Bone Tissue and Formation

Question 1

What are osteoprogenitor cells?

Answer 1

• they undergo mitosis and become osteoblasts

- precursor cells

Question 2

What are osteoblasts?

Answer 2

• they secrete collagen and organic components
• they form bone and once they are covered by matrix they become osteocytes
• are now no longer capable of cell division

Question 3

What are osteocytes?

Answer 3

• mature cells found on bone surface
• they maintain daily activities of bone tissue
• they are formed when an osteoblast becomes embedded in the material it has secreted

Question 4

What are osteoclasts?

Answer 4

• they develop from monocytes
• they settle on bone surface
• the function is bone re-absorption (destroy matrix)

Question 5

Describe the structure of bone tissue (osseous tissue)

Answer 5

• consists of widely spaced cells surrounded by matrix
  - -> matrix contains abundant mineral salts (esp hydroxylapatite) and collagen fibres
  - -> water 25%, protein 25% and minerals 50%
• there are two types of bone (compact + spongy)

Question 6

Describe the process of bone formation (ossification)

Answer 6

• bone forms by ossification
• mesenchymal (embryonic connective tissue) cells transform into osteo-progenitor cells
  
  • -> they undergo cell division and give rise to cells that develop into osteoblasts and osteoclasts
• this process begins 7th embryonic weeks and continues throughout adulthood

Question 7

What is intramembraneous ossifcation?

Answer 7

• it occurs within fibrous membranes of embryo, fetus, child, adult
• at sight where bone will develop, mesenchymal cells cluster and become vascularised
• they then differentiate into osteoprogenitor cells, then into osteoblasts, this cluster is the ossification centre
• osteoblasts secrete the organic matrix of bone until they are surrounded by it
  
  • -> these cells are now called osteocytes
• calcium and mineral salts are attracted and the tissue calcifies (hardens)

Question 8

What is endochondral ossification? (within cartilage)?

Answer 8

• a cartilage model forms by mesenchymal cells crowding together in the shape of a future bone
• a membrane (perichondrium) surrounds this
• the primary ossification centre is in the diaphysis (shaft)
• bone tissue develops and the cartilage degenerates, this leaves cavities that merge to form the medulla
• then epiphyses (ends) ossify

Question 9

Describe how a long bone grows

Answer 9

• controlled by human growth hormon (HGH), this is active after 2 years
• the epiphyseal plate is a thick layer of cartilage within the metaphysis
  
  • -> it separates the epiphysis from the diaphysis of the bone
• the epiphyseal plate has a proliferation of cartilage which calcifies
  
  • -> this results in the elongation of the diaphysis
• growth is equal at each end of the shaft
• later a secondary ossification centre forms in the epiphysis
  
  • -> each epiphysis now grows with the diaphysis
  • -> simultaneously growth of bone diameter results from addition of new bone tissue by periosteal osteoblasts around the outer surface
  • -> diaphysis continues to grow at each end
• eventually the epiphyseal cartilage cells stop dividing and bone replaces cartilage
• the once active epiphyseal plate becomes a thin line of bone called the epiphyseal line, the bone then ceases its growth
• ossification is usually complete by 25 years (earlier in women)
• the clavicle is the last bone to stop growing and some bones grow at different rates