4.1 Development of the nervous system Flashcards
The development of the nervous system begins around 3 weeks post-fertilisation, from the trilaminar (3-layer) stage (inner endoderm, middle mesoderm, outer ectoderm): • A strip of ectoderm (neural plate) along the middle of the dorsal surface begins to proliferate and differentiate
• Neural plate folds up laterally with more proliferation; tips of the folds differentiate into ________________
• Neural folds eventually fuse in the mid-dorsal line (neural crest is not involved in the fusion), forming the neural tube (with neural canal inside) running along the mid-dorsal line
There are thus two different sources of neural tissue in the embryo:
- ________________: Central nervous system (brain + spinal cord)
- _________: Peripheral nervous system
a different tissue (neural crest);
Wall of neural tube (neuroepithelium);
Neural crest
What do neuroblasts form?
All neurones with cell bodies in the CNS
what do glioblasts form?
Neuroglia (supporting cells) of the CNS (e.g. astrocytes, oligodendrocytes):
• Microglia (special form of macrophages) do not develop from the neuroepithelium, but rather from the mesodermal layer then migrates secondarily into the nervous system
what do epndymal cells form?
Lining cells of ventricles (of the brain) and central canal (of the spinal cord)
• Initially forms a thin lining to the neural tube
The wall of the neural tube during early development is made of only one layer of cells which are attached to both inner and outer membranes:
• Cells at the bottom are not attached to the outer surface as they contract towards the inner membrane before undergoing mitosis (divides into 2 daughter cells)
• Both daughter cells may enter the cell cycle again, or only one cell remains attached to the inner membrane and gradually grows bigger until it enters the cycle again
o Other daughter cell migrates away from the inner membrane and develops into a neuroblast or glioblast
- Neuroblasts: Outgrowth of neural processes from the cell to form _______________→ guided even further away from the inner membrane)
- Glioblasts: Similar process but ______________; tends to migrate more widely (further from the inner membrane)
- Ependymal cells: Remain attached to the inner membrane and forms the lining as they proliferate
After the ______________proliferates for a period of time, the entire area (precursor of grey matter containing most of the cell bodies) enlarges:
• Outer layer also gradually gets bigger as it becomes filled with axons (precursor of white matter)
dendrites (one of which differentiates into an axon ;
without dendrites or axons;
neural tube
what are the contents of the ependymal layer?
Inner layer surrounding the neural canal
what are the contents of grey matter ?
cell bodies
what are the contents of white matter ?
axons
what are the contents of the roof plate?
Dorsal extent of neural canal
what are the contents of floor plate?
Ventral extent of neural canal
There are groups of cells which lie around the neural tube and produce signalling molecules in a concentration gradient:
• Developing neurones respond to these signalling molecules depending on ___________________ → migration of cells is an important factor for the process to work normally
• Signalling molecules must be produced in a time where the developing neurones can respond (presence of appropriate receptors)
o Some signalling molecules are produced at more than one time during development → different effects at different times
• Both ________________ are guided by these signalling molecules (attraction or repulsion)
their position relative to the concentration gradient;
cell migration and direction of axonal growth
The ____________ develop into dorsal horns, while _____________ develop into ventral horns:
• Neural canal becomes smaller central canal due to ______________
• Dorsal and ventral roots join to form the spinal nerve, which passes out of the vertebral column to form the peripheral nerves
alar plates;
basal plates;
grey and white matter expansion
Dorsal horns
- Dorsal root ganglion is developed from the ____________
- Centrally directed axons of sensory cells pass through the _____________ to deliver their information into the dorsal horns
neural crest tissue;
dorsal root
Ventral horns
- Motor neurones in the ventral horn send their axons out through the ____________
ventral root
What does the anterior vesicle develop into at the start of the 4th week?
- end of 4th week: Prosencephalon (future forebrain)
- end of 5th week: Telencephalon (lateral), Diencephalon (midline part)
What does the middle vesicle develop into at the start of the 4th week?
- end of 4th week: Mesencephalon (future midbrain)
- end of 5th week: Mesencephalon (remains mostly as is)
what does the third vesicle develop into at the start of the 4th week?
- end of 4th week: Rhombencephalon (future hindbrain)
- end of 5th week: Pons (metencephalon)
Medulla (myelencephalon
The ventricular system emerges as the brain continues to develop:
• Hemispheres develop from telencephalon (each containing a ___________)
• Diencephalon contains the ______________
• Aqueduct (thin channel) develops through the __________, and the 4th ventricle opens up in a part of the __________________
• Developing cerebellum grows out from the back of the developing pons
lateral ventricle;
3rd ventricle;
midbrain;
hindbrain (pons and upper part of medulla)
where does the cephalic flexure occur?
Between most anterior primary vesicle and the middle vesicle
where does the pontine flexure occur?
Between the middle vesicle and 3rd vesicle
where does the cervical flexure occur?
At the point where the 3rd vesicle joins the developing spinal cord
The folding process begins by the 4th week of gestation (primary vesicle stage) with 3 folds
• Entire developing brain folds; developing __________________ enlarge until they grow backwards over the diencephalon and mesencephalon
• Cerebral cortex in the human embryo is so large that the only way which it can be accommodated within a cranial cavity small enough to pass through the birth canal is to fold it over very closely to other parts of the brain
cerebral hemispheres of the telencephalon
The adult configuration of the ventricular system consists of:
- _____________ in each hemisphere
- ___________ in the middle of the diencephalon
- ___________ passing through the midbrain
- 4th ventricle opening up in the ____________
C-shaped lateral ventricle;
3rd ventricle;
Aqueduct;
brainstem (pons and medulla)
The brainstem (midbrain, pons, medulla) is a tubular structure which develops similarly to the spinal cord (except eventual opening up of the 4th ventricle → substantial space):
• Roof plate: marks the ______________ of the neural canal (cells of the roof plate begin to proliferate → expands laterally)
o Allows the neural canal to become the ______________ in this region
This changes the relationship between the alar and basal plates (alar plate lies ___________ instead of dorsal to basal plate):
• Accounts for the development of the cranial nerve nuclei
• ____________ tend to lie medially in floor of 4th ventricle
• ___________ (including special senses) more laterally
• _________: landmark for division between alar and basal plates (autonomic nuclei for autonomic motor function found in this area)
most dorsal end;
4th ventricle;
lateral ;
Motor cranial nerve nuclei (basal plate);
Sensory cranial nerve nuclei (alar plate);
Sulcus limitans
Most of the brain conforms to the basic template of a grey matter core surrounded by white matter (also in the forebrain), but the ________________ is a layer of grey matter on the outside:
• These cells must migrate over a large distance for proper development
• Neuroblasts develop in the inner ventricular zone then some migrate a short distance into the subventricular zone (develop into structures like the ______________)
• Some neurones starting at the ventricular zone migrate all the way to the cortical plate close to the outer membrane (aided by ____________)
o Cell bodies of glial cells lie in the ventricular zone but possess ____________ which pass to the marginal zone
o Neuroblasts attach themselves to these processes and climb up to the outer parts of the neural tube
• Occurs in phases: ___________ in ventricular zone → ____________ up to cortical plate → pause → repeat (usually occurs 6 times)
o Produces a traditional 6-layered arrangement of cells in the adult cortex
o Much room for error as migration to the correct level is crucial to make the correct synapses for the correct function
cerebral cortex;
basal ganglia;
radial glial cells;
single processes ;
wave of proliferation; wave of migration
what is the main genetic cause of cognitive disorders in males
Fragile X syndrome
what does folic acid deficiency cause
spina bifida;
