Chapter 12- CNS and epidermis

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