Bone: Structure, Growth and Repair

Question 1

What is the structure of a long bone?

Answer 1

Epiphysis –> Metaphysis –> Diaphysis

Articular cartilage –> Spongy bone –> Yellow Marrow –> Epiphyseal line –> Compact bone (surrounded by periosteum) –> Medullary cavity (wall is endosteum and contains nutrient vessels)

Question 2

What are the four main types of bone cells and how do they relate to one another?

Answer 2

1. Osteoproginator (OPG) cells - develops into an osteoblast during growth/repair
2. Osteoblast - secretes osteoid that calcifies into bone (forms bone ECM)
3. Osteocyte - maintains bone tissue (bone cell)
4. Osteoclast - from WBC lineage (functions in resorption, the breakdown of bone ECM)

Question 3

Explain what the ECM of bone is

Answer 3

It comprises of water, 30% collagen fibres (provide tensile strength) and 55% crystallised mineral salts (calcium phosphate and calcium hydroxide form hydroxyapatite)

Question 4

What is compact bone?

See page 87 for diagram

Answer 4

It is dense and strong and is arranged with osteons and lamellae within that provide the ability to resist stresses such as compression, tensile and torsion. Compact bone also contains canaliculi that allows cell to cell communication and a passage for nutrients and waste products.

Question 5

What is spongy bone (trabecular bone)?

See page 87 for diagram

Answer 5

It ensures the bone is not too heavy and is arranged as a crosslinked network of bony struts. There is an irregular arrangement of lamellae. They contain osteocytes within the bone matrix and osteoclasts and osteoblasts are found on the bone surface within the endosteum. Osteocytes gain nutrients through diffusion.

Question 6

What is intramembranous ossification and give examples

Answer 6

It is when bone forms directly from a fibrous membrane.
It is involved in the formation of the clavicle, mandible and flat bones of the skull.

1. Development of ossification centre: osteoblasts secrete organic ECM (Mesenchyme start to undergo condensation)
2. Calcification: calcium and other mineral salts are deposited and ECM calcifies
3. Formation of trabeculae: ECM develops into trabeculae that fuse to form spongy bone
4. Development of the periosteum: mesenchyme at the periphery of the bone develops into the periosteum

Question 7

What is endochondral ossification and give examples

Answer 7

It is where bone forms from a hyaline cartilage “template”
It occurs in all bones except those in intramembranous ossification.

1. Development of cartilage model: mesenchymal cells develop into chondroblasts, which form the cartilage model. (development of hypertrophic chondrocytes)
2. Growth of cartilage model: growth occurs by cell division of chondrocytes (Hypertrophic chondrocytes signal perichondral cells to form osteoblasts)
3. Development of primary ossification centre: in this region of the diaphysis, bone tissue has replaced most of the cartilage (lots of bone being laid down)
4. Development of the medullary cavity: bone breakdown by osteoclasts forms the medullary cavity
5. Development of secondary ossification centres: these occur in the epiphysis of the bone (expands outwards creating spongy bone)
6. Formation of articular cartilage and epiphyseal plate: both structures consist of hyaline cartilage (The epiphyseal plate is a growth plate and it is the layer of cartilage that joins the metaphysis and epiphysis)

Question 8

What is appositional growth and give examples

Answer 8

It is when bone increases in diameter

1. Ridges in the periosteum creates a groove for periosteal blood vessel
2. Periosteal ridges fuse, forming an endosteum-lined funnel
3. Osteoblasts in the endosteum build new concentric lamellae inward toward the centre of the tunnel, forming a new osteon
4. Bone grows outward as osteoblasts in the periosteum build new circumferential lamellae. Osteon formation repeats as new periosteal ridges fold over blood vessels.