major early events 2

• What does the axial musculoskeletal system include?

  Vertebrae, ribs, muscles attached to them, and the dermis overlying them.
• What are the components of the axial musculoskeletal system during development?

  • Vertebrae
  • Ribs
  • Muscles attached to the vertebrae and ribs
  • Dermis overlying the vertebrae and ribs
  • Muscles of the limb (but not the skeleton of the limb)
• What are somites?

  Segments of tissue along the body axis, one either side of the neural tube.
• From which germ layer do somites form?

  Paraxial mesoderm.
• During which developmental stage do somites form?

  Gastrulation.
• What is the mesenchymal tissue formed during somite development called?

  Pre-somitic mesoderm (PSM).
• What does the pre-somitic mesoderm (PSM) become?

  Somites.
• Why does the length of the pre-somitic mesoderm (PSM) remain constant?

  Because as the anterior PSM becomes somites, more PSM is added at the posterior end.
• What is somitogenesis?

  The formation of somites.
• Is the number of somites formed for a species typically constant?

  Yes, the number of somites formed is constant for a species.
• What are the two main regions of the pre-somitic mesoderm (PSM)?

  • Anterior PSM
  • Posterior PSM
• How many vertebrae are usually formed from one pair of somites?

  One vertebra.
• How frequently does a pair of somites segment from the PSM in a chick embryo?

  Every 90 minutes.
• What regulates somitogenesis?

  • Molecular clock
  • Temperature-dependent
• What does segmentation of pre-somitic mesoderm refer to in somitogenesis?

  The process of dividing the pre-somitic mesoderm into somites.
• What gene is used to measure time on the molecular clock in somitogenesis?

  Hiry gene.
• How long does each cycle of Hiry expression last in somitogenesis?

  90 minutes.
• What event occurs every 90 minutes during somitogenesis?

  A somite forms every 90 minutes.
• What does the dermamyotome split further into?

  • Sclerotome
  • Dermomyotome
• What does the dermamyotome split into during somite development?

  • Myotome
  • Dermotome
  • Dermomyotome
• In what manner do all somites develop?

  All somites develop in an identical manner.
• What transition do cells at the anterior of the pre-somitic mesoderm (PSM) undergo?

  • Mesenchymal to epithelial transition
  • Form somites
• What structures are formed from each somite?

  Sclerotome, Myotome, and Dermotome.
• How do more anterior somites compare in development to more posterior somites?

  More anterior somites are more developed than more posterior ones.
• What is somitogenesis?

  The maturation of somites.
• What change does the pre-somitic mesoderm (PSM) undergo during somitogenesis?

  Mesenchymal to epithelial change.
• What structure forms from the pre-somitic mesoderm (PSM) after the mesenchymal to epithelial change?

  An epithelial ball called the Somite (SO).
• What happens to the next-most anterior somites after they mature?

  • Ventral-medial portion undergoes epithelial-mesenchymal transition.
  • Forms mesenchymal cells called Sclerotome (Sc).
  • Dorsal aspect remains epithelial called Dermomyotome (DM).
• What shape does the Dermomyotome (DM) resemble?

  A fitted sheet.
• What role do signals from the notochord play in somite development?

  They are responsible for inducing the sclerotome.
• Which protein controls the induction of the sclerotome by notochord signals?

  Sonic Hedgehog protein.
• What does the dermomyotome form in maturing anterior somites?

  A new layer called the Myotome.
• Do myotomal cells divide?

  No, myotomal cells never divide.
• From which structure are myotomal cells continually recruited?

  Dermomyotome.
• What does the Dermotome form in somite development?

  • Dermis
• What structure resegments in anterior-most somites?

  Sclerotome.
• How does the resegmentation of sclerotome affect muscle attachment?

  Muscle binds adjacent segments due to sclerotome fusion.
• What develops in the middle of the sclerotome during resegmentation?

  A fissure.
• Does the Myotome resegment during somite development?

  No, the Myotome does not resegment.
• What are the adult vertebrate derivatives of somites?

  • Axial Skeleton
  • Vertebrae
  • Ribs
  • Skeletal Muscle: Body and Limb
  • Dermis