Somite Derivatives: Muscle and Bone Formation

Gilbert - Chapter 14

Today’s Goals

• Become familiar with the mesodermal sub-populations

• Examine somite maturation, muscle and bone formation

Muscle and Bone Formation

• Which compartment of the somite will give rise to muscle?

• Which compartment of the somite will give rise to bone?

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)

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

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

MyoD Expression

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

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

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

Bone Formation

• In vertebrates, 3 systems must form

• Craniofacial bones

• Axial Skeleton

• Appendicular Skeleton

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

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

Bone Formation (Osteogenesis)

• 2 major modes

• Intramembranous (dermal) ossification

• Endochondral ossification

Intramembranous (dermal) ossification

• Directly convert mesenchyme into bone• Involves osteoblasts, osteoclasts• Typical of skull formation

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)

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