CNS Development Flashcards
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
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)
