Neural Developments & Malformations I

Question 1

the neural plate forms from what tissue?

what causes the neural plate to form?

Answer 1

• ectoderm
• the underlying notochord & paraxial mesoderm

Question 2

outline the sequence of events in neural tube formation.

when does neural tube formation begin and end?

where does neural tube formation occur?

Answer 2

day 20-22

in the cervical region.

steps:

1. neural plate forms from ectoderm, d/t notochord & paraxial mesoderm
2. neural folds - lateral elevations of the neural folds - develop
3. neural tube forms from fusion of neural folds

Question 3

what occurs after the neural tube has formed?

Answer 3

the neural tube extends cranially & caudally, and is in communication with the amniotic cavity

Question 4

what are the neural tube neuropores?

when do they close?

why is this important?

Answer 4

• are the site of communications between the neural tube and amniotic cavity.
• they close after formation of neural tube vascularization is complete
  
  • cranial neuropore: closes day 25
  • caudal neuropore: closes day 27

Question 5

what does the lumen and walls of the neural tube develop into?

Answer 5

• walls: the brain & spinal cord
• lumen: its innermost cavity, the neural canal, forms
  
  • the ventricles of the brain
  • the central canal of spinal cord

Question 6

what is spina bidifa?

what is its cause?

how does it present?

Answer 6

• definition: lack of vertebral arches
• cause: a failure of the closure of the neural tube
• presentation: three main presentations that lack in severity
  
  • spina bifida occulta
  • meningocele
  • meningomyelocele

Question 7

in normal anatomy, how far should the meninges extend?

Answer 7

up until the proximal-most part of the spinal nerve, near the intervertebral foramen

in other words: not past the peduncles

Question 8

spina bifida - prentation of

• spinal cord
• meninges
• other

Answer 8

• spinal cord - normal (or filum terminate connected to subQ)
• meninges - normal - i.e., don’t pass pedicles)
• other: hair skin patch overlies defect

Question 9

meningocele - prentation of

• spinal cord
• meninges
• other

Answer 9

• spinal cord: normal - no extrusion of neural tissue
• meninges: extrude dorsally
• other: mild disuprtion of bladder / bosel control

Question 10

meningomyelocele - presentation of

• spinal cord
• meninges
• other

Answer 10

• spinal cord: extruding neural tissue
• meninges: extrude dorsally + meningeal sac contains malformed cord
• other: LE dysfunction, loss of bowel & bladder control

Question 11

during what part of the vertebral column are meningocele & meningomyelocele most common?

Answer 11

lumbosacral

Question 12

what are the neural tube defects of the brain?

Answer 12

• craniochischisis
• encephlocele
• anencephaly

Question 13

craniorachischis

• cause
• presentation
• prognosis

Answer 13

• cause: neural tube fails to close at the cranial spinal cord - hindbrain junction
• presentation: abormal spinal cord located outside of body, m/c occipital region
• prognosis: rarely compatible with life

Question 14

encephalocele

• cause
• presentation
• prognosis

Answer 14

• prognosis: defect in skull (m/c occipital): allows brain to protrude through
• presentation: brain protrudes through occipital skull
• prognosis: smaller cases may be resolved surgically, many incompatible with life

Question 15

anencephaly

• cause
• presentation
• prognosis

Answer 15

• cause: malformation at anterior end of neural tube -> disruption of forebrain development
• presentation: absence of brain/calvaria
• prognosis:stillborn -> die within a few hours

Question 16

which part of the neural tube develops into the spinal cord?

Answer 16

the part caudal to the fourth pair of somites

Question 17

the wall of the neural tube develops into what three general zones?

Answer 17

• from deep -> superficial (in -> out)
  
  • ventricular
  • intermediate
  • marginal

Question 18

what is the ventricular zone lined with?

Answer 18

• neuroepithelium: psuedostratified columnar neuroepithelium, made of
  
  • ependymal cells - ultimately line central canal
  • glioblasts
  • neuroblasts

Question 19

what cells does the neuroepithelium give rise to?

Answer 19

(lining of ventricular zone of neural tube)

• ependymal cells: will line central canal
• glioblasts: differentiate into astroblasts & oligodenroblasts
• neuroblasts

Question 20

glioblasts

• migrate how?
• difffereniate into what cells?

Answer 20

• migration: move superficially into intermediate + marginal zones
• differentiate into:
  
  • astocytes: protoplastmic, fibrous
  • oligodendrocytes: myelinate axons in the CNS

Question 21

what are microglial cells?

Answer 21

macrophages of the CNS that migrate in from blood (are part of monocytic phagocyte system)

Question 22

what are radial glial cells?

why are they important?

Answer 22

• they are the first cell to form in the diferentiation of glioblasts
• serve as a scaffolding on which developing neurons can migrate in -> out of neural tube

Question 23

what directs & faciliates migration of developing neural cells? how?

what occurs to neurons as the migrate?

Answer 23

• migration directed by: neurochemical transmitters (chemo-attractants, repellents), which drive the movement of the growth cone, the tip of the developing axon immature
• migration facilitated by: radial glial cells, which serve as scaffolding
• as neurons migrate, they differentiate

Question 24

what part of the neural tube wall is the destination for immature neurons?

what occurs once the neurons arrive?

Answer 24

• the intermediate zone (middle zone)
• once neurons arive, they:
  
  • produce dendrites & axons that extend into the marginal zone
  • form aggregations: early nuclei

Question 25

what are aggregations?

Answer 25

groups of immature neurons in the intermediate zone of the neural tube wall (their destination). _are early forms of nuclei_

Question 26

what is the significance of the marginal zone?

Answer 26

* becomes invaded with axons & dendrites from neurons in the intermediate zone, after which synaptogenesis occurs * **gives rise to the white matter of the spinal cord**

Question 27

what is the significance of each neural tube wall zone?

Answer 27

* ventricular: site of massive neurolast mitosis * intermediate: destination for neurons * marginal: receives axons & dendrites from intermediate zone -\> forms white matter of spinal cord

Question 28

do neurons go mitosis after the ventricular zone?

Answer 28

no

Question 29

synaptogenersis * results in? * is triggered by? * is guided by? * involves what processes? * has what significance?

Answer 29

* results in: synapse formation * triggered by: **invasion of axons & dendrites into marginal zone** * guided by: * **neurotrophins**: permits neuronal growth & survival * **astrocytes** * involves**: significant neuronal cell death** (75% of neurons that just migrated) * significance: random pattern of cell death -\> **massive levels of plasticity:** * ​"everyones brain is different"

Question 30

what is the cause of the cell death in synaptogenesis?

Answer 30

**insufficient neurotrophins** neurotrophils promote neuronal growth & survival, & there is not enough for all neurons that migrated

Question 31

what is he sulcus limitans? what does it form?

Answer 31

* **a longitudinal groove** formed from the thickening lateral walls of the growing nueral tube/ divides the lateral tube into an: * **basal plate** - ventrally * **alar plate** - dorsally

Question 32

what do they alar plate and basal plate form?

Answer 32

* alar plate = afferents (sensory) * **dorsal horn** * dorsal median sulcus * basal plate = efferents (motor) * **ventral & lateral horn** * ventral median fissure

Question 33

from what does the ventral median fissure form?

Answer 33

the basal plate of neural tube

Question 34

syringomyelia * pathogenesis * demographics * presentation

Answer 34

* pathogenesis: associated with trauma, or charli I formation * demographics: adults * presentation: * a spinal cord cyst that is * fluid filled (but NOT CSF) * within gray matter * not contiguous with central canal \ * m/c in cervical & upper thoracic segments * loss of pain'temp sensations in upper extremities