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BIOL 1F25 (223)


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Daniel Glenday

Muscle MUSCLE TYPES Skeletal – syncitial differential splicing  different muscle fiber isoforms Cardiac – mechanical coupling (intercalated disks) Smooth – not syncitial, not striated; random actin-myosin arr. MYOEPITHELIAL – iris dilator, mammary, salivary, sweat glands; important for secretion **from ectoderm associated w/ above organs** DESCRIPTIVE EMBRYOLOGY Mesoderm is initially a bilayer PARAXIAL mesoderm  axial skeleton paraxial mesoderm skeletal muscle dermis initially forms SOMITOMERE “whorls” somitomere somitomeres (mesenchymal)  SOMITES (epithelial) somites first 7 remain somitomeres and form face/throat musc. 37 total somite pairs form in humans ventral, LATERAL PLATE mesoderm  splanchnopleuric  lateral plate cardiac/smooth muscle (dorsal, lateral plate  somatopleuric mesoderm) SOMITE DIFFERENTIATION – occurs anterior to posterior ventral – SCLEROTOME  axial skeleton, skull sclerotome myotome, myogenic medial – MYOTOME  myogenic cells  1. ventral hypomere (lateral/ventral body wall musculature) 2. dorsal epimere (spine muscles) dorsal – DERMOTOME  dermis dermotome Muscle HEAD MUSCULATURE: all muscle from paraxial mesoderm (somitomeres and somites) LIMB MUSCULATURE: derived from lateral somites only involves migration into the limb buds upon arrival, myogenic cells are arranged dorso-ventrally later, patterns are determined by connective tissue contact medial portion of somites gives rise to AXIAL muscles Skeletal muscle lineage: Smooth Muscle: epiblast  epiblast  PARAXIAL mesoderm  LATERAL PLATE mesoderm somitomeres/somites  Splanchnic mesoderm  dermamyotome  smooth musc. around gut myotome  myoblasts  (or mesenchyme  blood vessels) myotubes  myofibers Cardiac Muscle: epiblast  ANTERIOR LATERAL PLATE mesoderm  splanchnic mesoderm  surrounds heart tubes MUSCLE DIFFERENTIATION: myoblasts myogenic cells  myoblasts (distinct from fibroblasts) myotube myoblasts fuse into myotube (no more DNA replication) myofiber myofibrils form in myotubes  MYOFIBER Muscle MUSCLE DETERMINATION 1) TF’s that cause undifferentiated tissue to become myoblasts MyoD MYOD – for myoblast formation and myogenin expression Myf5 MYF5 – activates MyoD expression Myogenin MYOGENIN - for terminal differentiation and maturation MRF4 MRF4 - same as above
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