Download Chapter 12- CNS and epidermis

Now we focus on individual lineages
Chapter 12- CNS and epidermis
Recall lineages
Ectoderm-skin/nerves
Mesoderm-Blood, heart, kidney, bones
Endoderm- Gut and associated organs
Ectoderm has three fates
1.Epidermis
2.Neural crest cells
3.Neural tube
Fig. 12.3
Epidermis (skin)
Peripheral neurons,
facial cartilage
Brain and spinal chord
This process is called neurulation
Dorsal ectoderm becomes neural Primary
ectoderm to become neural plate Neurulation
to become neural tube
1. Folding
Two types of neurulation
1. Primary- “pinching off”
2. Secondary – hollow out a cord
2. elevation
Both are used in many creatures
Neural plate
Neural crest
epidermis
3. convergence
4. closure
Fig. 12.4- Amphibian embryo
Neural tube
Fig. 12.3
A few details at each step in primary neurulation
1. Folding
Mesoderm signals ectodermal cells to form neural plate
2. Elevation and
3. Convergence
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 closebrain development ceases
•Spina bifida – posterior tube fails to
close at human day 27
• 50% of spina bifida preventable with 0.4mg/day vitamin B12
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
Brain development is complex and laden with nomenclature
Fig. 12.10- human brain development
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
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
Axon
on cortical neurons
• 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
Nerve cells are protected to facilitate electrical signal conduction by:
In central nervous system
In peripheral nervous system
By myelin sheath produced by
oligodendrocytes
By myelination from
Schwann cells
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
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
Eye development requires the
specification of numerous tissues
Eye lens development forms by:
1. Lens vessicle folds onto itself to form ring
2. Interior cells
elongate across
cavity to produce
crystallin lens
fibers
Fig. 12.27
Fig. 12.29
3. Cells enucleate
A few words about epidermis (skin) development
Recall:
1.Epidermis
2.Neural crest cells
3.Neural tube
Epidermis (skin)
Peripheral neurons,
facial cartilage
Brain and spinal chord
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”
Fig. 13.32
Keratinocytes (continually shed)
Granular layer
TGF-a and FGF7 are
important factors in
skin development
Spinous layer
Malpighian layer
Basal layer
Cells
differentiate and
migrate toward
surface
Feather, hair and scales are
formed by epithelialmesenchymal interactions
between epidermis and
mesoderm