Basics of bone

Question 1

Structure of bone

Answer 1

Woven bone (immature bone)
Collagen fibre aligned randomly
No lamellae
More flexible, but weaker
Isotropic characteristic

Lamellar bone (Mature bone)
Cortical and cancellous bone with stress orientated collagen fibres
Anisotropic characteristic
Has Lamellae
Collagen fibres run parallel to each other
Herring-bone structure : Fibres in adjacent lamellae run at oblique angles to each other

Question 2

Bone lining cells Gatekeeper function

Answer 2

When stimulated by PTH, undergo cyclic adenosine monophosphate (cAMP) that expose the bone surface and allow osteoclasts to start resorption

Question 3

Lack of osteoclast and overactivity of osteoclast diseases

Answer 3

Lack of osteoclast: Osteopetrosis
Overactivity of osteoclast: Paget’s disease

Question 4

Function of inorganic and organic bone matrix

Answer 4

Inorganic matrix
Responsible for compressive strength of bone
Serves as reservoir for 99% of calcium, 85% of phosphorus and 40-60% of body sodium and potassium

Organic matrix
Constitutes of 90% of collagen type 1
Responsible for tensile strength of bone

Question 5

Blood supply system for bone

Answer 5

Nutrient artery system (high pressure system)
Enters mid-diaphysis
In child, vessels end on metaphyseal side of physis, contributing to the process of endochondral ossification

Metaphyseal- epiphyseal system
Periarticular vascular complex

Periosteal system (Low pressure system)
Enter at sites of major muscle attachments, normally supplying outer third of cortex
Dominant system in child
Responsible for circumferential growth

Normal direction of flow
Centrifugal: Inside to outside
If endosteal system is damaged, after reaming, periosteal system becomes dominant, flow become centripetal (outside to inside)

Question 6

Regulators of calcium and phosphate metabolism

Answer 6

Vitamin D
Enhance calcium and phosphorus absorption across small intestine
Enhance osteoclastic resorption from bone

Parathyroid
Secreted by chief cells of parathyroid glands

In kidney
Stimulation of hydroxylation of 25 (OH)- vitamin D3 in proximal tubules
Increasing reabsorption of filtered calcium in kidney
Promotion of urinary excretion of phosphate from kidney

In bone
Stimulation of osteoclast, producing bone resorption

Calcitonin
Secreted by parafollicular C cells of thyroid gland
Directly inhibits osteoclast