CNS Development Flashcards
(23 cards)
What are the three layers of the early neural tube?
Ectoderm
Mesoderm
Endoderm
Describe the early development of the neural tube and state when this occurs.
- At 3 weeks
- Proliferation of the ectoderm –> neural plate
- Neural plate thickens and folds up on the sides to fuse dorsally –> neural canal is formed in the space
- A little strip of cells at the dorsal tip of the neural fold forms the neural crests that run alongside the neural canal more dorsally.
Neural tube –> ALL CNS cells
Neural crest –> ALL PNS cells
What is the wall of the neural tube made of? What deoes it give rise to?
Neuroepithelium –> All cells of the CNS
What 3 types of cells differentiate from the neuroepithelium?
Neuroblasts -All neurons with cell bodies in the CNS
Glioblasts - Astrocytes, oligodendrocytes (neuroglia, NB microglia arise elsewhere)
Ependymal cells - Lining ventricles and central canal
What do neural crest cells differentiate into?
Sensory neurones of dorsal root ganglia and cranial ganglia.
Postganglionic autonomic neurones.
Schwann cells – myelinate axons in PNS.
Non-neuronal derivatives – e.g. melanocytes.
Describe the proliferation of the neuroepithelium.
- Cross-section of the neural tube at an early stage.
- There is ONE-layer of cells and most cells are attached to both the inner and outer membrane.
- The bigger cells towards the inside are undergoing mitosis.
Describe the differentiation of the neuroepithelium.
- Mitosis occurs at the inner side after moving downward from outside layer
- One of daughter cells migrates and develops into neuroblasts while the other stays attached, goes into the cell cycle again and gets bigger.
- They develop processes to direct them away from inner membrane and you end up with three layers which starts to form difference between white and grey matter:
- Mainly mitosis occurring
- Mainly cell bodies (grey)
- Mainly axons (white)
NB: Glioblasts can migrate like this to white matter too but do not develop axons (processes instead)- they can migrate to grey matter too. Oligodendrocytes mainly in white. Ependymal cells remain in ependymal layer.
What are the layers of the neural tube?
There are 3 clear layers:
- Inner – Ependymal layer – germinal layer.
- Middle – Grey matter – mantle layer.
- Outer – White matter – marginal layer.
What controls differentiation of the neural tube?
- Signalling molecules, secreted by surrounding tissues, interact with receptors on neuroblasts
- These control migration & axonal growth by attraction and repulsion
- Depends on concentration gradient & timing
- Concentration gradient – close to source of molecules = higher conc.
- Timing – only produce s. molecules when developing neurones have the correct receptors to receive them.
What 2 layers is the grey matter split into in the developing spinal cord ?
Alar plate - dorsally - made of interneurons - receive sensory info
Basal plate - ventrallly - motor neurons and interneurons - can send information out of the ventral roots
Describe three changes that occur in spinal cord development to form the matrue spinal cord.
Dorsally, alar (roof) plate –> eventually form dorsal horns
Ventrally, basal (floor) plate–> ventral horns
Laterally, neural crest cells –> sensory neurones of PNS in dorsal root ganglia
Describe flow of information in the mature spinal cord.
Dorsal root –> dorsal horn –> up to CNS (or reflex) –> ventral root –> motor neurone out
How does the brainstem develop from the spinal cord?
- The development of the 4th ventricle occurs:
- The roof plate proliferates rapidly and the dorsal part expands laterally so that the alar (roof) plate splits
- Dorsal/sensory nuclei = lateral
- Efferent/motor nuclei = medial
- Nuclei are now arranges in columns based on function
Describe the structure and development of the brain in the embryo at about 5 weeks.
- Only the anterior portion of the neural tube develops into the brain.
- Three primary vesicles form:
- Prosencephalon– future forebrain.
- Mesencephalon – future midbrain.
- Rhombencephalon– future hindbrain.
- Flexures formed: cephalic, pontine, cervical. These become more exaggerated upon development.
Over the next week, the most superior vesicle divides in two and the third (inferior) vesicle divides into two. This produces 5 secondary vesicles.
What are the 3 main flexures of the brain at development?
Flexures formed: cephalic, pontine, cervical.
These become more exaggerated upon development.
Describe the development of the brain at approx. 5 weeks.
- Forebrain develops into telencephalon (mostly cortex) + diencephalon (key as relay station for pathways -thalamus, hypothalamys, epithalamus and subthalamus)
- Hindbrain shows differentiation between pons + medulla
Describe the development of the brain at approx. 8 weeks.
- Hemisphere develops
- Cerebellum development begins - from back of pons
- Ventricular system develops - space within the neural tube becomes smaller relative to the wall forming centricular system
What are the zones of the developing cortex?
- Marginal
- Cortical plate
- Intermediate
- Subventricular
- Ventricular
Describe the development of the cortex.
NB: the grey matter is outside and white matter underneath in the brain (opposite for the spinal cord)
- Brain has a 6 layer structure (cerebellum - 3)
- Neurons develop from ventricular zone, follow processes of radial glial cells (act as a scaffold) and migrate to the marginal zone
- Radial glial cells have soma attached to inner membrane and send a single long process to the outer membrane
- Proliferation and migration occurs in waves until 6 layers have developed
Which cells act as a scaffold in cortical development?
Radial glial cells
List the processes which need to be coordinated in brain and spinal cord development to prevent disorders occurring.
- Proliferation
- Migration
- Differnetiation
- Axon growth
- Synapse formation
Describe these neural tube defects:
- Craniorachischisis
- Anencephaly
- Enceephalocele
- Iniencephaly
- Craniorachischisis - completely open brain and spinal cord
- Anencephaly - open brain and lack of skull vault
- Encephalocele - herniation of the meninges and brain
- Iniencephaly - occipital skull and spine defects with extreme retroflexion of the head
Describe these neural tube defects:
- Spina bifida occulta
- Closed spinal dysraphism
- Meningocele
- Myelomeningocele
- Spina bifida occulta (deficiency of folic acid) - closed asymptomatic NTD in which some of the vertebrae are not completely closed
- Closed spinal dysraphism - deficiency of at least two vertebral arches (on picture below covered by a lipoma)
- Meningocele - protrusion of the meninges (filled with CSF) through a defect in the skull or spine
- Myelomeningocele - open spinal cord (with a meningeal cyst)
