Development of the CNS Flashcards
What is the wall of the neural tube called?
neuroepothelium
What happens during the early development of the CNS?
The first thing that happens is that you get a proliferation of the ectoderm in the dorsal midline of the embryonic disc
This proliferation is called the neural plate
As this thickens, the neural plate starts to fold up on the sides
Eventually the two neural folds fuse dorsally to form a tube
The space in the middle of the tube is called the neural canal
You get a little bunch of cells at the tip of the neural fold called neural crests
The neural crests separate from the neural tube and doesn’t take part in the fusion but lies alongside it
At the end of this process you have a neural tube lying in the midline dorsally in the embryo and on either side of this you have two strips of neural crest tissue
The neural tube and the neural crest are two independent sources of nervous tissue
Neural Tube = all CNS cells
Neural Crest = all PNS cells
What are neuroblasts?
Precursors for all neurones that have their cell bodies within the CNS
What re glioblasts?
These become neuroglia - astrocytes and oligodendrocytes
What are ependymal cells?
Lining ventricles and central canal
These remain close to the inner membrane of the neural tube and they spread out and form a lining around the developing ventricular system
What are the different types of neural crest cells?
Sensory neurones of dorsal root ganglia and cranial ganglia
Postganglionic autonomic neurones
Schwann cells
Non-neuronal derivatives e.g. melanocytes
Describe the proliferation of the neuroepithelium.
There is an inner membrane at the bottom and an outer membrane at the top
Almost all these cells are attached to BOTH the inner and outer membranes - it’s just that the nuclei are in different positions
The fat cells at the bottom are going through mitosis
What are the different cells in the neuroepithelium?
Firstly, you get the cells withdrawing from the outer membrane towards the inner membrane and then it goes through mitosis
One of the daughter cells will stay attached to the inner cell membrane, it gets bigger and goes into the cell cycle again
The other daughter cell migrates away from the inner membrane and then develops into neuroblasts
They develop processes (one will become the axon) and these axons are directed away from the inner membrane again
As this occurs over and over you end up with THREE layers
There is one layer by the inner membrane where you get mitosis occurring, another layer where you mainly have cell bodies and another that has mainly axons
This is the beginning of the difference between grey and white matter
Glioblasts also show a similar pattern of differentiation and migration to neuroblasts but glioblasts can migrate into WHITE matter as well
REMEMBER: glioblasts DO NOT develop axons (but do develop processes)
Ependymal cells just remain in the ependymal layer
What are the layers of the neural tube?
The ependymal layer (germinal layer) surrounds the neural canal
then grey matter (mantle layer) and then white matter (marginal layer)
The roof plate and the floor plate are landmarks that show the most dorsal and the most ventral parts of the canal
What controls neural tube differentiation?
differentiation and migration is controlled by signalling molecules that are secreted either from the tissues surrounding the developing neural tube or sometimes secreted by cells within the neural tube
signalling molecules interact with receptors on the developing neuroblasts and also guide the developing axons
The signalling molecules can either cause attraction or repulsion and the direction is determined by the concentration gradient - close to the source of the signalling molecules there will be a higher concentration
The other important factor is TIMING - there is no point producing signalling molecules if the developing neurones don’t have the right receptors
Some of the signalling molecules have different effects at different
What are two significant features of the developing spinal cord?
- The neural canal is even smaller compared to the thickness of the wall
- The grey matter has split into TWO different types:
- Alar Plate - Dorsal
- Basal Plate - Ventral
The interneurons in the alar plates are becoming specialised to receive sensory information and this information comes from the developing dorsal root ganglia that have developed from the neural crest
In the basal plate you get some interneurons and the development of motor neurones
This means that the basal plate has a motor function - the axons leave the spinal cord to go towards muscles
How is the mature spinal cord different to the developing spinal cord?
The neural canal has become the central canal carrying CSF
Alar plates develop into the dorsal horns
Basal plates develop into ventral horns
The whole spinal cord is surrounded by a thick layer of white matter
Outline the development of the brainstem.
in the interim between the spinal cord and the brain - neural tube has split roof plate -> much bigger layer of ependymal layer->sensory neurons tend to be more lateral
In the region that becomes the brainstem, the first thing that happens is the development of the 4th ventricle
The roof plate starts proliferating rapidly and the dorsal part of the brainstem expands laterally
As the roof plate expands it pushes the alar plates aside so they are no longer dorsal to the roof plate - they are actually lateral to the roof plate
What are the divisions of the brain at 4 weeks after fertilisation?
Only the most anterior bit of the neural tube develops into the brain
You get differentiation of the wall of the anterior neural tube to form THREE primary vesicles:
Prosencephalon - future forebrain
Mesencephalon - future midbrain
Rhombencephalon - future hindbrain
The rest becomes the spinal cord
What happens to the brain at 5 weeks after fertilisation?
Over the next week, the most anterior vesicle divides in two and the third vesicle also divides in two -> this produces 5 secondary vesicles.
You get an enormous expansion of the top part of the developing forebrain and this telencephalon will become the cerebral hemispheres
There is less expansion in the lower part of the developing forebrain because that part becomes the diencephalon
The developing midbrain doesn’t expand very much
The developing hindbrain divides in two to becomes the pons and the medulla
What happens to the developing brain at 8 weeks?
As development continues you get more and more growth of the wall of the neural tube
The space within it has become smaller relative to the wall and this space becomes the ventricular system
Coming quite late, you have the first development of the cerebellum - out-pouching from the back of the pons
How do the flexures in the brain develop?
The vesicles aren’t in a straight line - there are THREE flexures : cephalic, pontine, cervical
As you go through development these flexures become exaggerated
By 8 weeks, the telencephalon has got so big that it’s starting to move back and cover the diencephalon
Outline the development of the cortex.
The brain has a core of white matter with grey matter around the outside
The grey matter consists of nuclei that have migrated from the inner membrane of the neural tube
The neuroblasts are proliferating near the inner membrane
Some of the neuroblasts will stay in the middle and form the basal ganglia
There are other neurones that then migrate towards the outer membrane - migration takes place by the neuroblasts attaching themselves to radial glial cells - these have their cell bodies anchored in the inner membrane and have a single long process that goes to the outer membrane
You get a wave of proliferation near the inner membrane and then a wave of migration towards the other surface and then one layer of the cortex is formed
Then you get another wave of proliferation and another wave of migration forming the 2nd layer of cortex
This continues until you have SIX LAYERS of cells within the cerebral cortex
Each layer of cells has different types of cells with different functions and different connections
What are dome developmental disorders?
Neural development involves several complex, timed processes - may be disrupted by genetic or environmental abnormalities
This occurs early in gestation
Schizophrenia is caused by a malfunction of neural development
What are some examples of neural tube defects?
anencephaly - open brain and lack of skull vault (usually still born)
encephalocele - herniation of meninges (and brain)
Deficiency of folic acid can lead to spina bifida - no closure of cordal part of tube
closed spinal dysraphism
menigocele
mylomeningocele
