Ossification And Bone Disease

Question 1

Hyaline cartilage model

Answer 1

• is a precursor of most bones
• it is a subsequently mineralised to form bone
• developing long bones grow by endochondral ossification

Question 2

Long bone development

Answer 2

• initial cartilage model
• collar of periosteal bone appears in the shaft
• central cartilage calcified (primary ossification centre formed) nutrient artery penetrates, supplying bone-depositing osteogenic cells
• medulla becomes cancellous bone. Cartilage forms epiphyseal growth plates - epiphyses develop secondary centres of ossification
• epiphyses ossify and growth plates continue to move apart lengthening the bone
• epiphyseal growth plates replaced by bone. Hyaline articular cartilage persists.

Question 3

Intramembranous ossification

Answer 3

• takes place within condensations of mesenchymal tissue and not by replacement of a pre-existing hyaline cartilage template
  flat bones develop by intramembranous ossification
• skull bones (e.g. parietal, occipital, temporal, frontal)
• mandible
• pelvis
• clavicle
  The process contributes to the thickening (not lengthening) of long bones

Question 4

Mesenchymal cells

Answer 4

• become osteopeogenitor cells which become osteoblasts

Question 5

Osteoclasts

Answer 5

• formed from the fusion of macrophages hence why they have up to 5 nuclei

Question 6

Osteogenesis imperfecta

Answer 6

• autosomal dominant disorders of connective tissues
• mutations in the gene for type I collagen
• it affects the skeleton (bowing of the limbs, thinning of bones), joints, ears, ligaments, teeth, sclerae and skin
  Medicolegal importance - multiple fractures can be mistaken for child abuse

Question 7

Effect of growth hormone (GH) on bone

Answer 7

• GH is synthesised and stored in the anterior pituitary
  Before puberty
• excessive GH causes gigantism through promotion of epiphyseal growth plate activity
• insufficient GH can affect epiphyseal cartilage and cause pituitary dwarfism
  In the adult
• excessive GH cannot cause gigantism because there are no epiphyseal plates instead may cause an increase in bone width by promoting periosteal growth (acromegaly)

Question 8

Effect of sex hormone on bone

Answer 8

• influence the development of ossification centre
• androgens and oestrogen are present in each sex
• they induce secondary sexual characteristics give rise to pubertal growth spurt
• precocious sexual maturity (can be brought about by sex hormone producing tumours) retards bone growth because of premature closure (fusion) of epiphyses
  If sex hormone is deficient, epiphyseal plates may persist later into life than they normally would, leading to prolonged bone growth and tall stature

Question 9

Effects of thyroid hormone on bone

Answer 9

• decreased thyroid hormone production (neonatal hypothyroidism) can be readily reversed by prompt administration of thyroxine
• if untreated the thyroid hormone deficiency can lead to an infant with permanent neurological and intellectual damage (cretinism) and other abnormalities such as short stature

Question 10

Osteoporosis

Answer 10

• enhanced bone reabsorption relative to formation
• is a metabolic bone disease which mineralised bone is decreased in mass to the point that it no longer provides adequate mechanical support
• is a risk factor for fractures in the elderly
• bone mass begins to decline in the 5th or 6th decade

Question 11

Osteoporosis

Answer 11

Type 1 (most common) - in postmenopausal women, due to an increase of osteoclasts number as a result of oestrogen withdrawal 
Type 2 - occurs in elderly persons, generally occurs after the age 70

Question 12

Achondroplasia

men = 51 inches, women = 49 inches

Answer 12

• one of the most common forms of short limb dwarfism
• have a normal mentation and average lifespan
  Achondroplasia
• autosomal dominant point mutation in fibroblast growth factor receptor-3 genes
• causes a gain in function
  Decreased endochondral ossification
  Inhibited proliferation of chondrocytes in growth plate cartilage
  Decreased cellular hypertrophy
  Decreased cartilage matrix production
  2 parents
• 25% chance the child will die
• 50% chance that the child will be heterozygous and have achondroplastia
• 25% chance the child will have a normal phenotype

Question 13

Metabolism of vitamin D

Answer 13

• most in synthesised in the skin by action of UV light
• ‘hydroxylation in the liver, then kidney –> 1,25 dihydroxyvitamin D3 –> increases calcium absorption by the small bowel promotes the mineralisation of bone

Question 14

Rickets and Osteomalacia

Answer 14

Rickets CHILDHOOD

• bones do not harden due to a vitamin D deficiency
• insufficient calcium deposition
• bones become soft and malformed
• distortion of skull bone (extreme)
• enlargement of costochondral junction of the rubs

Osteomalacia ADULTHOOD

• significant calcium deficiency or lack of vitamin D
• abnormally large amounts of non-mineralised bone
• no deformity of bones
• bone pain
• back ache
• muscle weakness

Question 15

Endochondral ossification

Answer 15

• the replacement of a pre-existing hyaline cartilage template by bone
• the way most of the bones of the body develop