Ossification And Bone Disease Flashcards
Where are the growth plates of bones located?
EPIPHYSIS
Briefly explain the development of long bones
- Periosteal collar forms on initial hyaline cartilage template by endochondral ossification
- Central cartilage CALCIFIES and nutrient artery penetrates to supply osteogenic cells (PRIMARY OSSIFICATION CENTRE)
- Medulla becomes cancellous bone and cartilage forms at EPIPHYSEAL growth plates (SECONDARY OSSIFICATION CENTRE)
- Epiphyseal growth plates ossify and continue to move apart, lengthening the bone
- Hyaline cartilage remains at articulating surfaces
Explain how growth occurs at the EPIPHYSEAL GROWTH PLATE
- Zone of PROLIFERATION -> chondrocytes actively divide forming columns and secreting matrix
- Zone of HYPERTROPHY -> cells enlarge and matrix is compressed into linear bands between cell columns
- Zone of CALCIFIED CARTILAGE -> enlarged cells degenerate and matrix calcifies (hydroxyapatite)
- Zone of RESORPTION -> small blood vessels invade region occupied by dying cells (calcified cartilage forms spicules between them and new bone is laid down by osteoblasts)
What is the zone of reserve cartilage?
Zone above the zone of proliferation where no cellular proliferation or active matrix production occurs
What is INTRAMEMBRANEOUS OSSIFICATION?
New bone is formed by condensations of MESENCHYMAL TISSUE
What type of bones form by intramembraneous ossification?
FLAT BONES such as skull, pelvis and clavicle
How does intramembraneous ossification contribute to growth of long bones?
Thickening of long bones by ossification of the periosteum
What is osteoid?
- Unmineralised component of bone (non calcified matrix)
- Precursor laid down by osteoblasts
- Formed by lack of vitamin D
Describe the cause(s) of OSTEOGENISIS IMPERFECTA
- Autosomal recessive (RARE 1 in 10000)
- Mutation in gene coding for TYPE I COLLAGEN (most abundant protein in body and present in matrix)
- Results in brittle bones that are prone to fractures
What is the medico-legal importance of osteogenesis imperfecta?
Possible confusion of multiple fractures caused by deliberate injury
What group of people are more susceptible to osteoporosis?
- Most common bone condition affecting the elderly
- Affects 1 in 3 women and 1 in 12 men in UK over the age of 50
What are the risk factors associated with osteoporosis?
- Age
- Menopause
- Hypogonadism
- Long term steroid treatment
- Poor dietary calcium and vitamin D
Why are females more susceptible to osteoporosis than men?
- Oestrogen and testosterone promote osteoblasts and suppress osteoclasts
- Loss of oestrogen post-menopause
- Less suppression of osteoclast activity and less promotion of osteoblast activity
- Rate of regeneration
Explain how steroids can be considered a risk factor for osteoporosis
- Steroids inhibit osteoblasts
- Regeneration
Describe the changes of bone mass that occur with ageing
- Bone mass density rises and peaks at mid-20s
- Slowly begins to decline
- Women have accelerated decline of bone mass post-menopause
What is the difference between osteoporosis and osteomalacia in regards to bone mass and density?
- Osteoporosis -> decrease in bone mass, same bone density
- Osteomalacia -> same bone mass, decrease in bone density
Explain the importance of vitamin D in ossification and bone strength
- Essential for normal ossification
- Absence causes formation of osteoid (poorly mineralised, pliable matrix)
- Affected bones are unable to support body weight and bend, as well as being prone to fractures
Give 2 examples of bone diseases that can occur due to lack of dietary vitamin D
- RICKETS in growing children as there is insufficient calcium to mineralise bone trabeculae (bones are weakened and bend as a result)
- OSTEOMALACIA in adults occurs similarly and results in decrease in bone mass
Outline 5 common fracture sites and explain why these bones are more susceptible to fractures
- Common sites are: > Neck of femur > Vertebrae (spine) > Ribs > Pubic ramus > Distal radius - More spongy cancellous bone in these areas (contain weakened trabeculae due to insufficient mineralisation)
What is the role of vitamin D in the mineralisation of bone?
- Calcium (Ca2+) absorption in small intestine
- Ca2+ resorption from bone (osteoclasts)
- Ca2+ reabsorption in loop of Henle in kidneys
Explain the homeostatic mechanisms involved in maintaining levels of Ca2+ ions
- ⬆️Ca2+ levels -> CALCITONIN stimulates osteoblasts to absorb Ca2+ into bone matrix
- ⬇️Ca2+ levels -> PARATHYROID HORMONE stimulates osteoclasts to break down bone matrix and release Ca2+
Why is osteoporosis considered as a risk factor for fractures but not as cause?
Associated with fractures as there is an increased chance of fracture due to weakened bones but is not necessarily the cause of fractures (FALLS)
Explain how HYPERGONADISM is considered a risk factor for osteoporosis
- Hormones from gonads act on osteoblasts and osteoclasts
- Lack of gonadal hormones decreases promotion of osteoblasts and decreases suppression of osteoclasts
- Results in reduced peak bone mass and bone mass declines more rapidly
How can conditions such as osteoporosis, rickets and osteomalacia be prevented?
- Increased dietary calcium
- Increased vitamin D from sunshine (early on in life)
- Increased mobility and exercise
- Management using hormone replacement (hypergonadism)
