L6- Bone physiology

What is the functions of the skeleton?

• Framework and levers

• Protection

• Mineral storage

• Fat storage

• Production of blood cells

How is compact bone organised?

Into osteons (also called Haversion system)

Describe the structure of the osteoid (extracellular matrix) of bone

• Collagen type 1 fibres

• Glycosaminoglycans (GAGs)

• Mineral salts

Bone cells (osteocytes). Lacuna is the small space containing the osteocyte. Extracellular matrix surrounds the lacuna

In compact bone, matrix arranged in lamellae (form osteon around central canal)

What is the function of collagen type 1?

Fibre bundles give tensile strength

Classify these bones and give an example of where they are found in the body

1. Long bone- limbs (e.g femur)

2. Flat bones- skull/ scapula

3. Irregular bones- Vertebrae

What are the 4 layers to a bone?

• Periosteum- fibrous layer covering outer surface of bone

• Compact bone (cortical bone)- Dense outer bone, made of osteons (haversian system)

• Spongy bone (trabecular/ cancellous)- contains trabeculae, spaces between trabeculae contain bone marrow

• Endosteum- lines surfaces, like trabeculae

• Bone marrow- occupies internal spaces within spongy bone

What runs through the central canal of an osteon?

Lymphatic vessels

What are osteoblasts and where are they found?

Found on bone surfaces, and produce bone matrix called osteoid (extracellular matrix)

When osteoid becomes mineralised it forms hard bone matrix

What is an osteocyte

Some osteoblasts become enclosed in the bone matrix they are forming, and become osteocytes. (Located in small spaces called lacunae)

Help maintain the surrounding bone matrix

What are canaliculi?

Small canals connecting adjacent lacunae (the small spaces where osteocytes are found)

What is the function of osteocytes?

Bone tissue is matrix is impermeable to nutrients, and so osteocytes allow communication and exchange of substances between cells

What is the function of osteoblasts?

Synthesize and secrete collagen and other components of the bone matrix

What are osteoclasts?

Breakdown bone, and help to regulate calcium levels

Release acids which dissolve calcium phosphate, digesting collagen and matrix proteins

What makes osteoclasts very active?

They are multinucleated

What happens as osteoclasts break down bone?

They leave behind mitogens (growth factors), which encourage osteoblasts to mature- show where bone needs to be built back up

What is osteopetrosis?

When the rate of bone breakdown and build up is not at the same rate, and osteoblast activity is too high. Dense bones

What is osteoperosis?

When the rate of bone breakdown and build up is not at the same rate, and osteoclast activity is too high. Weakened bones

What supplies the inner 2/3 of bone with blood?

Endosteum

What supplies the outer 1/3 of bone with blood, and how?

Periosteum vessels, via attachments pf muscles and tendons

Describe endochondral ossification

1. Bones develop from hyaline cartilage, lined with connective tissue sheath (perichondrium)

2. Ossification begins in the middle of long bones, starting with growth of blood vessels into perichondrium

3. This vascularisation stimulates osteoprogenitor cells to differentiate into osteoblasts

4. Osteoblasts produce osteoid, which are ossified and become bone collar

5. Bone cavity forms in centre of long bones- hypertrophic chondrocytes release alkaline phosphatase, precipitating calcium phosphate crystals (calcification)

6. Blood vessels infilitrate calcified matrix, and calcified cartilage matrix is replaced by bone

7. Ossification progresses towards ends of long bones as osteoclasts break down tissue in the middle of the bone to form the marrow cavity

Where does ossification start?

The middle of long bones

What determines how long the bone can grow?

Growth plates

What happens in the zone of proliferation?

Where chondrocytes multiply

What happens in the zone of hypertrophy and calcification?

Where chondrocytes get bigger and the matrix begins to mineralise

What happens in the zone of cartilage erosion?

Where the matrix is removed by chondroclasts and blood vessels invade

What happens in the zone of endochondral ossification?

Where the bone replaces the removed cartilage and bone elongation occurs

What is intermembranous ossification, and describe the stages

When bone is formed inside a membrane by osteoblasts

1. Cells gather and become osteoblasts

2. Osteoblasts make osteoid, which hardens and becomes bone

3. Some osteoblasts become osteocytes (bone cells)

4. This becomes spongy bone, with compact bone around the outside

Which type of bones does endochondral ossification for, and intermembranous ossification?

Endochondral- long bones

Intermembranous- Flat bones

Describe how bone remodels in response to loading

Osteocytes detect mechanical loading in the bone

Trabeculae become arranged in the direction of the greatest applied forces, making the bone better suited to withstand loading.