Topic 9: Skeletal System

Question 1

Skeletal System Functions

Answer 1

• Support
• Protection of vital organs.
• attaches muscles and allows movement
• red marrow produces ALL blood cells
• stores Ca++

Question 2

Bone Ossification

Answer 2

• before week 8 skeleton of embryo constructed of fibrous membranes + hyaline cartilage
• bone formation begins at ~8th week ⇒ 20 years

Question 3

Intramembranous ossification

Answer 3

• flat bones e.g. some skull bones, mandible, clavicle
• develop from fibrous CT membrane
• process:
• -osteoblasts produce spongy bone along CT fibres
• -periosteum forms and then osteoblasts under it lay down compact bone
• e.g. fontanels = unossified membranes in the skull that remain at births

Question 4

Endochondral ossification (most bones)

Answer 4

• hyaline cartilage used as a “model” (similar shape) for bone construction – then ossifies to form spongy bone
• periosteum forms and the osteoblasts beneath lay down compact bone
• articular cartilage and epiphyseal plates = cartilage that has not ossified

Question 5

Growth in Length

Answer 5

-endochondral growth - at epiphyseal plates
process:
-cartilage grows ⇒ matrix near diaphysis becomes bone
-growth in length stops when epiphyseal plate ossifies into epiphyseal line (closure)

Question 6

Growth in Diameter

Answer 6

-appositional growth
process:
-osteoblasts beneath periosteum secrete bone matrix (external bone surface)
-osteoclasts resorb bone on the endosteal surface ∴ size of cavity keeps pace with growing bone

Question 7

Factors Affecting Bone Growth/Remodeling

Answer 7

• Mechanical Stress
• Nutrition
• Hormones

Question 8

Mechanical Stress

Answer 8

typically due to muscle action
examples:
-moderate exercise ⇒ ⇑ osteoblast activity in areas of stress (more building than breakdown)
-injury that restricts movement ⇒ ⇓ osteoblast activity and no change in osteoclast activity ⇒ ⇓ bone mass
-weights or electric currents - may speed healing by ⇑ osteoblast activity

Question 9

Nutrition

Answer 9

• Ca2+, PO4- - required for bone matrix production
• vitamin C - required for collagen production
• vitamin D - ⇑ absorption of Ca2+ from intestine
• -If lacking vit. D ⇒ Rickets (soft bones)

Question 10

Hormones

Answer 10

• growth hormone (GH), thyroid ⇒ stimulate bone growth
• Estrogen and Testosterone
• -⇑ osteoblast activity
• -E (both sexes) - ossification of epiphyseal plate (to line)
• Calcitonin
• -inhibits osteoclast activity
• -⇑ Ca2+ movement from blood ⇒ bone; promotes bone growth
• Parathyroid Hormone (PTH)
• -⇑ osteoclast actvitiy and ⇓ osteoblast activity ∴ ⇑ blood Ca2+

Question 11

Osteoporosis

Answer 11

⇓ in bone mass
Risk factors:
-increasing age - ♀ + ♂ (⇓ sex hormones)
-post-menopause - ⇓ E 
-inadequate diet (⇓ vit D, Ca2+)
-illness, excess PTH
-too little/excessive exercise (stress - cortisol) 
-drugs e.g. cortisone, alcohol - ⇓ osteoblast activity, smoking - ⇓ estrogen

Question 12

Bone Repair:

Answer 12

process:

• begins with formation of a blood clot
• replaced by a callus - consists of fibrous network and fibrocartilage islets
• callus ossifies ⇒ intramembranous and endochondral ossification
• takes 4 - 6 weeks ⇒ cast required since movement can re-fracture new matrix