Introduction to the limbs

Question 1

What are the main functions of bone?

Answer 1

Support
Storage of minerals
Protection of soft tissues
Provide a system of levers for movement

Question 2

What are the 2 classifications of bone tissue?

Answer 2

Woven bone (immature)
Lamellar bone (mature)- cortical or cancellous

Question 3

What are the features of woven bone?

Answer 3

Immature bone
Found either in the growing skeleton or after a fracture
More random than lamellar bone
Weaker but more flexible than lamellar bone
Not stress-oriented

Question 4

What are the features of lamellar bone?

Answer 4

Mature bone
Found in the normal skeleton
2 types: cortical and cancellous.
Cortical or compact bone makes up 80% of the skeleton, has a slow turnover rate and high resistance to torsion and bending.
Cancellous bone (spongy, trabecular) is less dense, consists of spicules with marrow in between, and has a higher turnover rate than compact bone.

Question 5

Describes the microscopic structure of bone.

Answer 5

Bone is a supporting connective tissue composed of specialised bone cells (osteoblasts and osteoclasts) and bone matrix consisting of extracellular protein fibres (mainly collagen), less structured ECM and crystalline calcium phosphates deposited around the collagen fibres.

Question 6

What cell types are found in bone?

Answer 6

Osteocytes
Osteoblasts
Osteoprogenitor cells
Osteoclasts

Question 7

What are the features and functions of osteocytes?

Answer 7

Mature bone cells.
Lie within lacunae (pockets within bone matrix). Each lacuna contains one osteocyte which never divides. Lacunae are connected by canaliculi. The canals contain cytoplasmic extensions of the osteocytes and provide for metabolic exchange.
Osteocytes maintain the bone matrix (proteins and minerals) and are involved in bone repair. If a bone is damaged, the osteocytes are freed from their lacunae and differentiate into osteoblasts or osteoprogenitor cells.

Question 8

What are the features and functions of osteoblasts?

Answer 8

Produce new bone matrix.
The matrix is initially called osteoid before it is calcified.
Ultimately the osteoblasts become osteocytes.

Question 9

What are the features and functions of osteoprogenitor cells?

Answer 9

Effectively stem cells, which can differentiate into osteoblasts.
Small number present in bone.
Actively involved in fracture repair.
Most osteoprogenitor cells are in the periosteum, endosteum and vascular canals in the bone matrix.

Question 10

What are the features and functions of osteoclasts?

Answer 10

Also called giant cells.
Remove bone matrix.
Derived from monocytes- different lineage to other bone cells.

Question 11

Describe the metabolic turnover in bone.

Answer 11

Continuous metabolic bone turnover- osteoclasts resorb bone and osteoblasts produce bone.
Can go wrong, resulting in too little or too much bone.

Question 12

What does the bone matrix consist of?

Answer 12

Extracellular protein fibres, ground substance and lots of inorganic ions, mainly calcium phosphate, calcium hydroxyapatite and calcium hydroxide.

Question 13

What proportion of the weight of bone do inorganic salts in the bone matrix account for?

Answer 13

Two thirds.

Question 14

What is the function of collagen fibres laid down in bone?

Answer 14

Provide a supporting lattice for the hydroxyapatite crystals.
Collagen is strong, flexible, good at resisting tension twisting and bending, but poor at resisting compression.
Calcium phosphate crystals are hard but inflexible and brittle, but good at withstanding compression.
Combination of the 2 allows for excellent biomechanical properties of bone.

Question 15

What is the basic unit in compact bone?

Answer 15

The osteon or Haversian system.

Question 16

How are osteocytes arranged in compact bone?

Answer 16

Osteocytes are arranged in layers around a central Haversian canal. Each canal carries a blood and nerve supply to the osteon. The Haversian canals run parallel to the bone surface.

Question 17

How are osteons arranged in compact bone?

Answer 17

Osteons are arranged into lamellae. Most are arranged in a cylindrically in the longitudinal axis of the bone- concentric lamellae.
Interstitial lamellae fill in the spaces. Circumferential lamellae are adjacent to either the periosteum or endosteum and are produced during bone growth.

Question 18

Describe the arrangement of cancellous bone.

Answer 18

Lamellae are not arranged into osteons.
Matrix develops into an interlacing network of rods- trabeculae.
This is much lighter than compact bone, and provides and ideal environment for bone marrow cells.

Question 19

Describe the structure of the periosteum.

Answer 19

Superficial layer of compact bone is covered by periosteum.

Consists of a fibrous outer layer and an inner cellular layer.

Question 20

What are the functions of the periosteum?

Answer 20

Protects bone from surrounding tissues
Provides route for blood vessels and nerves
Actively involved in bone growth and repair

Question 21

Describe the structure and function of the endosteum.

Answer 21

Incomplete cellular layer lining marrow cavity.

Very active during bone growth and repair.

Question 22

When does stripping of the periosteum occur and what can it result in?

Answer 22

In fractures and with surgery.
Can result in deprivation of blood supply from bone. This can result in bone death (bone infarction) and bone infection (osteomyelitis).

Question 23

What are the 3 main sources of blood supply to a long bone?

Answer 23

Arteries enter bone mainly via the periosteum. Periosteal arteries enter the bone at a number of points.
Nutrient artery- enters near middle of bone, passing obliquely through the compact bone, and supplies the medulla (spongy bone and any bone marrow within).
Metaphyseal and epiphyseal arteries- supply the ends of bones. Only present in growing skeleton. Blood can “sludge” within them. May be why haematogenous bone infection (osteomyelitis) occurs in children but rarely in adults.)

Question 24

Describe the nerve supply of bone.

Answer 24

Accompanies the arteries supplying that bone.
Periosteum surrounding bones is very richly innervated with nerve fibres.
Most periosteal nerves are sensory, for pain and proprioception.
Blood vessels in most bones also have a strong sympathetic supply, regulating bone blood flow.

Question 25

When does the skeleton begin to form?

Answer 25

6 weeks fo foetal life.

Question 26

How long can skeleton growth continue?

Answer 26

Until 25 years of age in some bones.

Question 27

What are the 2 processes by which bones are formed from mesenchyme (embryonic connective tissue)?

Answer 27

Intramembranous ossification (directly from mesenchyme)
Endochondral ossification (mesenchyme differentiates to cartilage before forming bone

Question 28

Describe the process of intramembranous ossification.

Answer 28

Mesenchymal cells produce the matrix- osteoid.
Osteoid becomes mineralised with calcium salts deposited.
Mesenchymal cells differentiate into osteoblasts from an ossification centre
As ossification proceeds, some osteoblasts are trapped into bony pockets (lacunae) and transform into osteocytes
Blood vessels grow into the area to maintain the metabolism of the bone
The initial bone formed is cancellous but this can subsequently remodel into compact bone
Periosteum forms around the bone and “traps” and surface layer of osteoblasts.

Question 29

Which bones are intramembranously ossified?

Answer 29

Typically bones just below the skin surface (dermal ossification).
Skull bones, mandible, clavicle.

Question 30

What is the mechanism of ossification for most bones?

Answer 30

Endochondral ossification.

Question 31

Describe the process of endochondral ossification in long bones.

Answer 31

Mesenchymal cells differentiate into chondroblasts (cartilage forming cells), which form cartilage models of the bones.
Cartilage bone model grows by expansion of the cartilage matrix (interstitial growth) and new growth of cartilage at the outer surface of the bone (appositional growth).
In the middle of the bone model, cartilage calcifies and chondrocytes die. At the same time the periosteum delivers periosteal capillaries into the calcified centre to form the periosteal bud.
Periosteal bud differentiates to form osteoblasts, and a primary ossification centre consequently forms in the middle of the cartilage model. Primary ossification centre then grows in both directions to replace the cartilage model by bone. The body of a bone formed from a primary ossification centre is the diaphysis- increases in diameter by appositional growth.
Secondary ossification centres also form in other parts of the bone- the parts of bone formed from these centres are epiphyses.
The epiphyseal plate is the cartilaginous area between the diaphysis and the epiphysis, and persists until growth is complete.
Bones grow by the laying down of bone at each epiphysis, and continue to grow until epiphysis fuses with diaphysis.

Question 32

Which is the first ossification centre to form?

Answer 32

The first centre to form is at the distal femur and at birth, most babies have ossification centres at the distal femur, proximal humerus and proximal and distal tibia.

Question 33

What is the difference between endochondral ossification in long and short bones?

Answer 33

Most short bones ossify by endochondral ossification, but only via one primary ossification centre. The calcaneum has a secondary ossification centre though.
Long bones have secondary ossification centres.

Question 34

What factors is bone remodelling affected by?

Answer 34

Physical stress placed upon the bone

Metabolic and hormonal factors

Question 35

What is the turnover rate of bone?

Answer 35

Relatively high- 20% of the adult skeleton is remodelled each year.

Question 36

Describe the pattern of repair after a bone fracture.

Answer 36

Torn vessels bleed to produce a fracture haematoma.
Osteoprogenitor cells and relatively inactive osteoblasts from the periosteum and endosteum differentiate into active osteoblasts and migrate towards the fracture.
Migrating osteoblasts form a provisional type of bone and cartilage- external callus.
External callus bridges the end of the bones.
Internal callus also forms between bone ends.
Chondrocytes produce cartilage, which is then replaced by bone.
Initial bone formed is woven bone, which then re-structures to form cancellous (lamellar) bone.
Osteoblasts and osteoclasts remodel the cancellous bone over time, to give rise to cortical bone.