Embryology and Congenital Malformations Flashcards
From what layer of the embryo does the nervous system develop?
The ectoderm
At what point in development does the CNS appear?
Beginning of 3rd week
How is the neural plate formed?
Thickening of the ectoderm anterior to the primitive node
How are the neural folds formed?
Edges of the neural plate thickening and moving upwards
How is the neural groove formed?
The neural folds converge forming a central groove between the neural plates
What does the neural tube do physically?
Divides the future left and right sides of the embryo
How is the neural tube formed?
The neural folds migrate towards each other and fuse at the midline
What is characteristic of the anterior and posterior ends of the neural tube at the beginning of their development
They are open
What are the openings at the anterior and posterior ends of the neural tube called?
Neuropores
AT what stage does the anterior/cranial/rostral neuropore close?
Around day 25
At what stage does the posterior/caudal neuropore close?
Around day 27
Describe the process of the neural tube closure
It is essential for normal development and function
It begins at several points along the A-P axis, proceeding in cranial and caudal directions
The process begins at day 18
The process completes by day 27
Humans have up to 5 closure sites
At what day does the closure of the neural tube commence?
Day 18
At what stage does the closure of the neural tube occur by?
Day 27
Give three major neurael tube ddefects (NTDs) which are caused by failure of the neural tube to close normally
Anencephaly
Encephalocoele
Spina bifida
These are some of the most common congenital abnormalities of the central nervous system
Describe exencephaly/anencephaly
Occurs in one in 1500 births, is 4 times more common in women
The anterior neuropore fails to form, causing the brain tissue to degenerate. This is incompatible with life and the child is unlikely to survive full term
What is craniorachischisis?
A more severe form of exencephaly where both the anterior neuropore and the rostral neural folds fail to close
Describe encephalocele
Occurs in one in 4000 births
Causes by a herniation of cerebral tissue through a defect in the skull
This is due to failure of the rostral neural tube to close
Most frequent in the occipital region - varying degree of corresponding neurological deficits
Describe generally spina bifida
Caused by a defective closure of the caudal neural tube that affects tissues overlying the spinal cord
It is caused specifically by non-fusion of the vertebral arches. The neural tissue may or may not be affected, but the degree to which it is will determine the severity of the effects of the malformation
Describe spina bifida occulta
This is the most minor form of spina bifida
Causes by failure of the embryonic halves of the vertebral arch to grow normally and fuse
Occurs in L5 and L6 of 10% of otherwise healthy people
Often no clinical symptoms but may result in dimple with small tuft of hair
Describe spina bifida with meningocele
This is the rarest form of spina bifida and features a protrusion of the meninges and cerebrospinal fluid
Describe spina bifida with meningomyelocle
In this form the nerve roots and or spinal cord are included in the protruding sac
Results in loss of sensation and muscle paralysis that generally affects bladder and sexual function and the lower limbs depending on the level of the lesion
Often associated with hydrocephalus
Explain the link between spina bifida with meningomyelocle and hydrocephalus
The cowards movement of the vertebral column that pulls the spinal cord downwards during development causes the brain to be pulled down and block the drainage of CSF, resulting in hydrocephalus
Describe spina bifida in the myeloschisis form
This is the most severe form of spina bifida, the spinal cord in the affected area is open due to failure of the neural folds to fuse
Describe some preventative methods to counter spina bifida
Folic acid supplements can lead to a 50-75% decrease in risk
Maternal blood screening to test for high AFP (alpha-fetoprotein) in serum - AFP from foetal liver leaks into amniotic fluid and then into maternal blood at 16-20 weeks
Amniocentesis
Ultrasound
What are some risk factors for developing spina bifida?
Genetic predisposition
Nutritional factors e.g. too little folate, too much vit A (pregnant women should avoid liver) - potent teratogen
Environmental factors e.g. hyperthermia, certain drugs - teratogens - such as sodium valproate for epilepsy
What event marks the beginning of the development of the brain vesicles?
The close of the anterior neuropore at around 25 days
What three brain vesicles are present by the time of the posterior neuropore closure at 27 days?
Prosencephalon (forebrain)
Mesencephalon (midbrain)
Rhombencephalon (hindbrain)
What are the two parts of the prosencephalon present by 32 days?
Telencephalon
Diencephalon
What are the two parts of the rhombencephalon present by day 32?
Metencephalon
Myelencephalon
What the three primary brain vesicles?
Prosencephalon
Mesencephalon
Rhombencephalon
What are the 5 secondary brain vesicles?
- Telencephalon and
- Diencephalon (make up prosencephalon
- Mesencephalon
- Mesencephalon and 5. Myelencephalon
What is the cephalic flexure and when does it form?
Flexure between the midbrain and hindbrain
End of week 3
What is the cervical flexure and when does it form?
Flexure between the hindbrain and the spinal cord
End of week 4
What is the pontine flexure and when does it form?
In the hindbrain between the mesencephalon and the myelencephalon
Week 5
What structures make up or are formed from the telencephalon?
Cerebral hemispheres, hippocampus, basal ganglia
What structures make up the diencephalon?
Thalamus, hypothalamus, pituitary gland, pineal gland
What structures make up the mesencephalon?
Superior and inferior colliculi
What structures make up the metencephalon?
Cerebellum, pons
What structure makes up the myelencephalon?
Medulla
What do the lumen of the neural tubes form?
The ventricular system
What does the lumen of the telencephalon form?
The lateral ventricles
What does the lumen of the diencephalon form?
3rd ventricle
What does the lumen of the mesencephalon form?
Cerebral aqueduct
What does the lumen of the metencephalon form?
4th ventricle
What does the lumen of the myelencephalon form?
Central canal
When does the CSF begin to form?
Week 5
Where is the majority of CSF produced?
Choroid plexus in the 3rd 4th and lateral ventricles
Where does CSF drain to?
The subarachnoid space via openings in the roof of the 4th ventricle and is absorbed into the venous system
What is hydrocephalus?
Excess accumulation of CSF in the brain, resulting in enlarged brain and cranium - CSF cannot pass from lateral and 3rd ventricles into 4th ventricle to drain, often caused by a blocked cerebral aqueduct
What is the most common direct physical cause of hydrocephalus?
A blocked cerebral aqueduct
What are some general causes of hydrocephalus?
Genetic
Prenatal viral infection
Intraventricular haemorrhage
Spina bifida cystica
Describe the histopathology of the neural tube
Initially a single layer of rapidly dividing neuroepithelial cells, pseudostratified epithelium
Divide at ventricular surface
What cells of the CNS are produced by the neuroepithelium?
Neurons
Astrocytes
Oligodendrocytes
Ependymal cells
What CNS cell type is not produced by the neuroepithelium?
Microglia (the macrophages of the CNS) - these are mesenchymal cells that migrate into the CNS
What cells are formed from the neural crest cells?
Cells of the PNS, autonomic nervous system (neurons and glia)
Are sensory components of spinal nerves formed from neural tube or neural crest cells?
Neural crest
Are motor components of spinal nerves formed from neural tube or neural crest cells?
Neural tube
Are parasympathetic and sympathetic ganglia formed by neural tube or neural crest cells?
Neural crest cells
At what stage of development of the sulci and gyri of the brain start to develop?
26 weeks and become more complex as the brain develops
What is lissencephaly?
A rare brain disorder also known as ‘smooth brain’ that is caused by defective neuronal migration
The gyri and sulci fail to develop
Results in severe mental impairment, failure to thrive, seizures and abnormal muscle tone
Many affected children die before age 10
What is polymicroglia?
Excessive number of small gyri
Variable degree of neurological problems e.g. mental retardation, seizures, motor deficits etc.
Discuss mental impairment in the absence of obvious brain defects
It is often the case that there are no gross obvious brain lesions or defects. The mental impairment may be caused by:
Genetic e.g. Down’s syndrome
Radiation
Infection e.g. rubella, toxoplasmosis
Birth trauma
Postnatal insults e.g. head injury, infections - meningitis
A common cause that should always be considered is maternal alcohol abuse leading to foetal alcohol syndrome in 1:1000 births. Of these 1% are estimated to have some degree of mental impairment
When does the development of the nervous system begin?
Week 3
