Ossification And Bone Disease

Question 0

Describe the process of intramembranous and endochondral ossification as related to bone growth

Answer 0

Intramembranous ossification - from mesenchymal tissue or loose connective tissue e.g. flat bones
Periosteal ossification (increases girth of bone), bone development starts in highly vascularised connective tissue, primary centre of ossification, mesenchymal cells --> osteoblasts --> osteoid --> osteocytes
Endochondral ossification - from cartilage
Primary ossification centre formed first in diaphysis and later epiphysis, medulla becomes cancellous bone, epiphyses develop secondary centres of ossification, epiphyses ossify and growth plates continue to move apart (lengthening bone), epiphyseal growth plates replaced by bone, hyaline cartilage persists

Question 1

Define the term ‘endochondral ossification’

Answer 1

Endochondral ossification = replacement of a pre-existing hyaline cartilage template by bone

Question 2

Outline the consequences of four different and contrasting bone diseases for the affected individual and society

Answer 2

Osteogenesis Imperfecta (brittle bones) - fractures and deformity, multiple fractures present at or before birth, usually fatal
Osteoporosis - from middle age, thinning of bones, more porous, low bone density so susceptible to fractures
Rickets - calcium deficiency in children, bone matrix doesn't calcify properly, epiphyseal plate becomes distorted due to strains of body weight and muscle activity (bowed legs)
Osteomalacia - calcium deficiency in adults, softening of bone, partial decalcification of older matrix, fragility, increased risk of fractures

Question 3

Describe the genetic basis and histological changes in osteogenesis imperfecta, and the disease’s potential medicolegal importance

Answer 3

Autosomal dominant, mutation in gene for type I collagen - abnormal collagen synthesis by osteoblasts and fibroblasts, fragility, fractures
Medicolegal - confusion with multiple fractures caused by deliberate injury

Question 4

Explain the importance of Vitamin D in normal bone development

Answer 4

Vitamin D (sunlight, dietary) is essential for normal ossification as it is involved in the absorption of calcium and phosphate by the small intestine
Deficiency - poorly mineralised, pliable matrix (osteoid) is formed, bones bend. Decreases calcification of osteoid (prevents osteoblasts --> osteoclasts) = osteomalacia

Question 5

Describe the features of bones affected by rickets and osteomalacia, and the difference between the two conditions

Answer 5

Rickets - occurs in growing bones (children), bone matrix fails to calcify normally (osteoid), epiphyseal plate distorted by body weight and muscle activity, bones grow slowly and become deformed, prone to fractures
Osteomalacia - occurs in bone remodelling (adults), deficient calcification, softening of bone

Question 6

Explain the importance of dietary and behavioural factors in their prevention

Answer 6

Need enough vitamin D, calcium and phosphate

Infants with low or no sun exposure, particularly with dark skin, may develop rickets

Question 7

Describe the radiological and histological changes occurring in osteoporosis

Answer 7

Bones density is reduced so fractures are more likely
Osteoclasts activity > osteoblasts activity
Medullary canals in centre of bone become enlarged and gaps develop in lamellae

Question 8

List the most common risk factors for osteoporosis

Answer 8

Gender (female), age (>30), poor diet, insufficient exercise, smoking, genetic (whites), insufficient calcium intake, insufficient calcium absorption and vitamin D

Question 9

Explain the importance of osteoporosis as a risk factor for fractures in the elderly

Answer 9

Elderly are more prone to falls
Due to weaker, osteoporotic bone, fractures are more likely during a trauma such as falling (neck of femur - hip, vertebrae, forearm)

Question 10

Outline the cause and morphological features of achondroplasia

Answer 10

Genetic (autosomal dominant) causes failure of proliferation and column formation of epiphyseal cartilage - defect in endochondral bone formation (skull - intramembranous - is unaffected)
Epiphyseal growth plates are thin, few cells in proliferating zone, hypertrophic cartilage cells form irregular columns, zone of provisionally calcified cartilage is small and doesn’t provide adequate scaffolding for bone matrix deposition by metaphysical osteoblasts

Question 11

Outline the effects of abnormal levels of growth hormone and sex hormones on bone development

Answer 11

Increased GH - gigantism (growth of long bones)
Adult - acromegaly due to periosteal growth
Decreased GH - pituitary dwarfism (affects epiphyseal cartilage)
Increased sex hormone - retard body growth due to premature closure of epiphyses
Decreased sex hormone - tall stature due to epiphyseal plates remaining for a long time
Decreased thyroid - cretinism (physical and mental retardation)