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Roles of the Mesencephalon in Chick Embryos
The mesencephalon is known to give rise to the midbrain tracts of neurons, including those of the optic lobes and tectum. However, new evidence suggests that the mesencephalon is critical in forming a portion of the cerebellum and that the neurons derived from the mesencephalon are critical in preventing epileptic seizures.
The Dual Origin of the Avian Cerebellum
The "rule" that the cerebellum is formed solely from the metencephalon has recently been challenged. While the ontogeny of cerebellum formation in mammals has not been reconsidered yet, the chick/quail chimeric system indicates that the avian cerebellum is formed from both the metencephalon and the mesencephalon (Hallonet et al., 1990; Hallonet and Le Douarin, 1993). When quail mesencephalon is grafted into the region formerly occupied by the chick mesencephalon, portions of the cerebellum can be seen to arise from the quail mesencephalic cells (Figure 1).
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| Figure 1 The presence of mesencephalon cells in the cerebellum was shown by replacing chick mesencephalon with the corresponding region from the quail. (A) Summary drawing of the distribution of quail (red triangle) and chick (green triangle) Purkinje cells and quail (red circles) and chick (green circles) ventricular epithelial cells. The Purkinje cells overlie a ventricular epithelium of the same species type. III and IV indicate the third and fourth ventricles, respectively. Is is the isthmus. (B) Derivatives of the brain vesicles, seen schematically from the dorsal surface (A from Hallonet et al., 1990; B after Hallonet and Le Douarin, 1993) |
The mesencephalic contribution is limited to a V-shaped wedge that is inserted into the developing cerebellum by a series of complex morphogenetic movements (Figure 2). By grafting smaller pieces from specific regions of the mesencephalon and metencephalon, it was shown that the entire cerebellar cortex arises from the dorsal half of the neural tube. This is consistent with the general schema of the neural tube wherein the ventral region (basal plate) produces motor neurons, the neural folds produce sensory neurons, and the dorsal half (alar plate) produces the interneurons which integrate information.
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| Figure 2 Morphogenetic movements whereby mesencephalon cells become part of the cerebellum. OT, optic tectum; IS, isthmus. (After Hallonet et al., 1990.) |
Mapping the Mutant Phenotype by Mutant Chick Chimeras
Delineating the steps of genotype to phenotype is the most challenging part of Developmental Biology. It is even more of a challenge when the phenotype is behavioral. Recent evidence has shown that the phenotype for the Fayoumi epileptic chicken is due to abnormalities of the cerebellar cells. These chicken exhibit epileptic seizures when exposed to flashing lights or loud noises. This condition is inherited as an autosomal recessive, wherein the epileptic condition is seen in the homozygotes. The "mapping" of this trait was accomplished by transplanting different regions of the mutant neural tube into that of a wild-type neural tube. Batini and colleagues (1996) found although the prosencephalon was responsible for the abnormal EEG activity of the mutant chickens, the mesencephalon of the mutant strain was necessary and sufficient to generate all the epileptic manifestations (Figure 3). Thus, it appears that particular regions of the brain are critical in generating the seizures of this form of epilepsy.
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| Figure 3 Summary of the experiments indicating that the transfer of the epileptic seizures in Fayoumi epileptic chickens is controlled by the mesencephalon, while the abnormal EEG pattern is controlled by the prosencephalon. The red sections indicate those from the mutant donor embryo. (After Batini et al., 1996). |
Literature Cited
Batini, C., Teillet, M.-A., Naquet, R., and Le Douarin, N. M. 1996. Brain chimeras in birds: application to the study of a genetic form of reflex epilepsy. Trends Neurosci. In press.
Hallonet, M. E. R. and Le Douarin, N. 1993. Tracing neuroepithelial cells of the mesencephalic and metencephalic alar plates during cerebellar ontogeny in quail-chick chimeras. Eur. J. Neurosci. 5: 1145-1155.
Hallonet, M. E. R., Teillet, M.-A., and Le Douarin, N. M. 1990. A new approach to the development of the cerebellum provided by the quail-chick marker system. Development 108: 19-31.
First posted: Apr 02, 2003 Last edit: Apr 02, 2003 by garrett
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