Embryology and Development of the Nervous System Flashcards
Describe the process of neuralation
The nervous system is derived from a patch of ectoderm known as the neural plate.
The neural plate folds in on itself to produce a neural groove.
The groove folds completely and encloses completely to produce a hollow neural tube that is one cell thick
The neural tube breaks free of overlying ectoderm and becomes a stand alone structure.
Neural crest cells are formed from the superior cells of the neural tube
The closing of the neural tube tends to begin in the middle of the length of the neural groove. It ‘zippers’ shut both rostrally and caudally in addition to several other independent zippering events. The attached image shows the zipping patterns.
What happens if the neural tube fails to close in areas?
There are two main conditions that result from defects in neuralation:
Anencephaly
Is the most dramatic failure where the failure of the forehead neural tube to close results in no brain or skull -> still born
Spina bifida
Is the most common failure whereby there is incomplete closure of zone 5 of neuralation
- results in movement disorders and stunted development
- high levels of folate are protective of spina bifida
What role do neural crest cells play in the development of the nervous system?
Neural crest cells are derived from the superior aspect of the neural tube and are extremely mobile cells that migrate away from the neural tube to form a wide range of structures.
Neural crest derivatives include:
Peripheral nervous system
- Dorsal root ganglia
- Sympathetic and parasympathetic ganglia
- Enteric ganglia
- Schwann cells
- Melanocytes
- Muscle cartilage and bone of skull, jaws, face
and pharynx
• Dentine
What is particularly interesting about the migration of neural crest cells to form the enteric nervous ganglia?
Enteric migration to become enteric ganglia is the longest migration of cells in the body. They must replicate and migrate rapidly to keep up with a rapidly extending gut.
This mobility is lost in mature cells -> except for cancerous cells that somehow rediscover this ability and become metastatic
What is neuroepithelium?
Neuroepithelium is the term given to the single cell thick lining of the neural tube.
It separates the ventricular system (hollow inside) from the outgrowing nervous system. In the early stages of development the brain is largely empty space.
What is the ventricular zone?
Neuroepithelium adds layers of cells from the inside out to generate a brain cortex. All new neurons are born at the ventricular surface known as the ventricular zone.
This ventricular zone contains stems cells
New neurons migrate from this zone and contribute to the ‘thickening’ of a greater brain structure.
What are radial glia?
Radial glia are glial cells that direct new migrating neurons through the layers of the brain to their destination -> akin to a stake for a climbing plant.
Describe two disorders of development that result from mutations in genes that affect migration
Reelin Mutation
No convoluted cortex surface and reduced white matter due to lack of cortex cells
Lissencephaly (DCX)
DCX is the doublecortin gene -> smooth unfolded cortex due to lack of cells in the cortex
What are two developmental disorders that result in disease of the nervous system?
Fragile X syndrome
Is the most common cause of male autism resulting from FMR1 gene mutation of synaptic and dendritic development
Schizophrenia
Evidence of a strong developmental component in its genesis
What is induction?
Induction is the signalling between structures or tissues in development involving ligands and receptors.
**Allows the inducing of a particular response for the next stage in development. **
What role does the notochord and floor plate play in the organisation of spinal cord development?
The notochord is a mesodermal structure deep to the neural tube that induces the formation of the floor plate - a region of cells at the deepest aspect of the neural tube
The floor plate is a significant organiser of signalling molecules (e.g sonic hedgehog) that provide chemical gradients that establish topography of the neural tube.
These gradients induce the correct positioning and development of neurons – particularly ventral horn motor neurons and interneurons
How are interneurons induced in the spinal cord?
The inducement of interneurons arises from a chain of inducement starting from the notochord.
Molecular signalling from the ventral motor neurons induces the differentiation of interneurons.
Discuss axonal growth in development
Axons are not present when a neuron first forms during development; it grows out to meet its target cell. This target cell releases diffusible and membrane bound molecular signals that homes specific growing axons to it.
The first axons – known as pioneering axons – are critical as they provide a scaffold for later neurons to innervate their targets
These axons are towed into position by the dynamic cytoskeleton of growth cones at their distal ends.
What is involved in the refinement of synaptic connections?
The preliminary processes generate a crude nervous system that requires refinement. This involves permanent changes that establish a mature and functional nervous system.
It occurs both pre- and post-natally
Discuss the concept of a ‘critical period’ in synaptic refinement
How does this apply to the visual system?
Critical period refers to the time at which the refinement of synaptic connections are made according to the relative experience / activity of these connections = use it or lose it.
Visual system
At birth, within the visual cortex, terminals from one eye interdigitate with terminal of the opposite eye. They are normally equally represented in the visual cortex.
If you cover the one eye of a cat shortly after birth during the critical period of vision development, the covered eye has greatly reduced representation while the uncovered eye has significantly elevated representation.
The once-covered eye is effectively blind after the critical period.
Note: if both eyes are covered then there is no change to representations which indicates that it is the relative neural activity not absolute neural activity that drives synaptic refinement.
