Chapter 12- CNS and epidermis
Ectoderm-skin/nerves
Endoderm- Gut and associated organs
Mesoderm-Blood, heart, kidney, bones
Recall lineages
Now we focus on individual lineages
Ectoderm has three fates
Epidermis (skin)
Brain and spinal chord
Peripheral neurons, facial cartilage
2.Neural crest cells
3.Neural tube
1.Epidermis
Fig. 12.3 This process is called neurulation
Dorsal ectoderm becomes neural ectoderm to become neural plate to become neural tube
Neural plate
Neural crest
epidermis
PrimaryNeurulation
1. Folding
2. elevation
3. convergence
4. closure
Two types of neurulation1. Primary- “pinching off”2. Secondary – hollow out a cordBoth are used in many creatures
Fig. 12.4- Amphibian embryo
Fig. 12.3
Neural tube
1. Folding
2. Elevation 3. Convergence
4. Closure
A few details at each step in primary neurulation
Mesoderm signals ectodermal cells to form neural plate
Mesoderm signals ectodermal cells to form neural plateHinge cells (called medial hinge point cells) attached to notochordCell shape and cells movement contribute to elevation
and
Folds adhere to each other
Failure of complete closure results in neural tube defects
•Spina bifida – posterior tube fails to close at human day 27
•anacephaly – anterior tube fails to close- brain development ceases
Fig. 12.6
• 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 chickNote- 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
Notochord (then hinge cells) secretes sonic hedgehog to signal ventral portion of neural tube to become motor neurons
•Retinoic acid also plays a role
Epidermis (then roof plate) secretes TGF- family proteins (BMP-4 and –7, dorsalin, activin) to signal dorsal portion of neural tube to become sensory neurons
Roof plate
Hinge cells
Fig. 12.13- chick neural tube
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
Dendrites- connect to other neurons
• Cortical neurons connect to 10,000 other neural cells during 1st year post birth!!
•At birth, very few dendrites are present on cortical neurons
Axon
Input axons from other neurons
• Axons are part of the cell body that can extend several feet• Growth cone explores and moves into new regions of body
Growth cone
Nerve cells are protected to facilitate electrical signal conduction by:
By myelin sheath produced by oligodendrocytes
In central nervous system In peripheral nervous system
By myelination from Schwann cells
Vertebrate eye development
Fig. 5.15-the Pax 6 gene
Pax6 gene encodes protein that directs eye developmentNeural-tube specific enhancer
Pax gene expression
Fig. 5.14
Recall chapter 5- introduce DNA containing pax6 cDNA under control of an inducible promoter + a tissue-specific enhancer
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
Fig. 12.29
Eye lens development forms by:
1. Lens vessicle folds onto itself to form ring
Fig. 12.27
2. Interior cells elongate across cavity to produce crystallin lens fibers
3. Cells enucleate
A few words about epidermis (skin) development
Recall:Epidermis (skin)
Brain and spinal chord
Peripheral neurons, facial cartilage
2.Neural crest cells
3.Neural tube
1.Epidermis Epidermis (skin)
Brain and spinal chord
Peripheral neurons, facial cartilage
2.Neural crest cells
3.Neural tube
1.Epidermis
Epidermis becomes two layers, a periderm (which is shed) and a basal layer that gives rise to skin cells
EpidermisBasal layer
Periderm
Spinous layer
Termed “Malpighian layer”
(Shed)
Granular cells Keratinocytes
Keratinocytes (continually shed)
Spinous layer
Basal layer
Granular layer
Malpighian layer
Cells differentiate and migrate toward surface
Fig. 13.32
TGF- and FGF7 are important factors in skin development
Feather, hair and scales are formed by epithelial-mesenchymal interactions between epidermis and mesoderm