Embryology and Congenital Malformations Flashcards
what germ layer does the nervous system develop from?
Ectoderm-outer layer
wen does the CNS appear?
beginning of the 3rd week
what is the neural plate?
Thickening of ectoderm anterior to the primitive node
what are neural folds?
Edges thicken and move upwards to form neural folds
what is the neural tube?
Neural folds migrate towards each other and fuse at midline forming neural tube
which ends of the neural tube initially remain open?
anterior and posterior ends
when does the Anterior (cranial/rostral) neuropore close?
~25 days
when does the Posterior (caudal) neuropore close?
~27 days
describe the closure of the neural tube
Initiated at several points along A-P axis.
Proceeds in cranial and caudal directions.
Begins day 18
Completed by end of 4th week (~day 27).
name some neural tube defects
Anencephaly
Encephalocoele
Spina bifida
what is Anencephaly?
1:1500 births (~4x more common in females) Failure of anterior neuropore to close. Skull fails to form Brain tissue degenerates. Incompatible with life.
what is Craniorachischisis?
failure of anterior neuropore and rostral neural tube to close
what is Encephalocoele?
1:4000 births
Herniation of cerebral tissue through a defect in the skull.
Failure in closure of rostral neural tube.
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 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 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.
what is Spina bifida cystica?
Protrusion of spinal cord and/or meninges through the defect in the vertebral arches.
1:1000 live births
what is Spina bifida with meningocele
Rarest form
Protrusion of meninges and cerebrospinal fluid
easily removed with surgery
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 Spina bifida with
myeloschisis
Most severe form
Spinal cord in affected area open due to failure of neural folds to fuse.
leads to degeneration of spinal cord
what can prevent neural tube defects?
Folic acid supplements
50-70% decrease in risk
describe the prenatal diagnosis of neural tube defects?
Maternal blood screening
Indicated by high levels α-fetoprotein (AFP) in maternal serum
Best detected 16 - 20 weeks Amniocentisis- high levels AFP in amniotic fluid
Ultrasound
(Anencephaly from 12 weeks, spina bifida from 16-20 weeks)
what are the risk factors of 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)
when do the brain vesicles start to develop?
~25 days
when and where does Cephalic flexure occur?
End of 3rd week. Between midbrain (mesencephalon) and hindbrain (rhombencephalon).
when and where does Cervical flexure occur?
End of 4th week. Between hindbrain and spinal cord.
when and where does Pontine flexure occur?
5th week In hindbrain (between metencephalon and myelencephalon).
when and how does the CSF begin to form?
during 5th week
Produced predominately by choroid plexus (in 3rd, 4th, lateral ventricles)
Drains into subarachnoid space via openings in roof of 4th ventricle
Absorbed into venous system
what is Hydrocephalus?
Accumulation of cerebral spinal fluid
Results in enlarged brain and cranium
Frequently due to blocked aqueduct
Prevents CSF from lateral and 3rd ventricles passing into the 4th ventricle can’t drain properly
what is the initial cell layer of the neural tube?
neuroepithelial cells.
Pseudostratified epithelium.
Neuroepithelium produces all cells of the CNS except?
microglia – mesenchymal cells migrate into CNS
what forms the neurons and glia of CNS?
Neural tube
what forms the neurons and glia of PNS (plus non-neuronal cells)?
Neural crest
what forms the body and dorsal horn of the spinal cord?
Neural crest
what forms the motor neurones of the spinal cord?
Neural tube
what are the Positional changes of the spinal cord.?
3rd month – 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).
what forms the Sympathetic and parasympathetic ganglia ?
Neural crest
when are the sulk and gyro fully formed in development?
38 weeks
what is 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.
Many affected children die before age 10.
what is Polymicrogyria?
Excessive number of small gyri.
Variable degree of neurological problems (e.g. mental retardation, seizures, motor deficits etc)
