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
Polymicrogyria
- Excessive number of small gyri
- Variable degree of neurological problems (e.g mental retartedness, seizures, motor defecitsd etc)
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
Neural Crest Derivatives
- Sensory neurons
- Autonomic neurons (sympathetic and parasympathetic ganglia)
- Schwann cells + satellite cells of ganglia
- Aortic plexuses (nerves surrounding the aorta)
- Endocrine cells (chromaffin cells of adrenal medulla; calcitonin producing cells of carotid body; parafollicular cells of thyroid)
- Enteric nervous system
- Pigment cells (melanocytes) - all pigment except RPE
- Leptomeninges of anterior brain (prosencephalon and part of mesencephalon)
- Non- neural head structures (including bone, cartilage and connective tissue)
- Teeth (dental papilla; odontoblasts)
- Iris muscle and pigment; ciliary muscles
- Corneal stroma
- Inner ear development
- Regions of the heart +m walls of large arteries
- 4th germ layer
What does neural crest form
Spinal nerve development
Positional changes of the spinal cord
Rare congenital anomalies
Diastematomyelia
Intellectual impairment
Development of the nervous system begins
In week 3
Neural tube develops from
Embryonic ectoderm
Neural tube closes
End of 4th week
Defects in closure results in NTDs
Neural tube forms
Neurons and Glia of CNS
Neural crest cells
Form neurons and glia of PNS ( plus non-neuronal cels)
Expansion and bending of neural tube forms
3 primary and 5 secondary brain vesicles
Lumen of neural tube forms
Ventricular system
Congenital anomalies include
Structural defects
Failure of commissure formation
Abnormal cysts and clefts
