Embryology Flashcards
what embryonic layer develops into the nervous system?
ectoderm
what does thickening of the ectoderm anterior to the primitive node form?
neural plate
the edges of the ectoderm thicken and move upwards in the 3rd week forming what?
neural folds
what is formed when the neural folds migrate towards each other and fuse in the midline?
the neural tube
when does the anterior neuropore close?
18-20 somite stage (25 days)
when does the posterior neuropore close?
day 27
describe neural tube closure
> initiated at several pints along the AP axis
proceeds in cranial and caudal directions
begins at day 18
when is neural tube closure completed?
by the end of the 4th week
how may closure sites of the neural tube are in humans?
up to 5
name some conditions arising from neural tube defects
> anencephaly
encephalocoele
spina bifida
what causes meroencephaly?
failure of the anterior neuropore to close so the skull fails to form and the brain tissue degenerates
what is craniorachischisis?
failure of the neural tube closure along the entire neuroaxis
what causes encephalocoele?
failure in closure of rostral neural tube
what is the effect of encephalocoele?
herniation of cerebral tissue through a defect in the skull with a variable degree of neurological deficits
what part of the skull is encephalocoele most frequent?
the occipital region
in spina bifida where is the closure of the neural tube defective?
the causal end
what is the effect of spina bifida?
> affects tissues overlying the spinal cord
can affect the neural tissue
severity ranges from minor to major clinical symptoms
define spina bifida
non fusion of the vertebral arches
describe the cause of spina bifida occulta
failure of the embryonic halfs of vertebral arches to grow normally and fuse.
at what vertebral level does spina bifida occulta normally occur?
L5 and L6
what are the effects of spina bifida occulta?
> usually no clinical symptoms
> may result in a small tuft of hair
what is spina bifida cystica?
protrusion of spinal cord and/or meninges through the defect in the vertebral arches
what is spina bifida meningocele?
> rarest form
> protrusion of meninges and cerebral spinal fluid
what is spina bifida meningomyelocle?
> nerve roots and/or spinal cord involved
neurological deficit (sensation loss and paralysis)
associated with hydrocephalus
what is the most severe form of spina bifida cystica?
spina bifida myeloschisis
describe spina bifida myeoschisis
the spinal cord in the affected area is open due to failure of the neural folds to fuse
how is spina bifida incidence decreased in the UK
> folic acid supplements
maternal blood screening
amniocentisis
ultrasound
describe maternal blood screening in spina bifida
> high levels of alpha-fetoprotein in serum
> best detected at1 16-20 weeks
what are the risk factors for spina bifida?
> genetic predisposition
nutritional (too little foliate, too much vitamin a)
environmental (hyperthermia, drugs)
when does brain vesicle development begin?
with the closure of anterior neuropore
name the 3 primary brain vesicles
> prosencephalon (forebrain)
mesencephalon (midbrain)
rhombencephaon (hindbrain)
name the 5 secondary brain vesicles
> telencephalon > diencephalon > mesencephalon > metencephalon > myelencephalon
where is the cephalic flexure?
between the midbrain (mesencephalon) and the hindbrain (rhombencephalon)
where is the cervical flexure?
between the hindbrain and the spinal cord
where is the pontine flexure?
in the hindbrain between the metencephalon and myelencephalon
what makes up the forebrain (prosencephalon)?
> telencephalon (hemispheres, hippocampus and basal ganglia)
diencephalon (thalamus, pituitary, diencephalon, pineal)
mesencephalon (superior and inferior colliculi)
what makes up the rhombencephalon- hindbrain?
> metencephalon (cerebellum, pons)
> myelencephalon (medulla)
when does CSF begin to form?
during the 5th weeks
where is CSF produced?
by the choroid plexus in the 3rd and 4th lateral ventricles
what is hydrocephalus?
accumulation of CSF resulting in an enlarged brain and cranium. it is frequently due to a blocked aqueduct preventing CSF from the lateral and 3rd ventricles passing into the 4th ventricle
what can cause hydrocephalus?
> genetic
prenatal viral infection
intra-ventricular haemorrhage
spina bifida cystica
what cells initially make up the neural tube?
a single layer of neuroepithelial cells
what do neuro-epithelial cells differentiate into?
> neurons
astrocytes
oligodendrocytes
what forms microglia?
mesenchymal cells that migrate into the CSF
what two processes does the neural crest extend?
> peripheral process (body)
> central process (dorsal horn of spinal cord)
describe the positional changes in the spinal cord
> 3rd month it extends the entire length of the vertebral column
vertebral column and dura mater grow more rapidly so spinal cord is at progressively higher levels
spinal nerves become elongated forming cauda equina
what form the terminal filum?
pia mater
what are the sympathetic and parasympathetic ganglia formed by?
neural crest cells
describe the formation of the sulci and gyri
brain surface is initially smooth but rapid growth results in the development of sulci and gyri, the pattern becomes more complex as the brain develops
what is lissencephaly?
> smooth brain where the gryi and sulci fail to develop
> rare disorder
what causes lissencephaly?
defective neural migration
what is the result of lissencephaly?
> severe mental impairment
failure to thrive
seizures
abnormal muscle tone
what is polymicrogyria?
excessive number of small gyri resulting in a variable degree of neurological problems (mental retardation, seizures, motor deficits)
what is the effect of microcephaly?
> intellectual impairment > delayed motor and speech function > hyperactivity > seizures > balance problems
what are the effects of aegenesis corpus callosum?
> cognitive and social difficulties > intellectual impairment > seizures > hypotonia > can range from subtle to severe
what is porencephaly?
CSF filled cysts or cavities from a postnatal stroke or infection
what is the effect of porencephaly?
> delayed growth and development
seizures
hypotonia
intellectual impairment
what is schizencephaly?
large clefts or slits in the brain due to genetics, in utero stroke or infection
what is the effect of schizencephaly?
> paralysis
seizures
intellectual impairment
developmental delay
what is diastematomyelia?
where the spinal cord is split longitudinally into 2 pairs and bony/cartilaginous process fixes the cord in place
what is the effects of diastematomyelia?
> scoliosis > weakness of extremities > hairy patch over lower back > foot deformities > loss of sensation
what are some causes for intellectual impairment when there is no gross brain defect?
> genetic > radiation > infectious agents > birth trauma > postnatal insults > maternal alcohol abuse
