Skeleton

• Skeleton


  Hard internal/external framework which supports the softer parts of an organism
• Main Functions of Skeleton

  • Support
  • Protection
  • Locomotion
  • Production of RBCs + platelets
  • Storage of calcium + phosphate
  • Storage of fat / adipose tissue
• 3 Major Types of Skeletons

  • Hydrostatic Skeleton
  • Exoskeleton
  • Endoskeleton
• Hydrostatic Skeleton


  • Characteristic of soft-bodied invertebrates
  • Fluid-filled cavity / coelom surrounded by muscles of body wall
  • Fluid secreted within the body itself into the coelom which exerts a pressure on muscles surrounding it
  • Muscles not attached to structure and only act against each other
  • Combination of muscle contractions and hydrostatic/fluid pressure within coelom
  • Most effective in organisms with metameric segmentation
• Locomotion in Lumbricus (earthworm)

  1. Circular muscles at anterior end contract (extending anterior end forward)

  2. Chetae retracted to reduce resistance to forward motion

  3. Longitudinal muscles at posterior end contract and chetae extended to anchor into soil

  4. Contraction of circular muscles propagated along body

  5. Anterior circular muscles relax and longitudinal muscles contract, shortening anterior segments and pulling body forward

  6. Chetae extend to grip soil and prevent backsliding
• Exoskeleton


  • Made of chitin (non-cellular) secreted externally by epidermal cells
  • Chitin is light, tough and flexible
  • Can be reinforced with proteins or CaCO3 for more strength (never at joints)
  • Arthropods have jointed appendages with antagonistic flexor and extensor muscles attached to inside of exoskeleton
  • Chitin is permeable to H2O, some arthropods secrete waxy layer to minimise water loss
• Advantages of Exoskeleton


  • Efficient for locomotion and support in small organisms
  • Can support more weight relative to solid bone of same mass
• Problems of Exoskeleton


  • Cannot grow, periodic moulting/ecdysis must occur to allow for growth
  • Vulnerable to predators due to lack of protection
  • Inefficient locomotion due to lack of rigid surface for muscle attachment and contraction
  • Vulnerable to desiccation due to compromised epicuticle
  • Efficiency lost as size/mass increases, exoskeleton must be larger and thicker
  • Skin surface can no longer be a gas exchange surface, need to develop specialised structures
  • Sensory perception might be slightly impaired
• Locomotion in Insects - Walking

  1. Coordinated movement of 3 pairs of legs attached to each of 3 thoracic segments

  2. Bending and stretching of legs via antagonistic muscles

  3. 3 legs remain in contact with floor (tripod) while other 3 move forward

  4. Front leg pulls, middle leg supports, back leg pushes

  5. Claws/adhesive pads help insects climb vertical surfaces/walk upside down
• Locomotion in Insects - Flying

  1. Wings moved by contractions of antagonistic muscles on inside of exoskeleton attached to joint where wing joins body

  2. Both wings function as single unit

  3. Posterior pair of wings reduced for balance
• Endoskeleton


  • Internal scaffolding of bones of different shapes/sizes joined together at joints to allow a variety of movement
  • Muscles attached to outside of endoskeleton and contract to generate movement
• Parts of the Human Endoskeleton

  • Axial skeleton (cranium, vertebral column, rib-cage, sternum)
  • Appendicular skeleton (bones of the limbs and pectoral/pelvic girdles)
• Functions of Endoskeleton


  • Facilitates movement/locomotion by providing rigid surface for muscle attachment
  • Supports against gravity
  • Protects internal organs
  • Produces RBCs and platelets
  • Stores calcium, phosphate and fat
• Cartilage


  Firm, flexible material made up of living cells (chondrocytes) embedded in a matrix of collagen proteins
• Cartilage


  • Resists tensile/compressive forces and withstands temporary deformation
  • Reduces friction between bones in contact with each other
  • Provides flexible support and shock absorption
  • Found in joints, nose, ears, trachea, and as temporary skeleton of embryos
• Bone


  • Most abundant skeletal material, provides support, protection and metabolic functions
  • Composed of living cells embedded in a mineralized matrix and collagen fibres
• Composition of Bone


  • 20-30% organic material (ossein) - mainly collagen fibres and glycoproteins
  • 70-80% inorganic/mineral salts - mainly hydroxyapatite (Ca10(PO4)6(OH)2), plus Na, K, Mg, Cl-, F, HCO3 and citrate
• Structure of Long Bone (e.g. femur)

  • Periosteum - outer sheath of tough connective tissue
  • Diaphysis - hollow bone shaft composed of compact bone
  • Epiphysis - expanded head of bone composed of spongy bone
  • Yellow bone marrow - adipose tissue storage
  • Red bone marrow - production of RBCs and platelets
• Histology of Compact Bone


  • Consists of numerous cylinders (Haversian systems/osteons) surrounding a central Haversian canal
  • Lamellae - concentric rings of bone matrix deposited by osteoblasts
  • Lacunae - contain osteocytes surrounded by bone matrix
  • Canaliculi - cytoplasmic connections radiating from lacunae to Haversian canal
  • Cement/glue line - attachment of osteons and limit crack/fracture spread
  • Volkmann's canals - transverse canals linking Haversian canals to bone marrow
• Cells of Bone Tissue

  • Osteoblasts - lay new bone matrix as lamellae
  • Osteoclasts - break down and resorb bone
  • Osteocytes - mature bone cells embedded in bone matrix
• Haversian system / Osteon


  Central canal called a Haversian canal
• Components of Haversian system

  • Haversian canal
  • Lacuna
  • Canaliculi
  • Lamella
  • Cement/glue line
• Haversian canal


  • Contains artery, vein + lymphatic vessel
• Bone matrix


  • Deposited in concentric rings by osteoblasts
  • Increases strength
• Osteocytes


  • Become completely surrounded by bone matrix laid when asteo-blasts
  • Relatively dormant/inactive
• Canaliculi


  • Cytoplasmic connections which radiate out from lacunae + connect lacunae to each other + to Haversian canal
  • Also contain capillaries branching off from artery + vein which nourish osteocytes
• Cement/Glue Line


  • Attachment of outeons + limit cracks/fractures from spreading to other osteons
• Volkmann canal


  • Transverse canals which link Haversian canal to bone marrow in marrow cavity for communication, contain nerves, blood vessels + osteoblasts
• Osteoblasts


  • Lay new bone matrix as lamellae
  • Secrete organic collagen fibres
  • Typically present on trabeculae of developing bone
  • Formed from cells from inner region of periosteum (rich in blood vessels)
  • Eventually become surrounded + enclosed by bone matrix in spaces called lacunae, at which point they become Osteocytes
• Osteocytes


  • Like osteoblasts but less organelles
  • Reside within lacunae
  • Communicate with each other + canal via canaliculi
  • Can revert back to osteoblasts
• Osteoclasts


  • Erode/reabsorb bone so that new bone matrix can be laid
  • Respond to new stressors/damage
  • Inhibited by calcitonin (thyroid gland) which lowers Ca level in blood from bone
  • Antagonistic to parathyroid hormone which stimulates osteoclasts, increasing Ca released from bone into blood
• Spongy Bone


  • Lightweight due to many cavities
  • Strong since meshwork of thin fibres called trabeculae
  • Matrix has less inorganic material, organic material mainly collagen fibres
  • Red bone marrow in cavities between trabeculae which produce RBCs + platelets
  • Trabeculae oriented in direction in which bone is stressed, allowing it to withstand tension/compression effectively whilst keeping weight of bone
  • Present mainly in epiphyses of long bones
• Types of Joints


  • Sutural
  • Cartilaginous
  • Synovial
• Sutural Joints


  • Bones held together by fibrous connective tissue, no movement
• Cartilaginous Joints


  • Cartilage acts as a cushion between bones, limited movement
• Synovial Joints


  • Contain a cavity filled with synovial fluid
  • Move freely in 1 or more planes
  • Fluid contains mucin, which lubricates + reduces friction at joint surfaces; also supplies nutrients + bears load (fluid = incompressible)
  • End surface of articulating bones lined with cartilage
  • Bones held in position by ligaments forming joint capsule which encloses joint cavity
  • Inner surface of capsule lined with synovial membrane, which secretes synovial fluid
• Types of Synovial Joints

  • Ball-and-socket
  • Hinge
  • Gliding
  • Pivot
  • Saddle
• Ball-and-socket Joints


  • Rounded ball-like structure fitting into cup-shaped socket of another bone
  • Greatest R.O.M. in all 3 planes
• Hinge Joints


  • Allow movement along 1 axis/plane only
• Gliding Joints


  • Allow only sliding movements