1-DevelopmentMyogenesis
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Development and Myogenesis • • • • Tissue patterning Cellular construction of muscle Myogenesis Synaptogenesis Process outline • Primary myoblasts migrate from somites to muscle pools and fuse • Axons extend from neural tube to muscle pools • Secondary myoblasts migrate • Muscle pools separate • Pruning of multiple innervation Embryonic structure Gastrulation Alberts & Johnson MBoC 2002 Neurulation Stage 13 Stage 16 Stage 18 Stage 20 Proliferation of neural plate ectoderm Formation of neural tube Schroeder, 1970 Somites • Segmental mesoderm – Sclerotome (spinal column) – Dematome (skin) – Myotome (body & limb muscles) • Adjacent to neural tube Christ & Ordhal, 1995 Quail-chick chimeras • Nicole Le Douarin (1969) • “feulgen” staining show different chromatin struct. • Cells are similar enough to incorporate and develop • Follow specific cells through development Somite transplantation • • • • • • Open quail egg Inject India ink Dissect somite-glob Isolate somites Implant in chick 5-hrs post-op Nicole Le Douarin, circa 1987, via sdbonline.org/archive/dbcinema (now a dead link) Spatial mapping of somitemuscle Lance-Jones, 1988 • Rostral somiteproximal muscle L1 L2 L3 L4 L5 L6 L7 Spinal Segment Somite Cross-section through leg S27 S28 S29 S30 S31 S32 Sartorius Muscles Adductors Somite 29femotibialis, adductor Femorotibialis Iliotrochantericus Post Iliotibialis Iliofubularis • Somites don’t know which muscle they will become Somite 32iliotibialis, iliofibularis Neuronal outgrowth • Growth cone – Tubulin-mediated extension – Actin-mediated extension Forscher & Smith 1988 Molecular Cell Biology. 4th edition. Lodish et al Specificity of neural outgrowth • By coordinated timing? ie: first to arrive • By coordinated tracking? ie: M&N follow same path • Cynthia Lance-Jones & Lynn Landmesser – Retrograde label of motorneuron pools – Flip neural tube in developing chicks (before limb bud formation) Neurons know their muscles Inject dye into sartorius Look for it in neurons Normal T7-L3 flipped • Neuron’s muscle identity is set/committed early Neuronal guidance • Pioneer Axons – Segmentally transposed neurons find their original targets – ECM cues (GAGs, fibronectin) • Secondary Axons – Follow pioneers – Cell contact cues Molecular Cell Biology. 4th edition. Lodish et al Final guidance by muscle-cues • Lewis & al (1981) • Irradiate half the somitesno muscle formation • Nerves reach the limbs, but fail their final branching Irradiated Normal Silver staining shows neurons and their axons Differentiation and Fusion Myotube #1: some contractile protein (grey circles); nucleus (speckled blob) and nucleolus (egg yolk) Myotube #2: less contractile protein; nucleus Myoblast #1:mostly unorganized, some mitochondria (circles) Undifferentiated cells (2): potentially myoblasts or fibroblasts Myotube #3: Even less contractile protein Myotube/myoblast interface Myoblast : disorganized (speckled) cytoplasm. Mitochondria. Filopods extend completely into myotube Two cell membranes in intimate, continuous contact Myotube: regular pattern of dots are contractile myofibrils Myofibrillogenesis • ECM anchor/nucleation • β-actin & non-muscle myosin • Ruler proteins • α-actin & muscle myosin Dabiri et al., 1997 Myofibrillogenesis cartoon Primary and secondary myotubes Very closely spaced nuclei, many labeled in clusters. Primary myotube (mouse E15) labeled on E14 with 3H-thymidine Still dense nuclei; none labeled Sparse nuclei; many labeled Primary and secondary myotubes (E15.5) labeled on E14.5 with 3Hthymidine Nuclei becoming more dense; randomly labeled Secondary myotubes (E18) labeled on E17 with 3H-thymidine Myofiber generations • Primary – Migrate and fuse before innervation – Express myosin independent of nerve – May be constrained to be slow-twitch • Secondary – Dependent on innervation – Majority of adult muscle mass – Phenotype from nerve activity Muscle Spindle lineage • Encapsulated “intrafusal” fibers • Sensory • Nuclear bag • Nuclear chain • Dependent on innervation by DRG Milburn, 1973 Innervation • NMJ proteins uniformly expressed in myotubes • Innervation induces clusters • Agrin (torpedo organ axons) Normal NMJ Failed NMJ: scattered, light (low density), unorganized Synapse elimination • Stephen Turney & Jeff Lichtman • GFP/CFP/YFP-mouse – Confocal microscope – Neonatal variation in relative expression – Photobleaching • Sterrnomastoid muscle – Repeated imaging over days Multiple innervations share endplate • One GFP and one CFP axon co-localize and intermingle on a single fiber Synapse elimination • Polyneural innervation during development • Pruning after birth (Hebbian) • Selective laser ablation Turney & al., 2012 Neuronal competition • Larger/stronger neuron takes over • Smaller neuron will fill in if larger neuron is ablated Reserve/Satellite cells • Not all myoblasts fuse into myotubes (2-5%) • Muscle precursor cells – Outside cell membrane – Within basal lamina – Metabolically quiescent • Muscle-derived stem cells – Morphologically identical to MPC, 0.2-2% – Pluripotent Summary • Muscles migrate from somites during development – Fate determined by diffusible factors from outside the somite – Primary myogenesis is independent of innervation – Secondary myogenesis requires innervation • Nerves migrate from neural tube – Target muscle identified intrinsically – Individual axons compete for specific muscle fibers
