Development+of+the+Nervous+System

=**Development of the Nervous System**=


 * 1. Describe the basic germ layers and tissues that contribute to the formation of the structures found within the peripheral portion of the nervous system and the central portion of the nervous system.**

The notochord forms from an aggregation of mesoderm that invaginates and extends inwards from the primitive pit, which subsequently elongates to from the primitive streak. The notochord and paraxial mesoderm induce the overlying ectoderm to differentiate into the neural plate. The neural plate forms the neural folds, neural tube, and neural crest. The neural tube differentiates into the CNS, consisting of the brain and spinal cord. The neural crest forms most of the PNS and ANS, consisting of cranial, spinal, and autonomic ganglia.


 * 2. Describe the formation of the neural plate, neural groove, and neural tube. What induces or controls the development of these structures?  How does the general development of the cranial portion of the tube differ from the development of the caudal portion of the tube.**

The neural plate is a thickened area of ectoderm that is formed in response to signaling factors TGF-beta, activin, and FGFs released from the underlying notochord and prechordal plate. During the formation of the neural tube (neurulation), the lateral margins of the neural plate fold inward to form the neural folds.

The neural folds eventually fuse together to form the neural tube which detaches from the overlying ectoderm, allowing migration of cells from somites to go around the neural tube to form the vertebrae and neural arches. In approprtiate separation of neural tube from ectoderm can results in meningiceals or other meningeal malformations that can affect the spinal cord; in the worse case, failure of the neural tube to separate from the ectoderm can results in an exposed nervous system (fetus can survive in sterile amniotic fluid but will suffer infections neonatally). Failure of enough mesoderm migrating around the neural tube will result in neural arch defects such as spina bifida.

The fusion of the neural folds proceeds in cranial and caudal directions until only small areas remain open at both ends where they form the anterior and posterior neuropores. The cranial 2/3 of the neural plate and tube (as far caudal as the 4th pair of somites) develop into forebrain structures including the diencephalon and telencephalon. The caudal 1/3 of the neural plate and tube develop into the future spinal cord.


 * 3. Describe the development of the three developmental layers of the neural tube. What major areas of the nervous system are derived from each.**

Neural crest forms all around the margin of the neuron plate. Some form connective tissue cells and fibroblasts to augment the mesoderm.

The neural tube layer segregated into three developmental layers: the ventricular, intermediate, and marginal zones. The neuroepithelial cells in the ventricular zone form macroglia (astrocytes and oligodendrocytes) in the spinal cord and also undergo mitosis and migration to form the intermediate layer. The marginal zone composed of the outer parts of the neuroepithelial cells from the white matter of the spinal cord as axons grow into it from nerve cell bodies in the spinal cord, spinal ganglia, and brain. Some dividing neuroepithelial cells in the ventricular zone differentiate into neuroblasts and form an intermediate zone.

The lumen of the neural tube becomes the ventricular system of the CNS.

Microglia do not develop from neuroepithelium. Instead, they are derived from mesoderm and move into the neurotube after the blood supply forms. Everything other than microglia and blood vessels are derived from neuroepithelium.


 * 4. Describe the formation of the alar plate and basal plate. What major functional areas of the nervous system are derived from each.**


 * 5. Describe the positional changes that occur during the development of the spinal cord and vertebral column.**

Outside the neural tube, connective tissue forms the meninges and the mesoderm and neural crest form skeletal muscle, skull, etc. Neuro crest cells are multipotential cells – those that stay near the neural tube form the dorsal column cells, dorsal root ganglia, and sensory ganglia. Other neural crest cells migrate to form the postganglionic autonomic neurons such as sympathetic ganglia, prevertebral ganglia, post-ganglionic parasympathetic ganglia, adrenal medulla, melanocytes, satellite and Schwann cells, and primary sensory neurons.

Tall pseudostratified columnar cells make up the wall of the neural tube. Mitosis S phase occurs at the external surface of the neural tube. Proliferating cells move toward the lumen side as they undergo M phase, and progeny cells rise back up to the external surface as they undergo G phase.

Radial cells extend from the lumen to the external surface and act to direct the trajectory of migrating neuroblasts, allowing formation of cortical columns. Each generation of neurons climb up the radial glia such that the oldest cells are deepest in the cortex and each generation migrate past the previous generation, leaving back synaptic connections.


 * 6. Describe the development of the regions of the lower portion of the brainstem.**


 * 7. Describe the major features of the development of prosencephalon, diencephalon, and telencephalon.**


 * 8. Summarize the functional components of the cranial nerves and localize (theoretically) their respective nuclei in the brainstem.**


 * 9. Define and/or describe the elements of the choroid plexus, the tela chordoidea, and arachnoid villi.**


 * 10. Differentiate between the tectum and the tegmentum of the brainstem.**


 * 11. How does the structure of the cerebellum reflect its phylogenetic development.**


 * 12. What is the origin of the cells of the sympathetic nervous system? How does this contrast with the origin of the parasympathetic system.**


 * 13. Describe the morphological defects and developmental errors associated with:**


 * 14. Describe the relationships and importance of neural crest cells relative to:**