1. Somite Derivatives: Muscle and Bone Formation Gilbert - Chapter 14

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

3. Muscle and Bone Formation Which compartment of the somite will give rise to muscle? Which compartment of the somite will give rise to bone?

4. Myogenesis The generation of muscle cells Come from 2 cell lineages within the myotome –Primaxial - don’t mix with Lateral Plate Mesoderm –Abaxial - Do mix with Lateral Plate Mesoderm Muscles are induced to form by paracrine factors (ex. WNT’s, BMP’s)

5. Myogenic bHLH proteins (Basic Helix Loop Helix) Transcription factors in MRF family (myogenic regulatory factors) All bind similar sites on DNA, activate muscle genes Expressed only in muscle cells/precursors Sufficient to specify/commit a cell to the muscle lineage –A variety of cells types in culture transfected with myf-5 or myoD will become muscles

6. http://8e.devbio.com/image.php?id=155

7. MyoD Expression

8. Muscle cell fusion Muscle tissue is multinucleate Myoblasts (muscle precursors) fuse to form multinucleate myotubes At this point, cells are differentiated Become organized into a muscle fiber

9. Differentiation of Myotubes In order to begin differentiation, myoblasts must stop proliferating (dividing) –Depletion of certain growth factors allows myoblasts to exit the cell cycle and differentiate –If growth factors are present, myoblasts will continue to proliferate

12. Muscle Cell “Regeneration” in response to injury Muscle cells are differentiated - cannot divide to replace themselves after injury As in many tissues, replacement of lost tissue comes from stem cell populations associated with that tissue type Satellite cells - found by basement membrane of muscle fibers –Not well understood

13. Bone Formation In vertebrates, 3 systems must form Craniofacial bones Axial Skeleton Appendicular Skeleton

14. Anatomy of the Musculoskeletal System of Jawed Vertebrates The locomotor anatomy is composed of 2 systems: Axial: Vertebrae, Ribs, Associated musculature Appendicular: Paired appendages, Pelvic and pectoral Girdles, Associated musculature Must work together for an organism to function properly. MusculatureSkeleton

15. Bone formation 3 lineages of bone structures –Somites: axial structures (vertebrae, ribs) Scapula? –Lateral plate mesoderm: appendicular structures (limbs) Scapula –Cranial neural crest: craniofacial bones

16. Bone Formation (Osteogenesis) 2 major modes Intramembranous (dermal) ossification Endochondral ossification

17. Intramembranous (dermal) ossification Directly convert mesenchyme into bone Involves osteoblasts, osteoclasts Typical of skull formation

18. Endochondral ossification Convert mesenchyme to cartilage first, then converts to bone Typical of the formation of vertebrate ribs, limbs Involves chondrocytes (make cartilage), osteoblasts (make bone)

21. Zebrafish Development - Day 2 Draw 3 embryos, label structures Use lab printout and poster to identify structures and stage of embryonic development When finished examine cleared and stained chick embryo - stained for cartilage Remove marbles from fish tank