Flashcards in Development of the CNS Deck (31):
Describe the three main stages of the development of the neural tube?
There are three main layers (trilaminar); endoderm, mesoderm and ectoderm. The midline of the ectoderm proliferates and forms the neural plate.
The neural plate folds over and becomes the neural fold.
The edges of the neural fold begin to proliferate in a different way and form the neural crests which are parallel to the edge of the neural tube (formed by the two edges joining together) with a neural canal.
What does the neural tube develop into and otherwise known as?
Known as the neuroepithelium
Precursor of all the cells in the CNS
What does the neural crest develop into?
Precursor of all the cells in the PNS
What are the cells that the neuroepithelium can differentiate into?
Neuroblasts; all the neurones with cell bodies in the CNS (including motor neurones)
Glioblasts; astrocytes and oligodendrocytes (not microglial cells)
Where are microglial cells derived from and what do they do?
The mesoderm as they are of haemotopoietic lineage
They are macrophages of the CNS
What is a key feature of neural crest cells?
They are capable of migrating over long distances
What are the different cell types that develop from the neural crest?
Post-ganglionic autonomic neurones
Sensory neurones of the dorsal root and cranial ganglia
Non-neuronal derivatives e.g. melanocytes
Describe the proliferation of the neuroepithelium
There is a single layer of neural cells which are seen at different positions and are attached to the inner and outer membrane of the neural tube.
Cells contract downwards to enter the cell cycle
Describe the differentiation of the neuroepithelium into neuroblasts, glioblasts and ependymal cells
The cells which are nearest the inner membrane undergo mitosis. The daughter cell which is lowest stays attached to the inner membrane and undergoes the cell cycle again.
The other daughter cell migrates away from the inner membrane and starts to develop dendrites, one of which will become an axon (neuroblasts).
In the development of glioblasts, it is similar but there will be no axon and they will migrate into grey or white matter.
In the development of ependymal cells, the daughter cells remain close to the inner membrane and create a thin layer of cells lining the ventricular system.
What are the layers of the differentiating neuroepithelium called?
The ependymal/germinal layer
The grey matter/mantle layer; cell bodies located here
The white matter/marginal layer
What is the roof and floor plate of the neural tube?
It is showing the most dorsal and ventral parts of the canal (respectively).
What drive the process of differentiation and migration?
Signalling molecules produced by the tissue surrounding the neural tube (or cells within the neural tube).
Why is location and timing important in the process of neuroepithelium differentiation?
The source tissues secrete signalling molecules and form a conc. gradient, it depends where the cell is in relation to the gradient to the way it develops.
Cells must develop receptors on their membranes to interact with the signalling molecules, and there are signalling molecules which have different functions at different stages of development.
Therefore: concentration gradient and receptors are key factors.
Cell migration and axonal growth occurs by a process of attraction or repulsion to the molecules.
What does the grey matter/mantle layer develop into?
The dorsal part (alar plate) will develop interneurones, they will receive sensory information form the dorsal root ganglia developing in the neural crests.
The ventral part (basal plate) will develop stop interneurones but also motorneurones (axons go out into the periphery).
What is the dorso-ventral patterning?
The differentiation of neurone development in the dorsal and ventral parts of the neural tube into interneurones and motor neurones.
What is the signalling molecule that encourages the development of motor neurones?
Sonic hedgehog signalling molecule from the notochord (below the basal plate) affect cells in the basal plate (secreted as a conc. gradient) and encourages it to develop into motor neurones.
What is the signalling molecule that encourages the development of interneurones?
The ectoderm of the dorsal part of the neural tube secretes signalling molecules which encourage cells to remain as interneurones.
Describe the development of the brain at 4 weeks
The brain develops from the most anterior part of the neural tube.
Forms three primary vesicles (the rest develop into the spinal cord);
Mesencephalon/ Future midbrain
Describe the development of the brain at 5 weeks
The first and third primary vesicles enlarge and divide into two vesicles -> five secondary vesicles.
Forebrain; telencephalon (forms two cerebral hemispheres) and diencephalon
Hindbrain; pons and medulla
Describe the development of the brain at 8 weeks
Proliferation of the wall causes there to be smaller neural lumen which will become the ventricular system.
The developing cerebral hemispheres (two parts of the telencephalon) have a lateral ventricle each
Diencephalon has a third ventricle
Midbrain has the cerebral aqueduct in the midline
Hind brain has the fourth ventricle in the middle
What does the cerebellum develop from?
Out poutching of the wall of the pons late in development
What are the folds of the developing brain at 4 weeks?
What occurs in the folding of the brain over the next few weeks and at 8 weeks?
The flexures become exaggerated.
At 8 weeks, the telencephalon is so large that it covers most of the diencephalon and parts of the midbrain.
Describe the development of the brainstem?
The roof plate starts to proliferate rapidly and the alar plate splits and lies laterally.
The roof plate now forms the fourth ventricle.
The cranial nerve nuclei with motor function lie medially.
The nuclei with sensory function lie laterally.
What are the basal ganglia and describe the purpose of radial glial cells?
Basal ganglia are the neuroblasts which remain near the inner membrane.
The migration of the other neuroblasts to the outer membrane occurs by attaching themselves to radial glial cells and climbing up them. Radial glial cells have cell bodies attached to the inner membrane and long processes to the cortical plate (outer membrane).
What gives rise to the cortical layers and how many are there?
Successive waves of proliferation and migration gives rise to the six layers of the cerebral cortex
What is thought to be the cause of major cognitive problems?
Fragile X syndrome
Name some environmental factors affecting the development of the brain
Fetal alcohol syndrome
What is a contributing factor to having spina bifida?
Folic acid deficiency
Spina bifida is when the neural folds do not close properly at 3/4 weeks gestation causing an incomplete neural tube.
This will cause a divided vertebral arch (not fully developed over the spinal cord) and result in the spinal cord protruding out.
Characterised by lower limb weakness/paralysis and bladder and bowel control problems
How does too much or too little vitamin A play a part in brain development?
Retinoic acid is derived from vitamin A which is a signalling molecule and can affect the development