Chapter 4 Embryological Development of CNS

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Chapter 4 Embryological Development of CNS

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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Chromosomes and genes

• Humans have 46 Chromosomes
• 22 pairs are alike in both sexes
• 1 pair determines sex
• X (female) or Y (male)
• Genome is all DNA in all chromosomes
• A Gene is the sequence of DNA required for a
  product to be expressed (proteins, enzymes)

Chromosome during division
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Types of Division

• Mitosis
• For general body growth and function
• Regularly occurring for much of our body during
  our entire life
• Meiosis
• Special division during reproduction

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Gametogenesis (involves meiosis)

• Process of forming reproductive units
• Gametes
• Male Spermatozoa
• From puberty through adult life
• Female Ovum
• Completed prior to birth 2 million

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Zygote (fertilized egg)

• Produced from combination of male and female
  parent chromosomes
• Mitotic Division Begins
• New Cells called Blastomeres which form a Morula

Two-cell Stage
Four-cell Stage
Morula 3 days
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Morula

• Morula develops central cavity called Blastocyst
• Blastocyst attaches to uterine wall
• One week from fertilization to implantation in
  uterine wall
• Allows blastocyst to get nutrients and excrete
  waste products

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Blastocyst

• Embryoblast
• Blastocyst
• Trophoblast

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Blastocyst
Uterine stroma
Trophoblast cells
Embryoblast
Blastocyst cavity
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The first week
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The second week Bilaminar Embryo

• Embryo has two primary layers Epiblast
  Hypoblast

Cytotrophoblast
Amniotic Cavity
Epiblast
Hypoblast
Primary Yolk Sac
Exocoelomic Membrane
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Third week Trilaminar Embryo Develops
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Week 3

• Embryo Trilaminar three layers between amniotic
  cavity and yolk sac
• Ectoderm future covering (skin, nails, hair,
  but also CNS)
• Mesoderm future muscles, bones, heart
• Endoderm future digestive tract

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Week 3

• Primitive Streak Forms dorsally
• Forms neural tube, notochord (cartilaginous rod,
  future spine) and neural crest cells

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Presomite Embryo 18 days
Cut edge of amnion
Neural plate
Primitive pit
Primitive streak (mesoderm)
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Early Highlights

• Day 18 - Neural plate invaginates (encloses) to
  form neural groove
• Day 22 - Neural Tube Forms
• Becomes brain and spinal cord
• About the same time, Neural Crest Forms
• Becomes cranial and spinal nerve ganglia

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Presomite Embryo 20 days
Cut edge of amnion
Neural groove
Somite
Primitive streak
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Neural Tube

• Anterior 2/3 will form brain
• Caudal 1/3 will form spinal cord
• Day 25 - Cranial opening closes
• Brain has 3 sections
• Prosencephalon
• Mesencephalon
• Rhombencephalon
• Day 27 - Caudal end closes
• Problems cause neural tube defects

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Human Embryo 22 days
Neural fold
Optic placode
Somite
Cut edge of amnion
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Human Embryo 23 days
Cranial neuropore
Pericardial bulge
Caudal neuropore
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Week 5

• Prosencephalon Develops
• Telencephalon (cortex)
• Diencephalon (thalamus, hypothalamus)
• Mesencephalon Develops (mid brain)
• Rhombencephalon
• Metencephalon (pons, cerebellum)
• Myelencephalon (medulla)

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Telencephalon

• Optic Vessels - retinae, optic nerve
• Cerebral Hemispheres - Lateral Ventricle
• Medial Connection Corpus Callosum
• Olfactory Lobe
• Corpus Striatum
• (Caudate N. Lenticular N.)
• Cerebral Cortex
• Very primitive though 20 weeks

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Third Trimester

• All structures present at birth
• All structures become more distinct in Third
  Trimester
• Commissures develop

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Seven Steps of CNS Development

• Production of initial neurons and glial cells
• Migration of cells to definitive location
• Selective gathering of cells to functional group
• Cytodifferentiation (axon, dendrite, synaptic
  patterns)
• Selective death of some cells in groups
• Outgrowth of axons to specific target cells and
  establishment of connections
• Elimination of certain connections and functional
  stabilization of others

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Maturation of CNS

• At birth, all neurons you will ever have present.
• Only a few exceptions (neurons involved w smell)
• Process of myelination signals onset of mature
  function
• Slow process
• Partially completed completed by age 7
• Axons and dendrites not until teens
• Some areas continue to age 70
• Some cells have programmed cell death (Apoptosis)
• tadpoles lose their tails and pigeons' feet
  become unwebbed. Crucial in brain
• Note not all developmental language disorders
  present at birth.

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Myelin Sheaths

• Short Gaps (Nodes of Ranvier) on Axons
• Speed up neural activity
• In CNS, formed by Oligodendrocytes
• Type of Glial Cell
• In PNS, formed by Neurilemmal or Schwann cells

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Rate of Myelination Varies

• Spinal tract completed by 9th month
• Major motor tracts by 2 years
• Cerebrum and Cerebellum into the teens

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Abnormal Development

• Anencephaly
• Cerebral Hemispheres reduced or missing
• More common in Females
• Cranial Bifidum
• Bone fusion presented by brain or spinal cord
  protruding through skull

Anencephalic
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Anencephalic Newborn
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Spinal Bifida

• Spinal Bifida Cystica
• Portions of the meninges or neural tissues not
  enclosed by posterior vertebral arches
• Spina Bifida Occulta
• Dimple on spinal column on top of an opening in
  between vertebrae

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Other Developmental Conditions

• Hydrocephaly
• Enlarged head, brain atrophy mental deficiency
• Excessive production of CSF or obstruction of
  drainage pathways
• http//neurosurgery.seattlechildrens.org/condition
  s_treated/hydrocephalus.asp

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Causes of hydrocephalus
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Hydrocephalus Treatment

• Many adults had developmental hydrocephalus with
  no problems
• Others need urgent surgery

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Microcephaly

• Brain and Skull cap are small
• Face is normal
• Mental Retardation