week 7

• Developmental origins of muscle
• Gastrulation and initial mesoderm formation

  1. Mesoderm differentiation

  2. Paraxial

  3. Intermediate

  4. Chordamesoderm

  5. Lateral plate mesoderm

  
• Skeletal muscle


  • Develops from fusion of myoblasts
  • Long fibres, multinucleated (peripheral nuclei) fused cells
  • Evident striations
• Cardiac muscle


  • Develops by end-to-end joining of cardiomyoblasts
  • Intercalated discs form between adjacent cells
  • Single, centrally placed nucleus
  • Striations and branching evident
• Smooth muscle


  • Develops by individual mesodermal cell differentiation
  • Spindle-shaped cells – NO striations evident
  • Single, centrally placed nucleus

  
• Skeletal, cardiac and smooth muscles all working together
• Muscle types have differing capacity to repair and regenerate. Smooth muscle has the highest regenerative capacity, followed by skeletal muscle, and cardiac muscle has the lowest regenerative capacity.
• Cardiac and skeletal muscle cells undergo terminal differentiation, retaining their phenotype in adult organs

  
• Smooth muscle cells are highly versatile and can change their phenotype, being proliferative in growing organs like the pregnant uterus, and differentiating and proliferating at the same time in wound healing of blood vessels
• Smooth muscle growth can occur through hyperplasia (increase in cell numbers) and hypertrophy (increase in cell size)
• Connective tissue


  Composed of specialised cells and extracellular matrix (fibres and ground substances)
• Connective tissue components


  • Specialised cells
  • Extracellular matrix
• Cartilage

  • Composed of chondrocytes embedded in extracellular matrix rich in collagen, proteoglycans and glycosaminoglycans
  • Avascular and aneural
• Types of cartilage


  • Hyaline
  • Elastic
  • Fibrous
• Hyaline cartilage


  • Strong
  • Flexible
  • Resists compression
• Elastic cartilage


  • Strong
  • Flexible
  • Elastic
• Fibrous cartilage


  • Strong
  • Tough
  • Resists compression
• Locations of cartilage


  • Respiratory tract
  • Articular surfaces of joints
  • Costal cartilage
  • Developing skeletal structures
  • External ear
  • Epiglottis
  • Intervertebral discs
  • Pubic symphysis
• Bone


  • Composed of osteocytes embedded in calcified extracellular matrix
  • Has a rich neurovascular supply
• Osteons


  Structures in bone tissue where the neurovascular supply runs along the central canal
• Extracellular matrix in bone

  • Composed of collagen, other proteins that assist with bone calcification, and proteoglycans
  • Hydroxyapatite crystals are deposited as the bone matures and calcifies
• Specialised bone cells


  • Osteoblasts
  • Osteocytes
  • Osteoclasts
• Osteoblasts


  • Responsible for the formation of new bone
  • Synthesise and secrete uncalcified extracellular matrix (osteoid)
  • Regulate the calcification of osteoid
• Osteocytes


  • Completely encased in extracellular matrix
  • Maintain the mineral/protein composition of bone matrix
  • Connect to neighbouring osteocytes via cytoplasmic processes
• Osteoclasts


  • Large multinucleated phagocytic cells
  • Responsible for the breakdown and resorption of bone
  • Secrete acids and enzymes that digest extracellular matrix
• Functions of bone


  • Surround and protect soft tissues and organs
  • Act as levers and muscle attachment points
  • Important storage site for minerals
• Skeletal regions


  • Axial skeleton
  • Appendicular skeleton
• Mesenchymal cells


  Multipotent stem cells that give rise to connective tissues
• Mesenchymal cell lineages


  • Paraxial mesoderm
  • Lateral plate mesoderm
  • Cranial neural crest cells
• Chondrogenesis


  Mesenchymal stem cells -> Condensation -> Differentiated chondrocytes
• Cartilage is an important precursor for most bones in the axial and appendicular skeleton
• Types of ossification


  • Endochondral ossification
  • Intramembranous ossification
• Intramembranous ossification


  Mesenchymal cells differentiate into osteoblasts -> Tissue develops into mature bone
• Endochondral ossification


  Chondrocytes hypertrophy and undergo apoptosis -> Blood vessels invade and primary ossification centre forms -> Secondary ossification centre
• Epiphyseal growth plates


  • Composed of hyaline cartilage and persist throughout childhood and adolescence
  • Proliferation and differentiation of chondrocytes drives longitudinal growth of long bones until adult height is achieved
• Achondroplasia


  Reduced proliferation of chondrocytes in the epiphyseal plate impairs bone elongation and causes a short-limb phenotype
• Limb length discrepancies


  Limbs grow at different rates causing length discrepancies, can result from congenital defects or epiphyseal plate injuries
• Bone has a remarkable potential for repair following injury
• Cartilage has limited potential for repair following injury or disease
• Damaged hyaline cartilage is typically replaced by fibrocartilage - scar tissue