Bone Flashcards
What bones develop by intramembranous ossification?
Flat bones such as skull bones, scapula, pelvic bones, lateral end of clavicle.
The process also contributes to the thickening (not the lengthening) of long bones, at their periosteal surfaces (appositional growth).
Describe the process of intramembranous ossification
A small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells (a nidus).
The MSCs become osteoprogenitor cells (each developing more Golgi apparatus and rough endoplasmic reticulum).
The osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing Type I collagen (osteoid).
The osteoid mineralises to form rudimentary bone tissue spicules, which are surrounded by osteoblasts, and contain osteocytes.
The spicules join to form trabeculae, which merge to form woven bone, which is finally replaced by the lamellae of mature compact bone.
Describe the process of fracture repair
1) A blood clot (haematoma) is formed in which granulation tissue arises.
2) The procallus of granulation tissue is replaced by a fibrocartilaginous callus in which bony trabeculae are developing. (Sleeve of hyaline cartilage)
3) Endochondral and intramembranous ossification give rise to a bony callus of spongy/ cancellous bone.
Cancellous bone is replaced by compact cortical bone until remodelling is complete.
What is osteoporosis?
Osteoporosis is a metabolic bone disease in which mineralized bone is decreased in mass to the point that it no longer provides adequate mechanical support. Enhanced bone resorption relative to formation.
How does osteoporosis arise with increasing age?
Osteoporosis associated with aging results from incomplete filling of osteoclast resorption bays.
What is the difference between type 1 and type 2 osteoporosis?
Type 1 occurs in postmenopausal women.
Type 1 is due to an increase in osteoclast number, a result of oestrogen withdrawal.
Type 2 occurs in elderly persons of both sexes (senile osteoporosis).
Type 2 generally occurs after age 70 and reflects attenuated osteoblast function.
What are some risk factors for osteoporosis?
Genetic: Peak bone mass is higher in blacks than in whites or asians
Insufficient calcium intake: recommended value for postmenopausal women is 800 mg/day.
Insufficient calcium absorption and Vitamin D: decreased renal activation of Vitamin D with age may be a factor in populations without Vitamin D supplementation or with the elderly confined indoors.
Exercise: immobilization of bone (prolonged bed rest or application of a cast) leads to accelerated bone loss.
Physical activity is needed to maintain bone mass.
Cigarette smoking in women has been correlated with increased incidence of osteoporosis.
Describe the process of endochondral ossification
1) Initial cartilage model (miniature version of adult bone) (embryo)
2) Collar of periosteal bone appears in the shaft.
3) Central cartilage calcifies. Nutrient artery penetrates, supplying bone- depositing osteogenic cells. Primary ossification centre formed. (fetus, 8-12 weeks)
4) Medulla becomes cancellous bone. Cartilage forms epiphyseal growth plates. Epiphyses develop secondary centres of ossification. (postnatal)
5) Epiphyses ossify and growth plates continue to move apart, lengthening bone. (prepubertal)
6) Epiphyseal growth plates replaced by bone. Hyaline articular cartilage persists. (adults)
