SKELETAL SYSTEM

• Skeletons


  supportive systems that provide protection, support, and a place for muscle attachment.
• Hydrostatic skeleton


  (earthworm) muscles in body wall develop force by contracting against incompressible coelomic fluids
• Hydrostatic skeleton


  alternate contractions of circular and longitudinal muscles of the body wall
• Muscular hydrostats


  composed of incompressible tissues and complex arrangement of muscles for complex movement (elephant's trunk, mammal & reptile tongues, cephalopod tentacles)
• Rigid skeletons


  provide protection, support, and anchor points for pairs of opposing muscles
• Exoskeleton


  mollusccs and arthropods and some other invertebrates
• Exoskeleton


  composed of chitin
• Endoskeleton


  echinoderms, chordates and some cnidarians
• Endoskeleton


  composed of calcium
• Vertebrate endoskeleton


  composed of bone and cartilage
• Bone


  provides support, protection, reservoir for calcium and phosphorus
• Notochord


  supportive rod found in protochordates and developing vertebrates; derived from mesoderm
• Notochord 


  replaced by backbone except in jawless vertebrates
• Cartilaginous skeletons


  derived feature of jawless fishes and elasmobranchs from bony skeletons of their ancestors
• Most vertebrates have bony skeletons, with some cartilaginous parts.
• Cartilage


  soft pliable tissue that resists compression; variable in form
• Hyaline cartilage


  clear, glassy appearance with chondrocytes surrounded by a matrix; avascular
• Cartilage


  often found at articulating surfaces of bone joints; supporting rings of passageways in respiratory system
• Cartilage similar to hyaline cartilage in invertebrates

  radula of gastropods, lophophore of brachiopods
• Bone


  highly vascular living tissue; contains significant deposits of inorganic calcium salts
• Endochondral (replacement) bone


  develops from another form of connective tissue - usually cartilage
• Intramembranous bone


  develops directly from sheets of embryonic cells; face, cranium, clavicle, dermal bone
• Bone


  vary in density
• Spongy bone


  open interlacing framework of bony tissue; oriented to give strength
• Compact bone


  dense, open framework of spongy bone that has been filled in by additional calcium salts
• Compact bone


  calcified bone matrix arranged in sets of concentric rings called osteons
• Bone


  bundles of osteons interconnected with blood vessels and nerves
• Lacunae


  cavities between concentric rings of bone
• Osteocytes


  bone cells contained in lacunae
• Canaliculi


  tiny passageways that distribute nutrients in bone
• Bone as dynamic tissue


  consists of osteoclasts and osteoblasts
• Osteoclasts


  bone resorbing cells
• Osteoblasts


  bone building cells
• New osteons formed


  Old ones resorbed
• Parathyroid hormone: responsible for resorption
• Calcitonin


  hormone responsible for deposition
• Parathyroid hormone and calcitonin


  responsible for maintaining constant calcium level in blood
• Regions in vertebrate skeleton

  axial and appendicular
• Axial skeleton


  includes skull, vertebral column, ribs, and sternum
• Appendicular skeleton


  includes limbs and pectoral and pelvic girdles