉及大脑灰质结构，但有证据表明白质也可能受累。为了说明 AD 病理学中的胼胝体白质变化，在本综述中，我们总结了人类的体内成像研究，重点介绍了感兴趣区、体素形态计量学、弥散加权成像和弥散张量成像技术。我们的目的是确定 CC 的哪个部位、AD 的哪个阶段以及不同的 MRI 技术如何检测到胼胝体的变化。结果表明，CC 的前部（膝部和前部体部）以及后部（峡部和压部）的变化可能已经存在于 AD 的早期阶段。这些发现支持了这样一种假设，即两种机制——沃勒氏变性和髓鞘破坏——可能是导致 AD 患者检测到的区域特异性变化的原因。沃勒氏变性影响接收来自受 AD 病理影响最大的脑区（颞顶叶区域）的轴突的后部 CC 亚区。相反，髓鞘破坏过程影响较晚髓鞘形成的 CC 亚区，解释了 CC 萎缩中膝部较早受累的原因。
