Bone

Question 1

Bone

Answer 1

• Specialised, mineralised connective tissue acts strong and durable. Made up of 65% mineral( hydroxyapatite) and 33% organic material called osteoid made up of collagen fibres.
• Acts as rigid framework of body and protects underlying organs.
• Subject to constant turnover with periods of selective destruction and repair = remodelling. Important in growth of bones and maintaining mineral balance.

Question 2

Fibres and ground substance

Answer 2

Bone matrix composed of densely packed collagen fibres infiltrated with mineral( hydroxyapatite).
65% mineral 33% colleges fibres
- this arrangement gives bone strength but with slight elasticity.

Question 3

Cells

osteoblasts/ osteocytes

Answer 3

2% of bone. All derived from mesenchymal cells.
Osteoblasts - bone forming cells. Form the collagen and proteoglycans of the matrix. Deposits minerals on fibres. Lie on surface of bone and inner surface of the periosteum . Numerous in growing bones/ fractures. As they deposit new bone tissue they get trapped in lacunas and differentiate to osteocytes.
Osteocytes- osteoblasts trapped in bone matrix in lacunas. Cells have long processes lying channels within matrix( cannaliculi) which carry tissue fluid bringing in nutrition from blood vessels. This is necessary as matrix is too dense for tissue fluid to diffuse. Osteocytes monitor and maintains bone tissue around it especially mineral content ie Ca.

Question 4

Cells

Osteoclasts

Answer 4

Bone destroying cells. Large multi nucleated cells which sit on surface of bone in howships lacunae. The cytoplasm contains many lysosomes. They remove bone to maintain shape during growth or fracture and may be stimulated S part of disease process ie perio.
Fine balance of osteoblast and osteoblast activity maintains normal bone structure and function.

Question 5

Periosteum

Answer 5

Outer surface of bone covered by a layer of dense , fibrous connective tissue. All surfaces covered except those in moveable joints.
Has a number of functions inc:
- protection of bone
- nutrition of the bone
- attachment for muscles , tendons , ligaments
- new bone formation by layer of osteoblasts on inner surface.

Question 6

Blood , lymphatic vessels and nerves

Answer 6

Bone has all 3 of these structures running through it.

Question 7

Bone marrow

Answer 7

Found within spaces of cancellous bone.
Before birth and childhood all bone marrow is red and capable of producing all blood cell types. Over the next 20 years most bone marrow is gradually replaced by yellow bone marrow . This is a fatty substance with no blood cell forming function. Red bone marrow still found in adults: 
Sternum
Ribs 
Pelvis
Skull
Extremities of long bones

Question 8

Classification of bone

Answer 8

Bone is classified on the spaces within it.
2 types:
Compact/ dense
Cancellous / Spongy/ trabecular

Question 9

Compact bone

Answer 9

Hard and dense. 80% of bone mass. Usually on outer aspect of bone.
Made up of parallel tube shaped units called osteons( haversian systems) each of which is made up of a central canal with a series of expanding rings surrounding it. Aligned in same way force is applied. Central canal contains blood, lymph and nerve vessels and each canal linked with neighbouring one via perforating canals at 90 degrees. Blood/lymph brought into Haversian canals by volkmans canals. Series of cylindrical plates of bone arranged around each central canal is called lamellae( circumferential bone). Between adjacent lamellae of osteon is strings of lacunae and in each sits an osteocyte. Lacunae communicate via cannaliculi which allows circulation of interstitial fluid through bone. Between osteons are interstitial lamellae , remnants of old systems.

Question 10

Cancellous bone

Spongy/ trabecular

Answer 10

Makes up 20%. To naked eye looks like honeycomb. This has 2 main functions inc the spaces contain red bone marrow and then this makes the bone lighter.
Organised in osteons with osteocytes in lacunas with cannaliculi. Bone tissue isn’t as dense so osteocytes are never far from the internal bone surface and therefore nourished from diffusion via cannaliculi and no need for central canal of osteon.
Branching framework of pillars/ plates is not arranged randomly and honeycomb develops maximum strength along direction of maximum stress.

Question 11

Formation of bone

osteogenesis / ossification

Answer 11

Bone formation begins at birth and is not complete until 21.
2 methods:
1. Intramembranous ossification - bone forms in an area of loose connective tissue ( membrane model). Flat bones form this way. Sesamoid bones similar but bone formation starts in tendons. Mesenchymal cell- osteoblast- bone.
2. Endochondral ossification - bone forms in the site of a cartilage model. This model will be destroyed as bone forms. Long short and irregular bones formed this way.

Question 12

Processes of bone formation

Answer 12

1. Secretion of osteoid by the osteoblasts. Osteoid is made up of collagen fibres in proteoglycan matrix. Gradually replaces the original membrane, tendon or cartilage model.
2. Calcification / mineralisation of osteoid - this starts immediately after osteoid is laid down and continues until bones complete. This process controlled by osteoblasts. As bone mineralises and grows the osteoblasts get trapped in the matrix that they have made and become osteocytes.

Question 13

Growth of bone

Answer 13

Width of bones can be increased by bone being laid down on the surface of bone , combined with resorption of bone on inner aspect. This is appositional growth.
Bones can grow in length until puberty.
Hormonal regulation of growth:
Hormones regulate growth, shape and size of bone.
Growth hormone and thyroid hormones ie thyroxine control growth in childhood , testosterone and oestrogen control physical changes in puberty and calcitonin and parathormone control calcium levels.

Question 14

Repair of bone

Answer 14

Bone has excellent powers of repair.
Fracture will heal in weeks with right conditions.
Repair more rapid in children.

Question 15

Exercise/ diet and bone

Answer 15

Bones can thicken throughout life. This involves appositional growth.
Weight bearing action stimulates thickening of bone , strengthening it and making it less likely to fracture.
Healthy bones require adequate dietary intake of calcium and vitamins A, C and D. Smaller amounts of phosphate , iron and manganese.
Vit A - osteoblast activity
Vit C - collagen fibres
Vit D - needed for calcium and phosphate absorption from intestinal tract.