matrix contains thick bundles of Type I collagen - provides high tensile strength
No distinct perichondrium - little or no ability to regenerate
Resembles dense regular connective tissue, except FC contains chondrocytes in lacunae and fibroblasts
found in intervertebral disks and at sites of bone attachment
Resistant to compression
What are the two components of the ECM of bone?
1) Organic component = osteoid
2) Inorganic component = mineralized
Name 4 features of the organic component of bone ECM.
low amount of ground substance
high amount of Type I collagen fibers
low amount of diffusion
deposited first by osteoblasts
Name 5 features of the inorganic component of bone ECM.
consists of hydroxyapatite crystals (high calcium high phosphate)
very low amount of diffusion
deposited late by osteoblasts and trapped osteocytes (crystals form on collagen fibers extracellularly)
facilitated by the enzyme alkaline phosphatase which can enter the bloodstream (useful serum-marker)
What is a spicule?
a small connected piece of bone tissue
What is an osteoblast?
usually on the edges of bone tissue, lays down osteoid initially, derived from osteogenic cells
What is an osteocyte?
Within/encased/trapped in bone tissue lacunae; lays down mineralized crystals; derived from osteoblasts
What is an osteoclast?
large, multinucleate cell; derived from hematopoeitic stem cells; can digest/reabsorb/destroy bone matrix; important for bone remodelling/repair and for controlling blood levels of calcium
Explain the four steps to osteoclast function.
1. Cells adhere to bone very tightly via focal adhesions around the periphery of the cell with actin filaments emanating from them; form a central sealing zone where bone digestion/reabsorption will occur
2. Cells generate carbonic acid (H2CO3) using the cytoplasmic enzyme carbonic anhydrase.
3. Cells release protons via transmembrane channels onto the bone surface inside the sealing zone - acid degrades hydroxyapatite which releases calcium, picked up by capillaries to raise serum calcium levels.
4. Cells release lysosomal enzymes and metalloproteases that digest collagen and osteoid.
What is osteoclast function stimulated by?
Osteoblasts due to parathyroid hormone.
What is osteoclast function inhibited by?
Calcitonin (hormone) and bisphosphonates (drug = fosamax)
What tissue type is this?
What tissue type is this?
Describe the structure of compact bone.
Bone matrix is arranged in concentric layers (=lamellae) arranged around a central canal
Forms cylindrical columns of bone tissue (=osteon) with central/vertical Haversian canals for blood vessels; there are also horizontal Volkmann canals for blood vessels
Name and describe the two methods of bone formation?
1) Intramembranous Bone Formation (flat bones of the skull):
mesenchyme condenses; mesenchymal stem cells generate cells of the osteogenic lineage
bone forms directly within the mesenchymal condensations
2) Endochondral Bone Formation (all other bones):
a hyaline cartilage model of the bone forms first
blood vessels invade the cartilage and drag in osteoprogenitor cells (=periosteal bud)
increased oxygen tension due to vascularization leads to calcification of cartilage followed by chondrocyte cell death; bone tissue is laid down in its place (=ossification centre)
ossification centers form centrally in the shaft of long bones (=diaphysis) at the primary ossification center and at the ends (=epiphysis) at secondary ossification centers
reserve cartilage remains between the diaphysis and epiphyses (=growth/epiphyseal plate)
growth plate expands until early adulthood to increase the size of long bones
Very common disease, characterized by defects in articular/hyaline cartilage within the joint
Causes include injury or wear and tear
Cartilage has little regenerative repair and there is no cure, so severely affected joints need to be replaced.
Increase in bone porosity
50% incidence is women > 50 years
Increased risk of fracture
Bone remodeling is skewed towards greater reabsorption
Treatment options include anti-resorptives and anabolics to decrease the activity of osteoclasts