1.2 Development Of The Nervous System Flashcards
When does gastrulation occur?
17 - 19 days after fertilisation
What is gastrulation?
Gastrulation is the process whereby the outer layer of the bilayer germ disc (epiblast) invaginates at the primitive streak to produce a three layered germ disc
What are the 3 germ disc layers?
Ectoderm
Mesoderm
Endoderm
What is the function of the notochord?
To induce a change in the ectoderm to form the neural tube and the CNS
Forms the nucleus propulsus
What does the ectoderm go on to form?
Becomes skin and neural tissue as well as neural crest
What does the mesoderm go on to form?
Heart, vessels, muscles etc.
What does the endoderm go on to form?
Gut including accessory organs. The notochord is a crucial structure composed of endoderm. It inducts the overlying ectoderm to invaginate and form the neural tube
What is neurulation?
Process of formation of the neural tube
Describe the process of neurulation
Induced by the notochord Elevation of the neural folds
Fusion of the folds in the midline at mid cervical level with reconstitution of the surface ectoderm
When the folds fuse, neural crest cells detach and migrate their ultimate destinations
Neural tube zips up rostrally and caudally
What are the primary brain vesicles?
Prosencephalon
Mesencephalon
Rhombencephalon
What are the first developments of the brain?
3 swellings develop called the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).
Where is the cephalic flexure?
Between the forebrain and the midbrain
Where is the cervical flexure?
Between the developing hindbrain and the spinal cord
The prosencephalon subdivides into which structures?
Telencephalon
Diencephalon
What does the telencephalon become?
becomes most of cerebral hemisphere
What does the diencephalon become?
becomes thalamus, hypothalamus and optic nerve/retina
What does rhombencephalon subdivide into?
Metencephalon
Myelencephalon
What does the metencephalon become?
Forms the pons and cerebellum.
What does the myelencephalon go on to become?
Forms the medulla (the prefix myelo- refers to spinal cord, which the medulla joins on to)
What is the function of the notochord?
To induct the ventral (anterior) portion of the neural tube to become the basal plate
What is the basal plate?
A portion of the neural tube ventral to the sulcus limitans. Gives rise to the motor neurones. Development of basal plate induced from the ventral half of the neural tube by chemical factors released by the notochord
What is the alar plate?
The alar plate, or the roof plate is the dorsal half of the neural tube that forms in the absence of influences from the notochord.
Gives rise to the inter and sensory neurones
Describe how the pattern of distribution of sensory and motor systems is seen in the midbrain
The colliculi (sensory) sit posterior to the cerebral peduncles (motor)
Describe the distribution of the motor and sensory pathways in the medulla
The lemnisci (sensory) sit posterior to the pyramids of the medulla (motor)
What is the cauda equina?
A collection of spinal nerve roots descending from the lower spinal cord. Contains the spinal nerve roots from L2 to Co1
Describe the development of the cauda equina
- Initially, there is a one to one correspondence between the cord levels and the vertebrae levels
- However, the spine grows faster than the spinal cord, particularly at the lumbar levels (lumbar vertebrae are larger than thoracic and cervical ones)
- Therefore, the lower portions of the cord are stretched, drawing out the cauda equina
Suggest why neural tube defects predispose to hydrocephalus
This may be caused by tethering of the cord at the site of the defect
Thus, as the spine grows the cord cannot move within the vertebral canal, resulting in the brainstem (containing the fourth ventricle) being pulled down through foramen magnum and
becoming occluded
There are other mechanisms at play as well
What is spina bifida?
A neural tube disorder where the dorsal portion of the vertebrae fail to form
What is spinal dysraphism?
Failure of the neural tube to fuse in the midline. General term for neural tube disorder.
Why do all neural tube disorders result in the failure of the development of the posterior vertebral arches?
appears that the neural tube is at least partly
responsible for inducing the migration of the
sclerotome from somites to form the posterior bony
arch. Hence an anomaly in the neural tube may
lead to disordered formation of the posterior arch.
List the neural tube defects going form most sever to least
Craniorachischisis Anencephaly Myelocoele Myelomeningocoele Meningocoele Spina bifida occulta
What is craniorschischisis
Entire neural tube remains open
Failure of both the spinal cord and the brain to form
Incompatible with life
What is anencephaly?
Cranial neural tube fails to close
Failure of the brain to form
Short term life expectancy
What is a myelocoele?
Also known as rachischisis, this occurs when the spinal cord fails to develop. There is no posterior bony arch of the vertebrae and a CSF filled cyst with the open spinal cord is exposed to the outside world.
Frequent neurological deficits and susceptibility to meningitis due to the presence of exposed neural tissue
What is a myelomeningocoele?
A CFS filled cyst containing the spinal cord
Transilluminates relatively poorly
Children may have neurological deficits but less than associated with a myelocoele
Repair is necessary
What is a meningocoele?
Presence of a CSF filled cyst. The cord is sited within the vertebral canal. Transilluminates brilliantly. These children tend to have a good neurological prognosis. However, the cyst will need repair as it does
predispose to infection
What is spina bifida occulta?
The only anomaly is the lack of the posterior
vertebral arch. May manifest a sign such as a tuft of hair or a large naevus over the defect. Not associated with significant neurological problems. Occurs in about 10% of the population
What is used in the prevention of neural tube defects
Folic acid is prescribed to all pregnant women
- also reduces risk of palatal anomalies
400 micrograms daily, to be taken before from
around 3 months before conception and until week
12 of pregnancy
What are the neural crest cells?
Part of the ectoderm in a vertebrae of an embryo that lie on either side of the neural tube.
Multifunctional
Where do the neural crest cells derive from?
The point at which the neural folds fuse when the surface ectoderm is reconstituted
What cells are completely derived from the neural crest cells?
All neurones whose cell bodies are in the peripheral nervous system
• Primary sensory neurones
• Autonomic postganglionic neurones
• Enteric neurones
Schwann cells
Cells of the adrenal medulla (these are after all specialised
sympathetic postganglionic neurones)
Melanocytes
The leptomeninges (arachnoid and pia)
Head mesenchyme, which contributes to many tissues such as
pharyngeal arches
What tissues receive a significant contribution from the neural crest?
Thymus
Thyroid
Parts of the heart
Parts of the teeth
During their migration from the dorsal wall, where are some of the neural crest cells left?
The dorsal root ganglia (sensory neurones)
The sympathetic ganglia (sympathetic postganglionic
neurones)
The preaortic ganglia (sympathetic postganglionic neurones that receive input from splanchnic nerves)
The adrenal medulla (chromaffin cells, which are homologous to sympathetic postganglionic neurones)
The gut wall (enteric nervous system)
What is Di George Syndrome?
A disorder of the neural crest cells due to a small deletion in chromosome 22. Resulting in
• Immunodeficiency (due to involvement of the thymus)
• Facial anomalies (due to contribution of neural crest to
facial development)
• Heart anomalies
• Hypocalcaemia (involvement of parathyroids
More frequent cleft lip/palate
What is Hirschprung’s disease?
Lack of enteric neurones in sections of the large intestine leading to hypomotility and constipation
