Embryology Flashcards
The nervous system develops from
The embryonic ectoderm
CNS appears at the beginning of the
3 rd week
- Thickening of ectoderm anterior to the primitive node
- Edges thicken and move upwards to form the neural folds
Neural plate rolls up to form the
Neural tube
- Neural folds migrate towards each other and fuse at midline forming the NEURAL TUBE
- Neural tube initially remains open at anterior and posterior ends
- Anterior (cranial/rostral) neuropore closes 18-20 somite stage (~25 days)
- Posterior (caudal) neuropore closes ~ day 27
Neurulation in more detail
Neural tube closure
- Essential for normal development and function
- Initiated at several points along A-P axis
- Proceeds in cranial and caudal direction
- Begins day 18
- Completed by end of 4th week (~day 27)
- Up to 5 closure sites in humans
Failure of the neural tube to close properly
Neural tube defects (NTDs)
- Anencephaly
- Encephalocoele
- Spina bifidia
Exencephaly/Anencephaly (Meroencephaly)
- 1:1500 births (~4x more common in females)
- Failure of anterior neuropore to close
- Skull fails to form
- Brain tissue degenerates
- Incompatible with life
Encephalocoele
- 1:4000 births
- Herniation of cerebral tissue through a defect in the skull
- Failure in closure of rostral neural tube
- Most frequently in occipital region
- Variable degree of neurological deficits
Spina Bifida
- Defective closure of the caudal neural tube
- Affect tissues overlying the spinal cord
- Spina bifida= non-fusion of vertebral arches
- Neural tissue may or may not be affected
- Severity ranges from minor abnormalities to major clinical symptoms
Spina bifida occulta
- Most minor form
- Failure of embryonic halves of vertebral arch to grow normally and fuse
- Occurs in L5 and L6 vertebrae of 10% of otherwise healthy people
- Usually no clinical symptoms
- May result in dimple with small tuft of hair
Spina bifida cystica
- Protusion of spinal cord and/or meninges through the defect in the vertebral arches
- 1:1000 live births
Spina bidida with meningocele
- Rarest form
- Protusion of meninges and cerebrospinal fluid
Spina bifida with meningomyelocle
- Nerve roots and/or spinal cord included in the sac
- Neurological deficits- loss of sensation and muscle paralysis
- Area affected determined by level of lesion
- Often associated with hydrocephalus
Spina bifida myeloschisis
- Spinal cord in affected area open due to failure of neural folds to fuse
Prevention, prenatal diagnosis and risk factors
Incidence decreasing in Uk:
- Folic acid supplement (400•µg/day è 50-70% decrease in risk)•
- Prenatal diagnosis:
- Maternal blood screening
Indicated by high levels α-fetoprotein (AFP) in serum – AFP from foetal liver leaks into amniotic fluid then into maternal blood.
Best detected 16 - 20 weeks.
Risk factors
- Genetic predisposition
- Nutritional (e.g too little folate, too much vitamin A)
- Enviromental (e.g hyperthermia; taking certain drugs- e.g sodium valproate)
- Amniocentesis
(high levels AFP in amniotic fluid)
- Ultrasound
(Anencephaly from 12 weeks, spina bifida from 16-20 weeks)
Development of the brain and spinal cord
Folding of the CNS
Structure formed from the brain vesicles
Cortical folding
Lissencephaly
SMOOTH BRAIN
- 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
- Many affeted children die before age 10
What forms the ventricular system?
Hydrocephalus
Cellular differentiation in the neural tube
Neural tube produce most cells of the CNS
Morphological differentiation of the spinal cord
Peripheral nervous system develops from neural crest cells
What does neural crest form
Spinal nerve development
Positional changes of the spinal cord
Rare congenital anomalies
Diastematomyelia
Intellectual impairment
