describe the process of neural tube formation
NEURULATION (starts in week 3, lasts 10 days):
1. The notochord (created during gastrulation) signals overlying ectoderm to convert to neuroectoderm and thicken into slipper-shaped neural plate.
2. Lateral edges of neural plate elevate into neural folds (around a neural groove) which fuse to form neural tube
what are neuropores
Open ends of neural tube - cranial and caudal neuropores - which communicate with overlying amniotic cavity.
Cranial neuropore closes day 25 followed by caudal neuropore 3 days later.
describe the 3 layers into which the neural tube organises into - what are the mature derivatives of these
1. inner NEUROEPITHELIAL layer
2. intermediate MANTLE layer:
- consists of neuroblasts (primitive nerve cells) - later form grey matter of spinal cord
3. outer MARGINAL layer:
- contains nerve fibres emerging from neuroblasts - are later myelinated to form white matter of spinal cord
label this diagram of the primitive neural tube
what are the basal and alar plates and what do these give rise to
Continuous addition of neuroblasts to mantle layer forms ventral and dorsal thickenings:
- ventral basal plates (contain motor horn cells)
- dorsal alar plates (contain sensory horn cells)
what is the function of roof and floor plates
Are dorsal and ventral midline portions of neural tube that don't contain neuroblasts - act to:
- regulate dorsal and ventral patterning
- guide neuronal positioning
describe the formation of brain vesicles during dev. - what are their mature derivatives?
During neural fold formation, cephalic end of neural tube forms 3 dilations... become 3 primary brain vesicles after NT closure in week 4... then differentiate into 5 secondary vesicles by week 5
1. PROSENCEPHALON (forebrain) gives rise to:
a) TELENCEPHALON = cerebral hemispheres
b) DIENCEPHALON = optic vesicle, thalamus, hypothalamus & pituitary
2. MESENCEPHALON (midbrain) gives rise to:
c) MESENCEPHALON = midbrain
3. RHOMBENCEPHALON (hindbrain) gives rise to:
d) METENCEPHALON = pons and cerebellum
e) MYELENCEPHALON = medulla oblongata
describe the 2 flexures that form during brain dev.
Growth and dec. at cranial NT exceeds available space linearly... it must fold with neuroaxis not remaining straight:
- cephalic flexure: midbrain region
- cervical flexure: hindbrain-spinal cord junction
what is the neural crest
Cells of the lateral border of neuroectoderm tube which become displaced and enter mesoderm - undergo epithelial-to-mesenchymal transition
how do neural crest cells contribute to nervous system dev.?
Give rise to:
- cranial nerve and spinal (dorsal) root ganglia
- enteric ganglia
- sympathetic ganglia (chain and pre-aortic)
- schwann and glial cells
- leptomeninges (arachnoid and pia)
how do neural crest cells contribute to head, neck and midline dev.?
Give rise to:
- connective tisue and bones of face and skull
- C cells of thyroid gland
what is hydrocephalus and how does it occur?
Obstruction to flow of CSF causing fluid accumulation and enlargement of ventricular system, with raised intracranial pressure.
Can be congenital (most common in neonates with spina bifida) or acquired (e.g. tumour or infection).
how can hydrocephalus be treated
inserting ventriculoperitoneal shunt - allows drainage of CSF from one of cerebral ventricles to peritoneal cavity (important to prevent cognitive delay)
what is rachischisis and why does it occur
Failure of neural fold elevation... splayed NT
(incompatible with life)
what is anencephaly and why does it occur
absence of cranial structures, inc. brain, due to defect in cranial neural fold closure
(incompatible with life)
what is spina bifida and what are the 3 types
Splitting of vertebral arches that can occur anywhere along length of spine (most common in lumbosacral region) due to caudal defect in neural fold closure.
- spina bifida occulta = vertebral arch defect covered by skin, not involving underlying neural tissue (usually asymptomatic)
- meningocoele = protrusion of fluid-filled meninges through vertebral arch defect. Results in neurological deficits but not usually associated with intellectual disability.
- myelomeningocoele = protrusion of fluid-filled meninges + neural tissue through vertebral arch defect. Results in neurological deficits but not usually associated with intellectual disability.
why does hydrocephaly occur in 80-90% of severe NTDs?
Due to inappropriate tethering of spinal cord to vertebral column... vertebral column lengthens and tethered cord pulls cerebellum through foramen magnum... herniated cerebellum cuts off CSF flow
how can spina bifida be diagnosed pre-natally
- alpha-fetoprotein in maternal plasma