1. Development of the Nervous System Flashcards
1
Q
Describe the early development of the CNS.
A
- Firstly, there is proliferation of the ectoderm in the dorsal midline of the embryonic disc. This proliferation turns into 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 the neural crests
- The neural crests separate from the neural tube and doesn’t take part in the fusion but lies alongside
- At the end of the process you have a neural tube lying in the midline dorsally in the embryo and on either side of this you have 2 strips of neural crest tissue
The neural tube and neural crest are two independent sources of nervous tissue :
- Nural tube = all CNS cells
- Neural crest = all PNS cells
The wall of the neural tube is called the neuroepithelium
2
Q
What are the three types of cell comes from neuroepithelium?
A
-
Neuroblasts
- Precursors of neurones
- All neurones that have their cell bodies within the CNS
-
Glioblasts
- These become neuroglia - astrocytes and oligodendrocytes
-
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
NOTE: neuroblasts include motor neurones because they have almost all their axons in the PNS but their cell bodies are in the CNS so they come from the neuroepithelium
3
Q
How does proliferation of neuroepithelium occur?
A
- There is an inner membrane at the bottom and an outer membrane at the top
- Mitosis occurs in the inner layer
- Almost all these cells are attached to BOTH the inner and outer membranes - it’s just that the nuclei are in different positions
4
Q
Describe the differentiation of neuroepithelium
A
- Firstly, cells withdraw 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 cells migrate away from the inner membrane and develop into neuroblasts
- They develop processes (one will become the axon) and these axons are directed away from the inner membrane
- As this happens, you will end up with 3 layers:
- Ependymal layer: where mitosis is occuring
- Grey matter
- White matter
- Glioblasts show a similar pattern but they can migrate into WHITE MATTER as well and they do not develop axons (but do develop processes)
5
Q
Describe the layers of the neural tube in cross-section.
A
- The ependymal layer surrounds the neural canal
- The roof plate and floor plate are landmarks that show the most dorsal and most ventral parts of the canal
- The whole process of differentiation and migration is controlled by singalling molecules that are secreted either from the tissues surrounding the devloping neural tube or sometimes secreted by cells within the neural tube - signalling molecules interact with receptors on the developing neuroblasts
- The signalling molecules also guide the developing axonz - can either cause attraction or repulsion and the direction is determined by the concentration gradient (close to the source there will be a higher conc.)
- 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 stages of development
6
Q
Describe the cross-section of the spinal cord during development
A
- Grey matter:
- Dorsal = sensory
- Ventral = motor
- The neural canal is even smaller compared tp the thickness of the all
- The grey matter has split into 2 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
7
Q
Describe the cross secton of the mature spinal cord
A
- The neural canal has become the central canal carring CSF
- Alar plates develop into dorsal horns
- Basal playes develop into ventral horns
- The whole spinal cord is surrounded by a thick layer of white matter
8
Q
Describe the development of the brainstem
A
- The brainstem is a tubular structure but in the middle of it you have a 4th ventricle
- The development of the 4th ventricle makes a mess of the tubular structure (first thing that happens)
- 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
- Basal plate lies more medially
- Motor =medial
- Sensory = lateral
9
Q
Describe the development of the brain in the first 4 weeks.
A
- 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
- This is all around 4 weeks
10
Q
Describe the development of the brain at 5 weeks.
A
- The most anterior vesicle divides into 2 and the third vesicle divides into 2 - this produces 5 secondary vesicles
- You get an enormous expansion of the top part of the developing forebrain and forms the telencephalon
- There is less expansion in the lower part of the developing forebrain because that part becomes the diancephalon
- The developing midbrain doesn’t expand very much
- The developing hindbrain divided into 2 to become the pons and medulla
11
Q
Describe the developing brain at 8 weeks
A
- 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 outpouching from the back of the pons
- Development of the diancephalon, cerebellum begins to develop the third and fourth ventricles and the aqueduct
12
Q
Describe the folding of the brain at 4, 5 and 8 weeks
A
4 WEEKS
- The vesicles aren’t in a straight line - there are 3 flexures
- The flexures are name based on their position
- As you go through development these flexures become exaggerated
8 WEEKS
- By 8 weeks, the telencephalon has got so big, that it’s starting to move back over the diancephalon
13
Q
Describe development of the cortex
A
- 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 will stay in the middle and form the basal ganglia
- There are other neurones than then migrate towards the outer membrane
- The migration takes place by the neuroblasts attaching themselves to the radial glial cells - they have their cell bodies anchored in the inner membrane and have a single long process that goes to the outer membrane
- This is how one layer of the cortex is formed
- 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 have different types of cells with different functions and different connections
- Marginal zone is largely acellular (layer 1)
- Layers 2-6 find the lamina of the neurones
14
Q
Developmental Disorders of the Nervous System: How is schizophrenia caused?
A
Malfunction of neural development
15
Q
Developmental Disorders of the Nervous System: what does deficiency of folic acid lead to?
A
Spina bifida - closed asymptomatic NTD (neural tube defects) in which some of the vertebraw are not completely closed
