Development of the CNS Flashcards
Differentiation of the neuroepithelium (of the neural tube)
- Neuroblasts (all neurons with their cell bodies in the CNS)
- Glioblasts (glioblasts differentiate into astrocytes and oligodendrocytes)
- Ependymal cells (lining ventricles and central canal)
Differentiation of neural crest cells
CELLS OF THE PERIPHERAL NERVOUS SYSTEM
- Sensory neurones of dorsal root ganglia and cranial ganglia
- Postganglionic autonomic neurons
- Schwann cells
- Non-neuronal derivatives (eg: melanocytes)
Proliferation of neuroepithelium
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Differentiation of neuroepithelium
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Layers of the neural tube
- neural canal surrounded by ependymal layer (germinal layer)
- floor plate marks most ventral part of neural canal
- roof plate marks most dorsal part of neural canal
- ependymal layer surrounded by grey matter (mantle layer)
- grey matter surrounded by white matter (marginal layer)
Control of neural tube differentiation
- controlled by selective signalling molecules secreted sequentially by surrounding tissues->interact with receptors on neuroblasts to control migration and axonal growth by attraction and repulsion
- dependent on concentration gradient and timing
The developing spinal cord structure
- grey matter is split into the alar plate (more dorsal containing interneurons) and the basal plate (more ventral containing motorneurons and interneurons)
- neural crest cells form sensory neurons in dorsal root ganglion
The mature spinal cord structure
- central canal shrinks
- the basal plate develops into the ventral horn (motor)
- the alar plate develops into the dorsal horn (sensory)
- the neural crest develops into the dorsal root ganglia
The developing brainstem structure
- Early development is tubular, much like the spinal cord
- Lateral proliferation of the roof plate enlarges the neural canal to form the 4th ventricle-> causes basal plate containing motor nuclei to lie more medially in brainstem and alar plate containing sensory nuclei to lie more laterally in brainstem
- This arrangement of the plates dictates the pattern of cranial nerve nuclei
- sulcus limitans separates cranial nerve motor nuclei from the cranial nerve sensory nuclei (separates basal plate from alar plate)
The 3 primary vesicles of the developing brain (~4 weeks)
- Prosencephalon (future forebrain)
- Mesencephalon (future midbrain)
- Rhombencephalon (future hindbrain) which is largely the cerebellum, pons and medulla oblongata
The 5 secondary vesicles of the developing brain (~5 weeks)
- telencephalon (developing forebrain) containing the cerebral cortex
- diencephalon (developing forebrain) containing the thalamus and hypothalamus
- midbrain
- pons (developing hindbrain)
- medulla (developing hindbrain)
The developing brain structure (~8 weeks)
- ventricular system arises
- development of individual brain hemispheres with lateral ventricles within (lateral ventricles drain into third ventricle)
- midbrain contains aqueduct (drains into fourth ventricle
Folding of the developing brain
NAME OF FLEXURE DEPENDS ON POSITION
- cephalic flexure
- pontine flexure
- cervical flexure
Flexures become exaggerated as development occurs
The developing cortex structure
- Neuroblasts formed from neuroepithelium migrate towards pial surface along radial glia
- successive waves of migration form the cortical layers (6 layers)
- myelination gradually occurs to term and beyond, producing the 6 discrete layers of the cerebral cortex
- highly convoluted structure as gyri and sulci=increases brains surface area and cognitive ability (higher complexity of activity)
Developmental disorders
- Normal development depends on the coordinated completion of several complex processes (eg: proliferation, differentiation, migration, axon growth, synapse formation etc)
- Genetic mutation and environmental factors (eg: mother’s lifestyle, diet, teratogen exposure etc) can interfere with these processes
- developmental disorders largely relate to problems in the third trimester
Neural tube defects
DEFECTS A TOP OR BASE OF NEURAL TUBE
- craniorachischisis
- anencephaly
- encephalocele
- iniencephaly
- spina bifida occulta
- closed spinal dysraphism
- meningocele
- myelomeningocele
FOLIC ACID SUPPLEMENTATION ADVISED TO REDUCE NEURAL TUBE DEFECT RISK
Craniorachischisis
- most severe of the neural tube defects
- brain and spinal cord remain open
- presence of both anencephaly and spina bifida
Anencephaly
- neural tube defect
- open brain and lack of skull vault
- partial or complete absence of bones of the rear of the skull, the meninges and the cerebral hemispheres of the brain
Encephalocele
- neural tube defect
- herniation of the meninges and brain
Iniencephaly
- neural tube defect
- occipital skull and spine defects with extreme retroflexion (backward bending) of the head
Spina bifida occulta
- neural tube defect
- closed asymptomatic neural tube defect in which some of the vertebrae are not completely closed
- defect in the bony arch of the spine that has a normal skin covering
Closed spinal dysraphism
- neural tube defect
- deficiency of at least two vertebral arches, here covered with a lipoma
Meningocele
- neural tube defect
- protrusion of the meninges (filled with CSF) through a defect in the skull or spine
Myelomeningocele
- neural tube defect
- type of spinal bifida
- open spinal cord with a meningeal cyst
Radial glial cells
- astroglia (of astroglial lineage in the developing CNS)->type of macroglia
- develop vertically to act as scaffolding from the ventricles to the pial surface
Neural stem cells
- apparent population in hippocampus but very few in number (not enough)->no programming to enhance neural circuitry (repairing damaged or lost neurons)->thus not used therapeutically
- Damaged/lost neurons from neuron apoptosis during development, neurodegenerative disease etc
Developmental neurobiology in the repair of the nervous system
An understanding of the developmental mechanisms may lead to novel treatment of neurological disorders:
- neural stem cell use to replace lost neurons
- use of axonal guidance mechanisms to induce CNS regeneration
The developing brain structure
- originally forms from the most anterior end of the neural tube
- the primary and then secondary vesicles develop by differential growth of the neural tube wall
- at the same time as the ventricle development, folding occurs to produce flexures so the developing brain fits into the cranial cavity
- the neural canal develops into the ventricular system
The developing brain structure (~4 weeks)
- Flexure formation to define three levels of the embryonic brain
- Differential growth of neural tube wall to form primary vesicles
The developing brain structure (~5 weeks)
Differential growth of neural tube wall to form secondary vesicles from the primary vesicles
The developing brain structure (at term)
-growth continues with myelination of appropriate pathways post term (allows for functions such as standing up, walking etc)
