2. Embryology Flashcards
define gastrulation
Gastrulation is the process whereby the outer layer of the bilayer germ disc (ectoderm) invaginates at the primitive streak to produce a three layered germ disc
what are the 3 layers of the germ disk after gastrulation>
ectoderm, mesoderm, endoderm
what does the ectoderm become?
Becomes skin and neural tissue as well as neural crest
what does the mesoderm become?
Heart, vessels, muscles etc.
The notochord is a crucial structure composed of endoderm
what is the function of the notochord?
It inducts the overlying ectoderm to invaginate and form the neural tube
what does the endoderm become?
Gut including accessory organs
describe the process of neurulation
Neurulation is the process of formation of the neural tube
o Induced by the notochord
o Elevation of the neural folds
o Fusion of the folds in the midline at mid cervical level with reconstitution of the surface ectoderm
o Neural tube zips up rostrally and caudally
what happens to neural crest cells in neurulation?
When the folds fuse, neural crest cells detach and migrate their ultimate destinations
what happens if there is failure of the neural tube to zip rostrally or caudally?
If this process fails in the rostral direction can result in anencephaly
Failure in the caudal direction can lead to spina bifida
what are the 3 Primary brain vesicles?
The rostral neural tube displays a number of swellings which will become the major parts of the adult brain
- Forebrain (Prosencephalon)
- Midbrain (mesencephalon)
- Hindbrain (rhombencephalon)
What are the 5 secondary brain vesicles and what do they form from?
- Prosencephalon forms the telencephalon and the diencephalon
- Mesencephalon form the mesencephalon
- Rhombencephalon form the metencephalon and the myelencephalon
What are the mature derivatives of the secondary brain vesicles?
- Telencephalon: cerebral hemispheres
- Diencephalon: thalamus, hypothalamus and optic nerve/retina
- Mesencephalon: midbrain
- Metencephalon: pons and cerebellum
- Myelencephalon: medulla
what is a fundamental pattern to the organisation of the nervous system?
Motor structures tend to sit anteriorly (I remember this as you ‘motor forwards)
Sensory structures tend to sit posteriorly
What causes the pattern of motor anteriorly and sensory posteriorly?
This pattern exists due to the development of the basal (floor) and alar (roof) plates in the neural tube
• The notochord inducts the ventral (anterior) portion of the neural tube to become the basal (or floor) plate
o The basal plate gives rise to motor neurones
• The alar (or roof) plate (dorsal) forms in the absence of influences from the notochord
o Gives rise to inter- and sensory neurones
How is this fundamental pattern seen in the spinal cord?
Dorsal roots are sensory whilst ventral roots are motor
The dorsal horn contains sensory neurones whilst the ventral horn contains motor neurones
The dorsal columns (a sensory tract) sit posteriorly whilst the corticospinal tract (motor) sits anteriorly
How is this fundamental pattern seen in the medulla?
The lemnisci (sensory) sit posterior to the pyramids of the medulla (motor) (contain corticospinal tracts)
How is this fundamental pattern seen in the midbrain?
The colliculi (sensory) sit posterior to the cerebral peduncles (motor)
How is this fundamental pattern seen in the cerebral cortex?
The primary sensory cortex (sensory) sits posterior to the primary motor cortex (motor)
Why are there flexures in the CNS?
Growth & development at cranial NT exceeds available space linearly, so….it must fold up
What are the 2 flexures of the CNS?
- Cervical flexure (spinal cord- hindbrain)
- Cephalic flexure (midbrain region)
what leads to the development of the cauda equina?
o Initially, there is a one to one correspondence between cord levels and vertebral levels (the cord fills the entire vertebral canal)
o but, the spine grows faster than the spinal cord, particularly at the lumbar levels
o so, the lower portions of the cord are stretched, drawing out the cauda equina
How do 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
What do neural tube defects result from?
Failure of the neutral tube to close
- can occur cranially or caudally
What do cranial neural tube defects result in?
Anencephaly
What is anencephaly?
Failure of NT closure cranially
• Absence of cranial structures, including brain
• Incompatible with life
What do caudal neural tube defects result in?
Spina bifida
What is spina bifida?
A birth defect in which there is incomplete closing of the spine and membranes around the spinal cord during early development in pregnancy
What are the different severities of spina bifida from most severe to least?
- myelomeningocele
- meningocele
- spina bifida occulta
what are the different neural tube disorders from severe to least severe
Craniorachischisis Anencephaly Myelocoele Myelomeningocoele Meningocoele Spina bifida occulta
what is Craniorachischisis
- The entire neural tube remains open
- Hence failure of both brain and spinal cord to form
- Incompatible with life
what is Anencephaly?
- The cranial neural tube fails to close
- Hence failure of the brain to form
- These children may be born alive but do not live for long
what is Myelocoele?
- The spinal cord fails to develop ( i.e. the lumen of the neural tube is exposed to the outside world), usually associated with a CSF filled cyst
- These children frequently have neurological deficits and are susceptible to meningitis due to the presence of exposed neural tissue
which conditions appear to have a normal neural tube, but there is failure of development of the posterior vertebral arch?
Myelomeningocoele
Meningocoele
Spina bifida occulta
what is Myelomeningocoele?
- A CSF-filled cyst containing the spinal cord
- Transilluminates relatively poorly (due to [presence of solid tissue in the cyst)
- These children may have neurological deficits (less likely than disorders listed above)
- Repair is necessary
what is a Meningocoele?
o Presence of a CSF filled cyst
o The cord is sited within the vertebral canal
o Transilluminates brilliantly
o These children tend to have a good neurological
prognosis
o However, the cyst will need repair as it does
predispose to infection
what is Spina bifida occulta?
o The only anomaly is the lack of the posterior vertebral arch
o May manifest a sign such as a tuft of hair or a large naevus over the defect
o Not associated with significant neurological problems
o Occurs in about 10% of the population!
how can nuerual tube defects be prevented?
folic acid
o 400 micrograms daily, to be taken before from
around 3 months before conception and until week
12 of pregnancy
what does the neural tube form?
brain and spinal cord
what does the neural crest form ?
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
Melanocytes
The leptomeninges (i.e. the arachnoid and pia)
Head mesenchyme, which contributes to many tissues such aspharyngeal arches
what is the neural crest?
o Derived from the point at which the neural folds fuse when the surface ectoderm is reconstituted
o They become detached from the ectoderm and then migrate to their distant targets
which tissues receive a significant contribution from neural crest?
Thymus
Thyroid
Parts of the heart (e.g. spiral septum)
Parts of the teeth
what are the 2 disorders of the neural crest?
Di George syndrome
Hirschprung’s disease
what is Di George syndrome
• Immunodeficiency (due to involvement of the thymus)
• Facial anomalies (due to contribution of neural crest to
facial development)
• Heart anomalies
• Hypocalcaemia (involvement of parathyroids)
what is Hirschprung’s disease
- Lack of enteric neurones in sections of the large intestine
- This leads to hypomotility and constipation
