Skeletal system

• Basic components of skeletal system:

  • skeletal muscle
  • cartilage
  • tendon
  • ligament - connects bone to bone
• basic components of skeletal system

  A) skeletal muscle

  B) bone

  C) cartilage

  D) tendon

  E) ligament

  
• types of bones:

  • irregular bone - vertebrate
  • splanchnic bone - baculum
  • long bone - femur
  • sesamoid bone - (small bones embedded in a tendon) - patella
  • short bone - carpal bones
  • flat bone - scapula
  • pneumatic bone - paranasal sinuses
• Fibrous joints:

  • bones connected by fibrous tissue
  • no joint cavity
  • three types: suture, syndesmosis, gomphosis
• fibrous joints three types:

  • suture: immovable
  • syndesmosis: slightly movable
  • gomphosis: immovable
• cartilaginous joints:

  • bones connected by cartilage
  • no joint cavity
  • two types: synchondrosis and symphisis
• cartilaginous joints: two types:

  • synchrondrosis = immovable
  • symphisis = slightly movable
• synovial joints:

  • freely movable
  • cavities between bones are filled with synovial fluid
• synovial joints:

  • plane - gliding movement
  • pivot - rotation
  • hinge - flexion and extension
  • ball and socket - freely movable
  • condylar - angular
• bone structure:

  • epiphysis
  • metaphysis
  • diaphysis (shaft)
• long bone:

  
• compact bone:

  also known as cortical bone
• spongy bone:

  • AKA cancellous bone or trabecular bone
  • meshwork of supporting bindles of supporting fibers called trabeculae
• blood supply of bone:

  • nutrient artery and vein = some bones have more than one of each
  • periosteal blood vessels = cover entire length of bone shaft
  • metaphyseal - epiphyseal blood vessels = supply blood to ends of long bones
• bone is composed of matrix and several types of cells
• matrix:

  • contains collagen fibers (organic) and calcium salts (inorganic)
  • hydroxyapatite crystals = 60 % weight of bone
  • collagen fibers = 30 % weight of bone
• irregular collagen synthesis leads to skeletal disorders - osteogenesis
• cells in bone:

  • osteoprogenitor cells
  • osteoblasts
  • osteocytes
  • bone - lining cells
  • osteoclasts
• osteoprogenitor cells:

  • mesenchymal cells that divide to produce daughter cells - differentiate into osteoblasts
  • maintain population of osteoblasts
  • found in periosteum and endosteum
• osteoblasts:

  • line surface bone
  • make organic components of bone matrix
  • when completely surrounded by bone they become osteocytes
• oesteocytes:

  • matured bone cells
  • make up most of cell population
  • mechanosensors stimulate bone formation or degradation in presence of mechanical stimuli
• bone-lining cells:

  • former osteoblasts
  • cover bone that is not remodelling
  • regulate movement of calcium and phosphate in and out of bone
• osteoclasts:

  • bone resorbing cell
  • derived from same stem cell that produces monocytes and macrophages
  • regulate calcium and phosphate levels in body fluid
• blood cell production:

  bone is major site for haematopoiesis

  red blood cells, white blood cells, and platelets are produced in the red marrow
• functions of skeletal system:

  • support
  • movement
  • protection
  • storage
  • blood cell production
• bone formation:

  1. mesenchymal cells
  2. hyaline cartilage
  3. primary ossification centre
  4. spongy bone formation + blood vessel penetration
  5. secondary ossification centre + medullary cavity formation
  6. articular cartilage
• bone growth and development depend on a balance between bone formation and bone resorption
• organic mineral components of bone matrix are continuously recycled and renewed through remodelling
• osteoclasts are constantly removing bone matrix and osteoblasts are always adding to it
• stages of bone remodelling:

  • activation - quiescent osteoblasts
  • resorption - osteoclast progenitors differentiate into osteoclasts
  • reversal - mononuclear cells differentiate into macrophages
  • formation - osteoblast progenitors differentiate into osteoblasts
  • termination
• alterations in rate of bone formation and bone resorption can cause disease:

  • when osteoclasts remove calcium salts faster than osteoblasts deposit them , bones weaken
  • when osteoblast activity predominates, bones become stronger and denser
• normal bone growth:

  • normal growth cannot occur without a constant dietary source of calcium and phosphate salts
  • lesser amounts of other minerals (Magnesium, fluoride, iron, and manganese) are also required
  • vitamin C required for collagen synthesis and stimulates differentiation
  • vitamin D stimulates calcium absorption in gut
• normal bone growth hormonal factors:

  
• skeletal role in calcium homeostasis:

  • calcium = most abundant mineral in body
  • skeleton = calcium reserve
• regulation of calcium levels: low levels of calcium levels

  • parathyroid glands - secrete parathyroid hormone
  • bone response = osteoclasts release stored calcium ions
  • intestinal response = rate of intestinal absorption increases
  • kidney response = kidneys retain calcium ions
• regulation of calcium ion levels in plasma: high calcium levels

  • parafollicular cells in thyroid gland secrete calcitonin
  • bone response = osteoclasts inhibited while osteoblasts continue to lock calcium ions in matrix
  • intestinal response = rate if intestinal absorption increases
  • kidney response = kidneys allow calcium loss