Dividing the Central Nervous System
The physical division of the prospective brain from the prospective spinal cord is done by occluding the lumen of the neural tube at the boundary between these regions. As the neural folds close in the region between the presumptive brain and spinal cord, the surrounding dorsal tissues push inward to constrict the neural tube at the base of the brain (Figure 1; Desmond 1982; Desmond and Schoenwolf 1986). The prospective brain cells then secrete cerebrospinal fluid into the lumen, expanding the embryonic brain’s volume enormously. In chicks and zebrafish, if the fluid pressure in the anterior portion of an occluded neural tube is experimentally decreased, the brain forms abnormally and contains many fewer cells than normal (Desmond and Levitan 2002; Lowery and Sive 2005). The occluded region of the neural tube reopens after the initial rapid enlargement of the brain ventricles. The anterior-posterior patterning of the hindbrain and spinal cord is controlled by a series of genes that include the Hox gene complexes.
Figure 1 Occlusion of the neural tube allows expansion of the future brain region. (A) Dye injected into the anterior portion of a 3-day chick neural tube fills the brain region but does not pass into the spinal region. (B,C) Sections of the chick neural tube at the base of the brain before occlusion (B) and during occlusion (C). (D) Reopening of the occlusion after initial brain enlargement allows dye to pass from the brain region into the spinal cord region. (Courtesy of M. Desmond.)
Literature Cited
Desmond, M. E. 1982. A description of the occlusion of the lumen of the spinal cord in early human embryos. Anat. Rec. 204: 89–93.
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Desmond, M. E. and G. C. Schoenwolf. 1986. Evaluation of the roles of intrinsic and extrinsic factors in occlusion of the spinal neurocoel during rapid brain enlargement in the chick embryo. J. Embryol. Exp. Morphol. 97: 25–46.
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Desmond, M. E. and M. L. Levitan. 2002. Brain expansion in the chick embryo initiated by experimentally produced occlusion of the spinal neurocoel. Anat. Rec. 268: 147–159.
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Lowery, L. A. and H. Sive. 2005. Initial formation of zebrafish brain ventricles occurs independently of circulation and requires the nagie oko and snakehead/atp1a1a.1 gene products. Development 132: 2057–2067.
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