Embryology and Congenital Malformations Flashcards
When does the CNS begin to appear?
Beginning of 3rd week
What happens in the 3rd week?
- Thickening of ectoderm anterior to the primitive node – NEURAL PLATE
- Edges thicken and move upwards to form the NEURAL FOLDS
Describe the initial folding of the neural tube
Remains open at the anterior and posterior ends:
• Anterior (cranial) neuropore – closes 18-20 somite stage (25 days)
• Posterior (caudal) neuropore – closes day 27
When does the anterior neuropore close?
25 days
When does the posterior neuropore close?
Day 27
Describe the neural tube closure
- Initiated at several points along anterior-posterior axis
- Proceeds in cranial and caudal directions
- Begins day 18 and completed by the end of the 4th week
What are the neural tube defects caused by the failure of the neural tube to close?
- Anencephaly
- Encephalocoele
- Spina bifida
What is exencephaly / anencephaly (meroencephaly)?
Failure of the anterior neuropore to close, so neural tissue (brain and skull) left exposed to amniotic fluid causing it to degenerate
Incompatible with life
(Craniorachischisis – failure of neural tube closure along entire neuroaxis)
More common in females
What is encephalocoele?
Problem with fusion of skull so the cerebral tissue herniates through the skull
- Most frequent in occipital region
- Variable degree of neurological deficits
What is spina bifida?
- Defective closure of the caudal neural tube
- Affects tissues overlying the spinal cord
- Non-fusion of the vertebral arches
What are four different types of spina bifida?
- Spina Bifida Occulta
- Spina Bifida Cystica
- Spina Bifida with Meningocele
- Spina Bifida with Meningomyelocle
What is spina bifida occulta?
Failure of embryonic halves of vertebral arch to grow normally and fuse
What are the features of spina bifida occulta?
- Most minor form
- Occurs in L5 and L6 vertebrae in 10% of healthy people
- Usually no clinical symptoms
- May result in dimple with small tuft of hair
What is spina bifida cystica?
Protrusion of spinal cord and/or meninges through the defect in the vertebral arches.
What is spina bifida with meningocele?
Protrusion of meninges and cerebrospinal fluid as the bones do not close around the spinal cord
What is spina bifida with meningomyelocle?
- Nerve roots and/or spinal cord included in the sac
- Spinal cord in affected area open due to failure of neural folds to fuse
- Neurological deficits – loss of sensation and muscle paralysis
What are the features of spina bifida with meningomyelocle?
- Area affected determined by level of lesion (loss of sensation and muscle paralysis)
- Associated with hydrocephalus
- Most severe form of myeloschisis
Why is the incidence of malformations in the UK decreasing?
- Folic acid supplements
* Prenatal diagnosis - maternal blood screening, USS, amniocentisis
What do you look in maternal blood screening?
High levels α-fetoprotein (AFP) in serum – AFP from foetal liver leaks into amniotic fluid then into maternal blood
What can be seen in ultrasound scan for prenatal diagnosis?
Anencephaly from 12 weeks, spina bifida from 16-20 weeks
What is amniocentisis in prenatal diagnosis?
Diagnosis of chromosomal abnormalities and fetal infections, and also for sex determination.
Name three risk factors for congenital malformations
- Genetic predisposition
- Nutritional (i.e. too little folate, too much vit A)
- Environmental (i.e. hyperthermia; takin certain drugs – i.e. sodium valproate)
Name the brain ventricle developed in the 4th week
- Prosencephalon (forebrain)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain)
Name the five brain ventricles which are further developed from the initial three in the 5th week
Prosencephalon –>
- Telencephalon
- Diencephalon
- Mesencephalon
Rhombencephalon –>
- Metencephalon
- Myelencephalon
What are the three flexures during the folding of the embryo?
- Cephalic flexure
- Pontine flexure
- Cervical flexure
What is the cephalic flexure?
- End of 3rd week
* Between the midbrain (mesencephalon) and hindbrain (rhombencephalon)
What is the cervical flexure?
- End of 4th week
* Between the hindbrain and spinal cord
What is the pontine flexure?
- 5th week
* In hindbrain (between metencephalon and myecephalon)
What structures develop from the telencephalon?
- Cerebral hemispheres
- Hippocampus
- Basal ganglia
What structures develop from the diencephalon?
- Thalamus
- Hypothalamus
- Pituitary gland
- Pineal
What structures develop from the mesencephalon (midbrain)?
Superior and inferior colliculi
What structures develop from the metencephalon?
- Cerebellum
* Pons
What structures develop from the myelencephalon?
Medulla
What parts of the neural tube form the different parts of the ventricular system?
- Telencephalon -> lateral ventricles
- Diencephalon (bit of telen.) -> 3rd ventricles
- Mesencephalon -> cerebral aqueduct
- Metencephalon and myelencephalon -> 4th ventricle
- End of neural tube -> central canal
What connects the lateral and 3rd ventricles?
Intraventricular foramina
When does CSF begin to form?
5th week by the choroid plexus in the ventricles
How does the CSF drain?
Drains into subarachnoid space via opening in the roof of the 4th ventricle
Absorbed into venous system via arachnoid villi
What is hydrocephalus?
Condition which causes the accumulation of CSF, abnormal production and resorption, that causes an enlarged brain and cranium.
What is a common cause of hydrocephalus?
Frequently due to blocked cerebral aqueduct which prevents CSF from lateral and 3rd ventricles from passing into the 4th ventricle and therefore cannot drain out of the holes.
Name four causes of hydrocephalus
- Genetic
- Prenantal viral infection
- Intraventricular haemorrhage
- Spina bifida cystica
Describe the cellular differentiation in the neural tube
- Needs to make neuronal cells (neurons and glia) and connect them together
- Begins in early embryogenesis and continues into postnatal period
- Some regions (i.e. olfactory epithelium) add new cells throughout life
Describe the neural tube developing epithelium
- Initially a single layer of rapidly dividing neuroepithelial cells
- Pseudostratified epithelium
- Divide at the ventricular surface
Name the cells that develop from neuroepithelial cells
- Neuron
- Astrocyte
- Oligodendrocyte
One the neuroepithelial cells have finished dividing they become ependymal cells
What develops from the neural crest cells?
Peripheral nervous system (PNS) and the autonomic nervous system (neurones and glia).
They also form non-neuronal cells like melanocytes, adrenal medullar, meninges, facial bones/cartilage.
What does the dorsal root ganglia contain?
Cell bodies on sensory neurons
What do dorsal root ganglia develop from?
Neural crest cells
What are the two process in the spinal nerve development?
- Peripheral process -> body
* Central process -> dorsal horn of spinal cord (via dorsal roots)
What do the motor and sensory components form from?
- Motor component – motor neurons of spinal cord – formed by the neural tube
- Sensory from neural crest cells
What are the positional changes of the spinal cord that occur?
- 3rd month – spinal cord extends entire length of vertebral column.
- Vertebral column and dura mater grow more rapidly -> cord at progressively higher levels (newborn, ~L2 or L3; adult ~L1).
- Initially spinal nerves found at level of origin.
- Become elongated forming cauda equina (horses tail).
- Pia mater forms terminal filum (long fibrous thread).
What is cortical folding?
Surface of cerebral hemispheres are initially smooth and then begin to develop sulci (grooves) and gyri (elevations)
What is the benefit of cortical folding?
- Folding of brain save space and minimises brain volume
- Bring together brain regions that would have otherwise been far apart – optimises brain wiring a functional organisation
Name a congenital malformation caused by abnormal cortical folding
- Lissencephaly (no cortical folding)
* Polymicrogyria (over folding)
What are the features of lissencephaly?
- Rare brain disorder
- Caused by defective neuronal migration
- Gyri and sulci fail to develop
- Results in severe mental impairment, failure to thrive, seizures and abnormal muscle tone
What is polymicrogyria?
- Excessive number of small gyri
* Variable degree of neurological problems (i.e. mental retardation, seizures, motor deficits)
Name four other congenital abnormalities?
- Microcephaly
- Aegenesis corpus callosum
- Porencephaly
- Schizencephaly
What is microcephaly?
Head (circumference) is smaller than normal
- Intellectual impairment
- Delayed motor functions/speech
- Hyperactivity
- Seizures
- Balance/co-ordination problems
What are causes of microcephaly?
Genetic, drugs or infection (i.e. zika virus)
What is aegenesis corpus callosum?
Complete or partial absence of the corpus callous
- Effects range from subtle - severe
- Cognitive and social difficulties -> intellectual impairment, seizures, hypotonia
What is porencephaly?
CSF filled cysts or cavities within the cerebral hemisphere
• Delayed growth and development, seizures, hypotonia, intellectual impairment
What is the cause of porencephaly?
Usually postnatal stroke or infection
What is schizencephaly?
Large clefts or slits in the cerebral hemisphere
• Paralysis, seizures, intellectual impairment, developmental delay
What are causes of schizencephaly?
Genetic, in utero stroke, infection
What is diastematomyelia?
- Spinal cord split longitudinally into two parts
- Associated with vertebral abnormalities
- Bone or cartilaginous process ‘fixes’ cord in place
- Scoliosis, weakness of lower extremities, hairy patch over lower back, foot deformities, loss of sensation
Name five causes of intellectual impairment
Often no obvious gross brain defects
- Genetic (i.e. downs syndrome)
- Radiation
- Infectious agents (i.e. rubella, toxoplasmosis, cytomegalovirus)
- Birth trauma (neurons due to the lack of O2)
- Postnatal insults (i.e. head injury, infections (i.e. meningitis), lead exposure)
Common: maternal alcohol abuse -> foetal alcohol syndrome
