Bone Flashcards
What are the functions of bone
- Endoskeleton
- Reservoir for body calcium (contains 1-2kg calcium)
- Provides attachment to muscles and tendons
- Allowing movement of the body as a whole or as parts of the body
- Forming boundaries for organ protection
What are the 4 types of bone cells?
- Osteoprogenitor cell
- Osteoblasts
- Osteocytes
- Osteoclasts
What are osteoblasts and what is their function?
Where are they found?
They are bone-building cells.
They deposit inorganic salts and osteoid in bone tissue.
Synthesis osteoid to collagen matrix
They are abundant in endoplasmic reticulum and golgi complexes - secrete a lot of protein
They are found at sites where the bone is growing, preparing, or remodeling.
E.g at the site of a fracture
What are Osteocytes and what is their function?
And where are they found?
They maintain bone tissue.
They are osteoblasts that lose the osteoid secreting ability and differentiate to osteocytes - they have no ability to divide
What are Osteoclasts and what is their function?
And where are they found?
These cells break down bone and release calcium and phosphate.
Acid release and enzymes dissolve bone minerals and break down collagen.
They are giant cells with up to 50 nuclei.
Found in areas of bone where there is active growth, repair, or remodeling.
What is the composition of bone?
Water - 10%
Organic protein matrix - 30%
Inorganic hydroxyapatite - 60%
What are some examples of long bones?
Femur, Tibia, Fibula
What are some examples of short bones?
Metacarpals, Carpals
What are some examples of flat bones?
Sternum, Ribs, Scapula
What is the development of bone tissue called?
And when does this begin?
Osteogenesis or Ossification
Begins from birth but not complete until the 21st year of life (on average)
What is the purpose of bone remodeling?
- To maintain the strength and integrity of the skeleton
- To remove old bone and replace it with new bone (mineralisation)
How much of bone is replaced each year?
5-10%
What are Osteoprogenitor cells?
They unspecialised bone stem cells.
They are the only cells that undergo cell devision, the resulting cells develop into osteoblasts.
They are found along the inner portion of the periosteum, in the endosteum and in the canals within bone that contain blood vessels.
What factors effect bone growth?
Minerals (Calcium, fluoride, manganese, phosphorus, magnesium)
Vitamins (Vitamin A, C, D, K and B12)
Hormones (Growth hormone, Insulin-like growth factors, Thyroid hormones, Insulin, Sex Hormones, Parathyroid hormones, Calcitonin)
Exercise
Aging
What is demineralisation?
Loss of bone mass - also includes loss of calcium and other minerals from bone extracellular matrix
What happens during bone remodeling?
- It involves bone resorption which is the destruction of bone extracellular and bone deposition which is the formation of bone extracellular matrix.
- Osteoclasts remove minerals and collagen fibres from bone.
- Osteoblasts add minerals and collagen fibres to the bone.
- Remodelling takes place at different rates in different regions of the body.
- Remodelling also removes injured bone and replaces it with new bone.
What are the benefit of bone remodelling?
- Fixes injured bone
- Grows back thicker and stronger
- More resistant to fracture
What triggers bone remodelling?
- Exercise
- Sedentary lifestyle
- Changes in diet
- Injury
Describe the composition of bone
10% water
30% organic protein matrix
60% inorganic hydroxyapatite
Organic protein matrix –> collagen fibres.
Collagen matrix provides flexibility and strength. Main type is known as collagen 1 which makes up 90%.
These fibres are topped with the inorganic hydrixypatite, calcium phosphate. This makes it light but strong
What are osteoids?
Osteoids make up 1/3 of the matrix and are made up of collagen fibres and ground substances (proteoglycans and glycoproteins).
Gelatinous substance which when laid down and inorganic salts are deposited in it, it hardens and forms mineralised bone
Describe the different bone cells
Osteoblasts - building
- Found on the surface of bone
- They deposit inorganic salts and osteoid into bone tissue to build new mineralised bone
- Present at sites where bone its growing, repairing or remodelling e.g. at the sites of a fracture, in centres of ossification of immature bone
- They are fromed from osteoprogenitor cells
- Synthesise collagen matrix
- Can differentiate into osteocytes
- As the deposit new bone around them they become trapped and in tiny pockets called lacunae in the growing bone and differentiate into osteocytes.
Osteocytes
- Mature bone cells than do not divide
- Monitor and maintain bone tissue, and are nourished by tissue fluid in the canaliculi
Osteoclasts
- Involved in breakdown of bone (bone resorption)
- Form a cavity by releasing acid (break down bone mineral) and enzymes (break down collagen)
- Preosteoclasts are activated to form osteoclasts
- Osteoblasts then come to the cavity and rebuild the bone
Describe bone remodelling and why it happens
Osteoclasts form a cavity
Osteoblasts synthesises the formation of osteoid
Osteoid hardens and becomes mineralised bone
Purpose - maintain integrity and strength of skeleton - removed old bone and replaced with new (mineralisation)
Surface area of trabecular bone is greater than cortical resulting at remodelling occurring at a higher rate, due to it being the location of fractures
How does bone mass change with age?
Does it vary depending on other factors?
- Bone mass is higher in men
- Depends on gender, activity, race
- From birth there is a steady growth upwards, this is attainment of bone mass
- It then reaches a peak, consolidation (around 35 years old)
- In later life for me, there is linear bone loss where the rate of breakdown is higher than the synthesis of mineralisation
- In later life for women, there is a rapid decrease in bone mass after menopause due to lower oestrogen levels which cause over activity of osteoclasts
How does oestrogen effect the bones?
Oestrogen slows down activity of osteoclasts, preventing making new cavities. It does this by binding to the the RANK receptor on the osteoclast, preventing the RANK ligand which activates osteoclasts to bind. By preventing the activation of the osteoclast, the formation of making a cavity is inhibited.
After menopause, oestrogen levels decrease, this causes osteoclasts to speed up rapdily causing are larger level of bone loss.
