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Nervous System development


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What is the primordium of the nervous system & from what germ layer is it derived?
Primordium = Neural tube

Derived from Ectoderm
What is neurulation?
Formation of the neural tube - in two phases (primary/secondry)
When does Primary neurulation occur?
End of week 3 to early week 4
What begins Primary neurulation?
Formation of neural plate
What is the Neural Plate?
Thickened area in center of ectoderm cranial to primitive node.
What causes the neural plate to form?
Interaction between Notochord and Overlying Ectoderm
What is the name of neural plate adjacent to notochord?
Floor Plate
What happens to the Floor Plate?
Cellular changes occur so it acts as Median hinge point
What results from forming a Median Hinge Point?
Bending of the neural plate so a midline Neural Groove forms, w/ Neural Folds on either side.
What happens to the neural Folds?
They Fuse and form the neural tube
But what happens just before the neural folds fuse?
Neural Crest cells separate by epithelial-mesenchymal transformation.
What happens to the ends of the neural tube?
They stay open for a while to ensure flow of amniotic fluid til vessels develop
What kind of tissue is
-Neural tube
-Neural crest
Tube = epithelium

Crest = mesenchyme
What does the Neural Tube NT become?
-Spinal cord
What does the lumen of the NT become?
The ventricles of the brain
What portion of the CNS is formed by:
-Primary neurulation?
-Secondary neurulation?
Primary = Brain thru S2

Secondry = S3 thru Coccyx
Where does 2ndary neurulation begin?
At the Caudal eminence
What tissue participates in 2ndary neurulation? What does it form?
Undifferentiated mesenchyme - becomes the Neural CORD.
How does the neural Cord associate with the Tube?
The Cord grows toward the tube and they become continuous.
What is essential for complete CNS formation?
BOTH primary and secondary neurulation.
Disease caused by Error in 2ndary neurulation:
Symptom of Myelodysplasia:
Pigmented skin covers defect
2 alternate names for errors of primary neurulation:
Dysraphic errors
Neural tube Defects
Where are Dysraphic/neural tube defects located? Why?
Midline and Dorsal b/c they are caused by Faulty closure of neural folds.
Where do Dysraphic defects usually occur?
At neuropores
What supplement affords some protection against neural tube defects?
Folic acid supplements
What % of Fetal vs. Infant deaths caused by congenital conditions are from nervous system defects?
Fetal: 75%

Infant: 40%
What % of all malformations in develpmnt are due to nervous system defects?
What % of embryos affected by nervous system defects are lost during week 4-8?
4 Neural Tube defects involving BRAIN:
-Cranial Meningocele
-Cranium Bifidium
What is Meroanencephaly?
Absence of the Calvaria
Rudimentary mass instead of brain
Only brainstem
How is meroanencephaly diagnosed?
-Incr alpha fetoprotein levels
What are the common characteristics of Meningoencephalocele?
-Bifid Cranium skull defect
-Herniated Brain/Meningies
-Hydrocephaly, polydactyly, polycystic kidney
What are the 2 deficits associated with meningoencephalocele?
What is Cranial Meningocele?
A small defect in the skull
-No brain herniation
-Just herniated meningeal sac filled w/ CSF
What is Cranium bifidium?
An opening between the skull bones
-Usually asymptomatic
What is the collective name for Neural tube defects involving the spinal cord?
Spina bifida
What does Aperta mean?
Who do we see more of each in?
-Spina bifida
Meroanenceph = Females 3:1

Spina bifida = Males
What is Myeloschisis?
Vertebral defect - neural arches are unfused so the neural plate is open dorsally
How common is myeloschisis?
Not at all - very rare
What are the majority of spina bifida cases?
Meningomyelocele - 90%
What are the characteristics of Meningomyelocele?
-Verebral defect
-Cystic swelling that can be covered with skin or membrane
-Herniated spinal cord and meninges
What defects are associated with meningomyelocele?
-Club foot
What deficits are associated with meningomyelocele?
-Urinary incontinence
What is the 2nd most common spina bifida? What %?
How is it different from Meningomyelocele?
Meningocele - 10% of cases
-Only meningeal herniated (not spinal cord)
What is meningocele characterized by?
-Vertebral defect
-Herniated meningeal sac filled w/ CSF
What term classifies both Meningocele or Meningomyelocele?
Spina Bifida Cystica
What is Spina Bifida Occulta?
A vertebral defect limited to 1/2 vertebral levels and often asymptomatic
What percent of population has spina bifida occulta?
What is Diastematomyelia?
A bone spur in spinal canal associated w/ hair tuft/dimple - can tether spinal cord and prevent normal ascension in spinal canal.

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