Morin C L, Dolina S, Robertson R T, Ribak C E
Department of Anatomy and Neurobiology, University of California, Irvine 92717.
Cereb Cortex. 1994 Mar-Apr;4(2):119-28. doi: 10.1093/cercor/4.2.119.
BALB/c mice lack a corpus callosum in about 11% of the population. Two inbred substrains of BALB/c mice, epilepsy-prone (EP) and epilepsy-resistant (ER), have been examined to determine whether these substrains differ in regard to corpus callosum morphology. Further, this study addressed the issue of whether misrouted cortical axons form an aberrant pathway instead of the corpus callosum. Initial studies that examined fresh brain tissue of adult animals revealed normal corpora callosa in all ER mice but deficient or absent corpora callosa in all EP mice. Subsequently, Dil crystals were placed in the motor cortices of aldehyde-fixed brains of 2-week-old animals to investigate cortical projections in both inbred substrains of mice. Fluorescent microscopy revealed that all of the ER animals had normal corpora callosa, whereas all EP animals exhibited either reduced corpora callosa (partially callosal) or an absence (acallosal) of this structure. Both acallosal and partially callosal EP mice displayed an extensive, aberrant projection to the basal forebrain as well as bilateral projections to midline and intralaminar thalamic nuclei. The fibers projecting to the basal forebrain arose from the cortex, coursed toward the midline before turning ventrally along the midline, and appeared to terminate in the medial septal nucleus and the nucleus of the diagonal band. ER animals lacked this aberrant cortical projection to the basal forebrain. Electron microscopic results obtained from EP mice indicated that labeled axons in this aberrant pathway formed axosomatic, axodendritic, and axospinous synapses with the neurons in the medial septal/diagonal band complex. The function of the aberrant projection to the basal forebrain remains unknown but it may provide an abnormal excitatory input to a region that provides cholinergic and GABAergic input to the cerebral cortex and hippocampus. The additional projections to midline and contralateral intralaminar thalamic nuclei in EP mice may function to intensify the synchronization of bilateral discharges.
在大约11%的群体中，BALB/c小鼠缺乏胼胝体。对BALB/c小鼠的两个近交亚系，即癫痫易感（EP）和癫痫抗性（ER）亚系进行了研究，以确定这些亚系在胼胝体形态方面是否存在差异。此外，本研究还探讨了皮质轴突错路是否会形成一条异常通路而非胼胝体的问题。最初对成年动物新鲜脑组织的研究显示，所有ER小鼠的胼胝体正常，但所有EP小鼠的胼胝体发育不全或缺失。随后，将Dil晶体置于2周龄动物经醛固定的脑的运动皮质中，以研究这两个近交亚系小鼠的皮质投射。荧光显微镜检查显示，所有ER动物的胼胝体正常，而所有EP动物的胼胝体要么减少（部分胼胝体），要么缺失（无胼胝体）。无胼胝体和部分胼胝体的EP小鼠均表现出向基底前脑的广泛异常投射以及向中线和丘脑板内核的双侧投射。投射到基底前脑的纤维起源于皮质，在转向腹侧沿中线前行之前先向中线走行，并且似乎终止于内侧隔核和斜角带核。ER动物缺乏这种向基底前脑的异常皮质投射。从EP小鼠获得的电子显微镜结果表明，这条异常通路中标记的轴突与内侧隔/斜角带复合体中的神经元形成了轴-体、轴-树突和轴-棘突触。向基底前脑的异常投射的功能尚不清楚，但它可能会为一个向大脑皮质和海马提供胆碱能和GABA能输入的区域提供异常的兴奋性输入。EP小鼠中向中线和对侧丘脑板内核的额外投射可能起到加强双侧放电同步性的作用。
