Chapter 12- CNS and epidermis

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Chapter 12- CNS and epidermis
Now we focus on individual lineages
Recall lineages
Ectoderm-skin/nerves
Mesoderm-Blood, heart, kidney, bones
Endoderm- Gut and associated organs
Ectoderm has three fates
Epidermis (skin)
1.Epidermis
Peripheral neurons, facial cartilage
2.Neural crest cells
3.Neural tube
Brain and spinal chord
Fig. 12.3
This process is called neurulation
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Neural plate
Dorsal ectoderm becomes neural ectoderm to become
neural plate to become neural tube
Primary Neurulation
Neural crest
1. Folding
epidermis

• Two types of neurulation
• Primary- pinching off
• Secondary hollow out a cord
• Both are used in many creatures

2. elevation
3. convergence
4. closure
Fig. 12.4- Amphibian embryo
Neural tube
Fig. 12.3
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A few details at each step in primary neurulation
1. Folding
Mesoderm signals ectodermal cells to form neural
plate
2. Elevation
3. Convergence
and
Mesoderm signals ectodermal cells to form neural
plate Hinge cells (called medial hinge point
cells) attached to notochord Cell shape and cells
movement contribute to elevation
Fig. 12.6
4. Closure
Folds adhere to each other
Failure of complete closure results in neural
tube defects

• anacephaly anterior tube fails to close-
• brain development ceases

• Spina bifida posterior tube fails to
• close at human day 27

• 50 of spina bifida preventable with 0.4mg/day
  vitamin B12

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Secondary neurulation
A cord is first made, then hollowed out
Example- posterior end of chick Note- rest of
chick uses primary neurulation
Further neural tube differentiation
1. Anterior-posterior axis
Anterior portion of neural tube forms three
vesicles 1. Forebrain 2. Midbrain 3.
Hindbrain
Brain volume increases 30-fold between days 3 and
5 of development
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Brain development is complex and laden with
nomenclature
Fig. 12.10- human brain development
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2. Dorsal-ventral axis
Fig. 12.13- chick neural tube
Epidermis (then roof plate) secretes TGF-b family
proteins (BMP-4 and 7, dorsalin, activin) to
signal dorsal portion of neural tube to become
sensory neurons

• Notochord (then hinge cells) secretes sonic
  hedgehog to signal ventral portion of neural tube
  to become motor neurons
• Retinoic acid also plays a role

Roof plate
Hinge cells
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Neuronal types

• Brains consists of 1011 neurons (nerve cells)
  and 1012 glia (support cells)

• The long-held belief that neurons were fully
  determined at birth is incorrect-
• Evidence for neuronal stem cells exists

Cells lining neural tube can give rise to neurons
or glia cells
Fig. 12.22- A motor neuron
Input axons from other neurons
Growth cone

• At birth, very few dendrites are present on
  cortical neurons

Axon

• Cortical neurons connect to 10,000 other neural
  cells during 1st year post birth!!

Dendrites- connect to other neurons

• Axons are part of the cell body that can extend
  several feet

• Growth cone explores and moves into new regions
  of body

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Nerve cells are protected to facilitate
electrical signal conduction by
In central nervous system
In peripheral nervous system
By myelination from Schwann cells
By myelin sheath produced by oligodendrocytes
Pax gene expression
Vertebrate eye development
Pax6 gene encodes protein that directs eye
development
Neural-tube specific enhancer
Fig. 5.15-the Pax 6 gene
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Recall chapter 5- introduce DNA containing pax6
cDNA under control of an inducible promoter a
tissue-specific enhancer
Fig. 5.14
Observe additional eyes
Pax6 mutants lack eyes in flies, mice and humans

• Sonic hedgehog dictates formation of two eyes
• Mutants produce one eye (cyclopia)

Fig. 6.25- a cyclopic lamb
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Eye development requires the specification of
numerous tissues
Eye lens development forms by
1. Lens vessicle folds onto itself to form ring
Fig. 12.29
2. Interior cells elongate across cavity to
produce crystallin lens fibers
3. Cells enucleate
Fig. 12.27
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A few words about epidermis (skin) development
Recall
Epidermis becomes two layers, a periderm (which
is shed) and a basal layer that gives rise to
skin cells
(Shed)
Periderm
Epidermis
Basal layer
Granular cells
Keratinocytes
Spinous layer
Termed Malpighian layer
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Fig. 13.32
Keratinocytes (continually shed)
TGF-a and FGF7 are important factors in skin
development
Granular layer
Spinous layer
Malpighian layer
Basal layer
Cells differentiate and migrate toward surface
Feather, hair and scales are formed by
epithelial-mesenchymal interactions between
epidermis and mesoderm