Bio 127 - Section III Organogenesis

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Bio 127 - Section IIIOrganogenesis

• Paraxial and Intermediate Mesoderm
• Gilbert 9e Chapter 11

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Organogenesis Encompasses

• The Emergence of the Ectoderm
• Neural Crest Cells and Axonal Specificity
• Paraxial and Intermediate Mesoderm
• Lateral Plate Mesoderm
• Endoderm

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Student Learning Objectives

• 1. You should understand that the mesoderm forms
  all of the organs between the ectodermal wall and
  the endodermal tissues.
• 2. You should understand that the paraxial
  mesoderm forms structures at the back of the
  embryo surrounding the spinal cord, including the
  somites and their derivative cartilage, bone,
  muscle and dermis.
• 3. You should understand that the intermediate
  mesoderm forms the structures of the urogenital
  tract, including the kidneys, gonads, ductwork
  and the adrenal cortex.
• 4. You should understand that the mesoderm helps
  both the ectoderm and the endoderm form their own
  tissues.

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The mesoderm forms during gastrulation and
neurulation, same as ectoderm
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Major lineages of the mesoderm
Somite Terminology sclerotome vertebral and
rib cartilage
myotome muscles of back, rib cage, abdomen
dermamyotome
dermal cells, limb muscle
syndetome most
dorsal, tendons
arthrotome most central,
vertebral
joints/discs,
proximal ribs
unnamed most posterior, dorsal
aorta
and intervertebral
arteries
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Paraxial mesoderm is made up of head mesoderm and
somites
Well look closely at the somites....
The head mesoderm forms the muscles and
connective tissues of the head and eyes. It even
forms under the direction of different
transcription factors and suffers different
disease states.
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Somitogenesis

1. Establishment of periodicity
2. Fissure formation (separation)
3. Epithelialization
4. Specification
5. Differentiation

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Somitogenesis Periodicity
Every 90 minutes in chick (less exact in mice)
Total of 50 in chick 65 in
mice 500 in snakes Notch and Wnt
signals oscillate like a clock FGF signals
sweep rostral-to-caudal in wave
Periodic formation of somites is inherant to the
cells of the mesoderm
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Delta-Notch are expressed at presumptive
boundaries
Delta-Notch dictates WHERE a somite will form
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Notch controls the wavelike expression of hairy1
Where Notch is expressed Hairy-1 stays on
long-term
The posterior edge is the edge that signals
separation
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Fissure Formation Separation from unsegmented
mesoderm
The FGF wavefront sets up an oscillation in Wnt
and Notch signaling as it passes
Notch expression gives final position of Hairy-1
Hairy-1 causes Ephrin expression which repels
neighbors (remember how Ephrin repelled motor
axons here also)
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Epithelialization of somites
That same posterior edge starts mesenchymal to
epithelial transition
- N-cadherin - rho family - actin change
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Specification of paraxial mesoderm
occurs early due to Hox expression....
transplants form what they would have in original
position
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Determination and differentiation in somites

• All of the cells of the somite are competent to
  form all of the derivative cell types
• cartilage, bone, muscle, tendons, dermis,
  vascular cells, meninges
• Their fate depends on their position near the
  neural tube, notochord, epidermis and
  intermediate mesoderm

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Determination and differentiation in somites
First step is notochord induction of sclerotome
Epithelial to mesenchymal transition causes them
to migrate to form vertebral cartilage, leaves
dermamyotome epithelium
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Determination and differentiation in somites
The second step is the segregation of dermamyotome
Central and bilateral myotome surrounds dermatome
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Determination and differentiation in somites
Dermatome forms back dermis, brown fat

• - Primaxial myotome forms back and intercostal
  muscles
• Abaxial myotome forms abdominal muscle, tongue,
  limbs
• Central myotome proliferates madly and makes
  most cells

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Figure 11.12 Primaxial and abaxial domains of
vertebrate mesoderm (Part 2)
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Mechanisms of Tissue Formation from Somites

• Myogenesis Muscle Formation
• Osteogenesis Bone Formation
• Vascular Replacement in the Dorsal Aorta
• The Syndetome Tendon Formation

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Myogenesis Muscle Formation

• The paraxial, abaxial and central somite
• Cells in the center give rise to satellite cells
• maybe stem cells, maybe committed progenitors
• remain viable for the life of the organism
• exit cell cycle upon injury and differentiate to
  muscle
• Classic skeletal muscle differentiation
• paracrine signals induce MyoD, Myf-5
• TFs for muscle genes and for themselves!

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Myogenesis Muscle Formation
Adult muscle cells (myotubes) are large and
multinucleated
muscle satellite cells dont express MyoD until
injury
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Myogenesis Muscle Formation
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Myogenesis Muscle Formation
In culture it doesnt matter what species
you place together they will fuse.
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Osteogenesis Bone Formation

• Four different sources of bone
• Somites form the axial skeleton
• Lateral plate mesoderm form the limb skeleton
• Cranial neural crest forms the bones of face and
  head
• Mesodermal mesenchyme in patella, periosteum
• Two different processes
• Endochondrial ossification in the first two
• Intramembraneous ossification in the second two

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Osteogenesis Bone Formation
Endochondrial literally means within cartilage
bone model
- vertebrae - ribs - pelvis - limbs
hypertrophic chondrocytes leave cell
cycle, enlarge, calcify their ECM and then
apoptose
Shh
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Osteogenesis Bone Formation
The step-wise progression continues out away from
the center growth plates
The calcified ECM plus Ihh cause bone
cells (osteoblasts) to differentiate from somite
progenitors
The center is remodeled by osteoclasts from the
blood to form marrow
Bone growth ceases when the secondary center
finishes up
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Osteogenesis Bone Formation
No calcium
Normal bone formation
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Endochondrial Ossification of Vertebrae
1. Sclerotome mesenchyme are attracted by
notochord and neural tube secretions
2. As motor axons extend toward muscles they
go through sclerotome and split
them rostral-to-caudal
The caudal end of one then recombines with the
rostral end of the next to form the bone model
and then bone
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Vascular Replacement in the Dorsal Aorta
Blood vessels are a single layer of
endothelium surrounded by multiple layers of
smooth muscle
The dorsal (or descending) aorta forms a primary
model by vasculogenesis and then both the
endothelium and smooth muscle are replaced by
somite. (the same thing happens to the
ascending aorta by neural crest cells!)
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The Syndetome Tendon Formation
Tendon joins bone to muscle. The last row of
sclerotome is induced by the overlying myotome to
differentiate into those connectors.
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Formation of the Kidneys from Intermediate
Mesoderm

• The adult kidney is very complex
• A single nephron has 10,000 cells, 12 cell types
• Each is positioned exactly for its job relative
  to others
• The embryo increasingly needs to filter blood
• IM mesoderm 1st forms organizer, the pronephric
  duct
• This tissue then induces three stages of kidney
• The first two are transitory, the third persists

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General scheme of development in the vertebrate
kidney
Pronephros is functional in fish, amphibians, not
in mammals, then degenerates
Mesonephros is functional in some mammals,
including humans, degenerates in females, forms
epididymous and vas deferens .
Nephric duct is the primitive organizer Wolffian
Duct
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Metanephros formed by reciprocal induction with
Wolffian Duct
Intermediate mesoderm mesenchyme develops
into kidney, while....
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Figure 11.26 Kidney induction observed in vitro
....the Wolffian Duct matures into the collecting
duct