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The Somites and Their Derivatives

The Somites and Their Derivatives. Gilbert - Chapter 14. Today’s Goals. Become familiar with the mesodermal sub-populations Examine paraxial mesoderm and the formation of somites Examine somite maturation, muscle and bone formation. Derivatives of the Somites. Somites form:

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The Somites and Their Derivatives

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  1. The Somites and Their Derivatives Gilbert - Chapter 14

  2. Today’s Goals • Become familiar with the mesodermal sub-populations • Examine paraxial mesoderm and the formation of somites • Examine somite maturation, muscle and bone formation

  3. Derivatives of the Somites • Somites form: • Cartilage of the vertebrae and ribs • Muscles of the rib cage, limbs, abdominal wall, back, and tongue • Tendons that connect muscle to bone • Dermis of the dorsal skin • Some Vascular cells that will contribute to blood vessels of the aorta

  4. Somite Maturation • Somites initially form an epithelial ball, but soon begin to form specialized tissues within the somite (subpopulations) • These subpopulations are committed to forming certain cell types

  5. Somite Maturation & Fate Mapping • Sclerotome: ventromedial portion • Loses epithelial characteristics • Becomes mesenchyme • Will form Vertebrae and ribs • Dermamyotome: dorsal portion • 2 cell populations arranged as epithelial sheets • Dermatome: forms connective tissue of dermis • Myotome: Form myoblasts (muscle precursors)

  6. * *

  7. Maturation of Somite: Fate mapping of Dermomyotome, Sclerotome Dermomyotome Sclerotome

  8. More about Dermamyotome • Some myoblasts stay close to the neural tube • Form muscles near the vertebrae and between ribs • Primaxial (Nowicki et al. 2003) • Other myoblasts migrate out of the somite and into the Lateral Plate Mesoderm • Form muscles of the abdomen, body wall, limbs and tongue • Abaxial (Nowicki et al. 2003)

  9. Somite Maturation • The most recently formed somite is the least mature • Epithelial ball • As you move further anterior, somites are more mature, cells are specialized, differentiating

  10. Determination of Somitic Subpopulations • Specification of the somitic subpopulations depends upon signals from neighboring tissues • “Local” signals vs. “Global” pattern • Local signals are consistent at all levels of the A-P axis • These signals come from opposite directions and can inhibit one another

  11. Sclerotome Determination • Signals from the notochord and floor plate of neural tube induce the ventral portion of the somite epithelium to become sclerotome • SHH! • How could we test whether SHH from these tissues is really involved in signalling sclerotome formation?

  12. Sclerotome Determination • Sclerotome cells are specified to express Pax-1 • Member of a family of “Paired box” genes • Pax-1 through Pax-9 • Transcription factors • All have 128 amino acid paired domain that acts as DNA binding motif • In mouse model, null mutants (knockouts) for the Pax-1 genes have defects in formation of the vertebrae, portions of the scapula

  13. Determination of Dermatome • Signals from the dorsal neural tube (roof plate) specify the dorsal-most somite to become dermatome • NT-3 (neurotrophin 3), Wnt1 • Design an experiment to test this

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