Neuro Embryology Flashcards
What does the Ectoderm form?
Epidermis
Nervous system
What does the Mesoderm form?
Bone
Muscle
Blood cells
etc
What does the endoderm form?
Gut
Pharynx
Lungs
What is the hypoblast?
Lining of the yolk sac
What does the epiblast form?
Generates all 3 germ layers from the primitive streak
How is the neural plate formed?
Thickening of ectoderm anterior to the primative node
primitive streak at one end of the trilaminar disc; neural plate at the other
After its formation the neural plates edges thicken and move upwards.
What does this form?
Neural folds
What is the groove in the centre of plates between the converging neural folds called?
What does this do?
Neural groove
divides the future left and right sides of the embryo
How is the neural tube formed from the neural folds?
Neural folds migrate towards echoer and fuse at the midline.
Neural tube zips up
The neural tube remains open at the anterior and posterior ends.
When do these ends close?
Anterior (cranial) neuropore
-about day 25
Posterior (caudal) neuropore
-about day 27
Describe the morphological cell changes in neurolation
Neural plate forms by notochord signalling ectodermal cells to become columnar
Neural tube starts to fold by cells becoming pyramidal, then triangular (notochord signals)
Fusion is by wound healing mechanisms
How does the ectodermal cells forming the neural tube separate from those that go onto form the epidermis?
Once the neural tube is fused this breaks off from the ectoderm and the remaining cell layer will form the epidermis
Describe neural tube closure
Essential for normal development and function.
- Initiated at several points along A-P axis
- Proceeds in cranial and caudal directions
- Begins day 18
- Completed by end of 4th week (around day 27)
- Up to 5 closure sites in humans
Failure of neural tube to close properly leads to neural tube defects (NTDs).
Name the 3 we need to know about
Anencephaly
Encephalocoele
Spina bifida
These are some of the most common congenital abnormalities of the CNS
What is Anencephaly?
AKA Exencephaly or Meroencephaly
1:1500 births (4x more common in females
Failure of anterior neuropore to close so telencephalon never develops.
Skull doesnt form over area because brain doesnt form
What is craniorachischisis?
Craniorachischisis is the most severe form of neural tube defect in which both the brain and spinal cord remain open to varying degrees. It is a very rare congenital malformation of the central nervous system.
Failure of anterior neuropore and rostral neural tube to close
What is encephaloceole?
1:4000 births
Herniation of cerebral tissue through a defect in the skull
Failure in closure of rostal neural tube
Most frequent in occipital region
Variable degree of neurological deficits (depending on size and location)
What are the two forms of encephalocoele?
Occipital- encephalocoele
Fronto-nasal encephalocoele
What is spina bifida?
Defective closure of the caudal neural tube.
Affects 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
What is Spina Bifida occulta?
Most minor form
failure for embryonic halves of vertebral arch to grow normally and fuse
Occurs in L5 and L6 vertebrea of 10% of otherwise 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
Three types:
- Spinal bifida with meningocele
- Spina bifida with meningomyelocle
- Myeloschisis
What is spinal bifida with meningocele?
Rarest form
Protrusion of meninges and cerebrospinal fluid
What is 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
What is myeloschisis?
Most severe form of spina bifida.
Spinal cord in affected area open due to failure of neural folds to fuse.
-> exposed to amniotic fluid as a result causing degeneration
How are Neural tube defects diagnosed?
Prenatal Diagnosis:
-Maternal blood screening
(Indicated by high level alpha-fetoprotein (AFP) in serum)
-Amniocentisis
(high levels AFP in amniotic fluid)
-Ultrasound
(Ancephaly from 12 weeks, spina bifida from 16-20 weeks
How can AFP be detected in maternal blood for Neural tube defect diagnosis?
AFP from foetal liver leaks into amniotic fluid then into maternal blood.
Best detected at 16-20 weeks
What reduces risk of neural tube defects?
Folic acid supplements
400ug/day causes 50-70% decrease in risk
What are the risk factors for neural tube defects?
Genetic predisposition
Nutritional (e.g. too little folate, too much vitamin A)
Environmental (e.g. hyperthermia, taking certain drugs ( e.g. sodium valproate))
Why shouldnt you eat liver when pregnant?
High level of vitamin A
What are the 3 primary brain vesicles?
Prosencephalon
Mesencephalon
Rhombencephalon
What does the Prosencephalon divide into?
Telencephalon
Diencephalon
What does the Rhombencephalon divide into?
Metencephalon
Myelencephalon
What are the 5 secondary brain vesicles?
telencephalon diencephalon mesencephalon metencephalon myelencephalon
When does the cephalic flexure develop?
Where does it develop?
End of 3rd week
Between midbrain (mesencephalon) and hindbrain (rhombencephalon)
When does the cervicle flexure develop?
Where does it develop?
End of 4th week
Between hindbrain and spinal cord
When does the pontine flexure develop?
Where does it develop?
5th week
In hindbrain (between metencephalon and myelencephalon)
What does the telencephalon go onto form?
Cerebral hemispheres
Hippocampus
Basal ganglia
What does the diencephalon go onto form?
Thalamus
Hypothalamus
Pituitary gland
Pineal
What does the mesencephalon go onto form?
Midbrain
Superior and inferior colliculi
What does the metencephalon go onto form?
Cerebellum
Pons
What does the myelencephalon go onto form?
Medulla
What does the lumen of the telencephalon form?
Lateral ventricles
What does the lumen of the diencephalon form?
3rd ventricle
What does the mesencephalon lumen form?
Cerebral aqueduct
What does the metencephalon and myelencephalon lumen form?
4th ventricle
When does CSF start to be produced?
During 5th week
Produced predominantly by choroid plexus in 3rd and 4th lateral ventricles
Describe hydrocephalus during development
Hydrocephalus = Accumulation of CSF
Results in enlarged brain and cranium
Frequently due to blocked aqueduct
Prevents CSF from lateral and 3rd ventricle passing to 4th ventricle
What are the causes of hydrocephalus during development?
Genetic
Prenatal viral infection
Intraventricular haemorrhage
Spinal bifida cystica
What is the cell type of the neural tube?
Initially a single layer of rapidly dividing neuroepithelial cells
Then pseudostratified epithelium
Divide at ventricular surface
Neuroepithelium produces most cells of the CNS.
What cell type does it not produce?
Microglia
Mesenchymal cells that migrate into CNS
Describe the positional changes of the spinal cord during development
3rd month - extends the entire length of the vertebral column
Vertebral column and the fixed dura mater grow more rapidly -> cord at progressively higher levels (L2 or L3, L1 in adults)
Initially spinal nerves found at level of origin.
Become elongated forming cauda equina
Pia mater forms terminal filum
What are sympathetic and parasympathetic ganglia formed by?
Neural crest cells
How are sulci and gyri formed?
Surface of cerebral hemispheres initially smooth
Rapid growth results in the development of sulci and gyri.
Pattern becomes more complex as brain enlarges
What is 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 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)
What is microencephaly?
“Small Brain”
Genetic, Drugs, Infection
Intellectual impairment, delayed motor functions/speech, hyperactivity seizures, balance and coordination problems etc
What is Aegenesis corpus callosum?
Corpus callosum fails to develop.
Can occur alone or with other abnormalities
Effects range from subtle to severe
Cognitive and social difficulties -> intellectual impairment, seizures, hypotonia etc
What is Porencephaly?
CSF filled cysts or cavities
Usually from postnatal stroke or infection
Delayed growth and development, seizures hypotonia, intelectual impairment
What is Schizencephaly?
Large clefts or slits in the brain
Genetic, in utero stroke, infection
Paralysis, Seizures, intellectual impairment, developmental delay
What is Diastermatomyelia?
Split cord malformation
Spinal cord split longitudinally into 2 parts
Usually associated with vertebral anomalies
Bony or cartilaginous process “fixes” cord in place
Scoliosis, weakness of lower extremities, hairy patch over lower back, foot deformities, loss of sensation
What are the causes of intellectual impairment?
Often no obvious gross brain defects
- Genetic (e.g. Down’s)
- Radiation
- Infectious agents (e.g. rubella, toxoplasmosis, CMV)
- Birth trauma
- Postnatal insults (head injury, infections (e.g. meningitis), lead exposure)
MATERNAL ALCOHOL ABUSE
-Foetal alcohol syndrome
When does the development of the nervous system begin?
Week 3
What forms the neurons and glia of CNS?
Neural tube
What forms the neurons and glia of the PNS?
neural crest cells
plus non-neuronal cells
