NS 1: Development of NS Flashcards
When does the CNS begin development?
at the beginning of the 3rd wk as the neural plate= an ectodermal thickening in the mid-dorsal region, in front of the primitive node.
what regulates notochord formation?
the primitive node= elevated region around cranial end of primitive streak.
what is the primitive streak?
groove formed in the epiblast at the caudal end of the embryo at the bilaminar germ disc stage through which epiblast cells migrate to form endoderm and mesoderm during gastrulation- formation of trilaminar disc from bilaminar disc.
what is the remnant of the notochord in the adult?
the nucleus pulposus= central gelatinous portion of IV disc.
how is the notochord formed?
by prenotochordal cells migrating through the primitive pit (depression in primitive node) to form a solid cord of cells..
what does the definitive notochord serve as the basis of?
midline
axial skeleton
neural tube
describe the formation of the neural tube
signalling molecules from the notochord cause thickening of neuroectoderm to form the neural plate, whose lateral edges soon elevate to form the neural folds. These continue to elevate with further development, approach each other in midline and fuse to form the neural tube.
Fusion starts in cervical region and then proceeds both cranially and caudally.
name given to depressed midregion in neural tube formation?
neural groove
how long does it take for the neural tube to be completely closed?
10 days
name given to openings of neural tube formed during fusion that communicate with the amniotic cavity?
anterior and posterior neuropores
when do the anterior and posterior neuropores close?
anterior= day 25 posterior= day 28
what happens if the anterior neuropore fails to close?
anencephaly- absent brain and calvaria
what happens if neural tube fails to close caudally?
spina bifida
what can the mother take to prevent NT defects in pregnancy?
folic acid- for 3 months pre-conceptually and for the 1st trimester
what can be measured in the maternal serum to assess for neural tube defects?
how else can NTDs be assessed for?
alpha-fetoprotein, if increased may signify NT defect
USS- by 12 wks defects in closure of vertebral arches ca be detected.
what is the vertebral arch?
the region of the vertebra comprising the pedicle (between the VB and the transverse process) and the lamina (between the transverse process and the spinous process).
what is spina bifida occulta?
neural tube defect where abnormal closure of neural folds in caudal region produces defect in vertebral arches (lack of fusion) covered by skin, and DOES NOT normally involve neural tissue.
patch of hair may overly affected region
may be incidental finding on back X-ray
define spina bifida
all neural tube defects affecting the spinal region, producing a defect as the bony arch of 1 or more vertebrae has failed to fuse dorsal to the SC.
name given to NTD where only fluid-filled meinges protrude through defect in vertebral arches and skin?
meningocele
what is a myelomeningocele?
protrusion of meninges and neural tissue through a defect in the vertebral arches and skin, forming a cyst-like sac, as a result of neural tube failing to close properly in caudal region.
what is rachischisis?
failure of neural folds to elevate, so remain as a flattened mass of neural tissue, so no neural tube formed.
If spina bifida, what would cause intellectual disability?
would result secondary to hydrocephalus, spina bifida alone results in neurological defecits but usually not associated with intellectual disability.
what is hydrocephaly often related to when occurring in children with severe NTDs?
presence of an Arnold-Chiari malformation- herniation of part of cerebellum (from metencephalon) into foramen magnum, which obstructs CSF flow.
why can herniation of the cerebellum occur in spina bifida, producing hydrocephaly?
tethering of SC to VC due to its abnormal development. As VC lengthens, tethering of SC pulls cerebellum into foramen magnum, cutting off CSF flow.
how can hydrocephalus be treated?
insertion of a ventriculoperitoneal shunt- allows CSF drainage from 1 of cerebral ventricles to peritoneal cavity.
why must spinal roots elongate during development?
VC grows faster than spinal nerve when trunk of body develops, and roots still exit at their IV foramen.
where is the needle inserted to tap CSF in a lumbar puncture e.g. to assess for bacterial meningitis?
at the lower lumbar level (L4-L5), to avoid the lower end of the SC.
what do the dilations of the neural tube at its cranial end become after neural tube closure?
the 3 primary brain vesicles: prosencephalon, mesencephalon and rhombencephalon
what are the seconday brain vesicles and when are they formed?
at 5 wks of development telencephalon diencephalon mesencephalon metencephalon myelencephalon
why do flexures form in the neural tube?
growth and development of cranial NT excess available space linearly so it must fold up
these form simultaneously to the appearance of the 3 dilations of the NT= the primary brain vesicles
what are the 2 flexures of the neural tube?
cervical flexure: at the SC-hindbrain junction
cephalic flexure: in midbrain region
so neuraxis doesn’t remain straight
this is why rostral and caudal ends are different when talking about the cerebrum and the brainstem.
what is the diencephalon characterised by?
outgrowth of the optic vesicles
what is the cavity of the rhombencephalon?
the 4th ventricle
what is the cavity of the diecnephalon?
the 3rd ventricle
what are the cavities of the cerebral hemispheres?
the lateral ventricles
what forms the aqueduct?
the lumen of the mesencephalon, which connects the 3rd and 4th ventricles and becomes very narrow.
role of ventricular system?
cushion brain and SC within their bony cases
what can block the ventricular system causing hydrocephalus?
infection
tumour
describe the 3 layers of the early neural tube
from inside out:
neuroepithelial layer
mantle layer- neuroblasts formed from neuroepithelial cells. layer later forms gray matter of SC- comrpises cell bodies
marginal layer- nerve fibres emerging from neuroblasts, takes on white appearance due to nerve fibre myelination, so white matter of SC formed- contains nerve cell axons and glial cells.
neuroblasts are continually added to the mantle layer of the NT. what does this result in?
a ventral and dorsal thickening being produced at each side of the neural tube.
ventral= basal plates- contain ventral motor horn cells
dorsal= alar plates- form sensory areas of SC.
what marks the boundary between the alar and basal plates formed from thickenings of neural tube?
the sulcus limitans
what are the roof and floor plates of the neural tube?
the dorsal and ventral midline portions respectively which do not contain neuroblasts, but serve as pathways fro nerve fibres crossing from 1 side to the other.
what are neural crest cells?
cells of the lateral border of the neuroectoderm tube which appear during neural plate elevation and become displaced to enter the mesoderm and undero an epithelial to mesenchymal transition
what are neural crest cell derivatives of the NS?
dorsal root ganglia (sensory ganglia) cranial nerve ganglia SNS(chain and pre-aortic) and PNS ganglia schwann cells glial cells leptomeniges (arachnoid and pia)
derviatives of neural crest cells other than in the NS?
chromaffin cells melanocytes conotruncal septum (heart) C cells of thyroid dermis (face and neck) odontoblast CT and bones of face and skull
what can disrupt the migration of neural crest cells?
alcohol
genetic factors
from what part of the NT are ventricles formed?
the brain vesicles
example of a defect caused by defect of neural crest cell migration affecting 1 structure?
Hirschsprung’s disease: aganglionic megacolon, failure of colonisation of GI mucosa with neural crest cells. meaning formation of aganglionic segment which is unable to relax, producing a functional colonic obstruction.
PNS ganglion cells in SM and myenteric plexuses of rectu, absent.
example of a disease where defect in neural crest cell migration affects multiple structures?
DiGeorge syndrome: CATCH 22, example of a T cell deficiency- primary immunodeficiency, cardiac abnormalities, abnormal facies, thymic hypoplasia, cleft palate, hypocalcaemia.
chromosome 22 abnormalities- deletion
neural crest cells populate mesenchyme forming prominences giving rise to face
T- thymic hypoplasia/aplasia- thymus derived from 3rd pharyngeal pouch and neural crest cells contribute the mesenchyme into which the endoderm from pharyngeal pouches migrates, the mesenchyme makes the CT necessary for gland differentiation
C- cleft palate- failure of 1 or both palatal shelves to grow towards the midline and/or fuse with its partner
H- hypocalcaemia- absence of PT glands (3rd and 4th pharyngeal pouches) which secrete PTH which stimulates Ca2+ release from bone, reabsorption by kidneys and absorption from gut via calcitriol
