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:– 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

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

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)

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Maturation of Somite: Fate mapping of Dermomyotome, Sclerotome

Dermomyotome

Sclerotome

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)

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

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

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?

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

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