Bone I

Question 1

describe the ECM of bone

Answer 1

• organic portion = predominantly collagenous proteins
  
  • mainly type I collagen
• inorganic portion is mainly calcium phosphate in the form of hydroxyapatite crystals

Question 2

describe osteogenic cells

Answer 2

• spindleshaped connective tissue cells (derived from embryonic mesenchyme stem cells) in the periosteium and endosteum that are capable of differentiating into osteoblasts

Question 3

describe osteoblasts

Answer 3

• derived from osteoprogenitor cells; are basophilic
• morphologically may appear as a cuboidal layer (high secreting activity) or flattened (declining secretions)
• cytoplasmic processes contact them with other osteoblasts and osteocytes

Question 4

describe the phases of osteoblasts

Answer 4

• during synthesis, osteoblasts have well developed rER and Golgi
• PAS positive granules in cytoplasm = precursors of GAGs
• eventually matrix secretion entraps the osteoblast in a lacunae with its cytoplasmic processes extending into canaliculi
  
  • osteoblast ceases its secretory function and becomes an osteocyte

Question 5

describe osteocytes

Answer 5

• osteocytes = mature bone cells derived from osteoblasts
• they are housed in their own lacunae, maintain communication and exchange nutrients and metabolites via gap junctions between their narrow cytoplasmic processes extending through the canaliculi
• increased amounts of condensed nuclear chromatin, reduced amounts of rER and smaller Golgi vs osteoblasts

Question 6

describe osteoclasts

Answer 6

• multinucleated giant cells (up to 50 nuclei) resulting from the fusion of blood-derived monoblasts
• Howship’s lacunae are depressions on the bone surface that house osteoclasts
• these lacunae result from the osteolytic activities of osteoclasts

Question 7

describe the 3 regions of osteoclasts

Answer 7

1. ruffled border
   
   • finger-like evaginations along Howship’s lacunae depict sites of active bone resorption
2. Clear zone
   
   • region of cytoplasm that surrounds the ruffled border
   • microfilaments anchor osteoclast to the boney surface and help isolate osteoclastic activity
3. basal region
   
   • houses the organelles and numerous nuclei of the cell

Question 8

describe the bone resorption activity of osteoclasts

Answer 8

• lysosomal enzymes (from Golgi) and H+ ions released into the confined space between bone matrix and the osteoclast’s peripheral clear zone
• decalcified bone matrix is then broken down by acid hydrolases, collagenous and protelytic enzymes, and the cell resorbs the organic and inorganic material

Question 9

describe the periosteum and endosteum

Answer 9

• non-calcified CT layer covering external and internal surfaces of bone, except over articular surfaces
• composed of 2 layers:
  
  • outer fibrous layer consists mostly of a dense collagenous CT containing blood vessels
  • inner layer is more cellular and contains osteoprogenitor cells (periosteal/endosteal cells)

Question 10

describe bone growth and remodeling

Answer 10

• elongation of bones is a result of interstitial growth of cartilage at epiphyseal growht plate
• bone increases in width by appositional growth (osteoprogenitor cells in the periosteum)

Question 11

describe immature (primary or woven) bone

Answer 11

• immature bone (IB) is the first compact bone
  
  • contains numerous osteocytes and irregularly arranged collagen
• low mineral content
• remodeled and replaced by mature bone (MB)

Question 12

describe mature (secondary or lamellar) bone

Answer 12

• replaces primary bone and made up of Haversian system, or osteons
• calcified matrix is arranged in regular concentric layers of lamellae surrounding a Haversian canal that transmits blood vessels, nerves and some loose CT
• osteocytes in lacunae are located between the lamellae and occasionally within them

Question 13

name the 2 types of mature bones

Answer 13

• compact bone
  
  • very dense and heavy
• spongy (cancellous) bone
  
  • filled with spaces that are interconnected
    
    • in long bone space filled with red (hemopoietic cells) or yellow (fat) bone marrow

Question 14

describe compact bone

Answer 14

• functional unit = Haversian system or osteon
• have numerous canaliculi that permit communication between lacunae and with the Haversian canals

Question 15

describe Volkmann’s canals

Answer 15

• connect adjacent Haversian canals to each other and to endosteum at the mattrow cavity on the inside periosteum on the outside
• allow for passage of neurovasculature through compact bone

Question 16

describe intramembranous ossification

Answer 16

• begins when mesenchymal cells condense to form a primary ossification center, from which osteoblasts differentiate and begin secreting osteoid
• osteoblasts become trapped in their own matrix and become osteocytes
• ossification centers expand into spicules as more osteoblasts aggregate
  
  • bone trabeculae (fused spicules) is the name given to the bone developing at these sites