Tissue Structure

What is a tissue?

• A collection of similar cells that are grouped together in an organised manner to fulfil a particular function

• Structure and function are linked

What are tissues made from?

Cells in a tissue are arranged within an extracellular matrix which holds them in place, and forms the bulk of the tissue

What is the extracellular matrix?

• The ECM is a structure outside the cells

• Provides structural support

• Made of proteins

What is the function of the extracellular matrix?

• Supports cells

• Maintains tissue structure

• Provides strength and flexibility

How do extracellular matrix proteins vary between tissues?

The ECM proteins are shaped to match the function of the tissue:

• Strong fibers in bones for support

• Flexible fibers in skin for stretchability

Where do extracellular matrix proteins come from?

• ECM proteins are produced and modified by cells

• This allows tissues to adapt and maintain their properties.

Bones

What is the structure of long bones?

Contain 2 types of bone tissue:

1. Compact (cortical) bone » dense and forms a hollow cylinder

2. Cancellous bone » within the cortical bone, less dense, honeycomb appearance

• Contain cells

• Contain numerous canals in which blood vessels, lymph vessels and nerves are located

Bones

Why might bones contain 2 types of bone rather than being entirely compact bone?

If entirely compact, bones would be too heavy and too brittle

Bones

What components does the extracellular matrix contain and what function does it have?

• Contains collagen

  • A structural protein that provides flexibility and resistance to breaking

  • Collagen fibers are deposited by cells and form a framework that gives the bone strength

• Mineral component, mainly calcium phosphate

  • Makes bones rigid and hard

  • Acts as a reservoir for Ca2+ » so if you become calcium deficient, Ca2+ can leak out of bones

Bones

2 types of bone cells

• Osteoblasts

• Ostcleoblasts

Bones

Osteoblasts

• Form bone

• Secret collagen

• Organise the mineralisation process to give bones hardness

Bones

Osteoclasts

• Break down (resorb) bone to remodel it onto an optimal shape

• Maintain bone health

Bones

Why is the balance between osteoblasts and osteoclasts important?

• The balance maintains normal bone structure and function

• Allowing the bone to respond to growth, repair, and mechanical loading.

Bones

How does bone remodelling change during life?

• During growth (e.g., in children), osteoblast activity is higher to build bone

• The balance between osteoblasts and osteoclasts shifts with aging, hormonal changes, reduced activity, or conditions like osteoporosis.

Bones

What factors can disrupt the balance between osteoblasts and osteoclasts?

• Osteoporosis

• Reduced mechanical loading

• Hormonal changes

• Corticosteroid drugs

Bones

What cells does the bone marrow contain

• Stem cells

  • Hematopoietic stem cells » make leukocytes, erythrocytes and thrombocytes

  • Mesenchymal stem cells » multipotent

• Adipose tissue

Bones

What allows bones to flex at the joints?

• The ends of bones are coated with articular cartilage

• These are smooth and lack the mineral component

• Which allows bones to bend

Skeletal Muscle

Where are skeletal muscles located?

Skeletal muscles are attached to bones via tendons

Skeletal Muscle

Structure

• Skeletal muscles consist of bundles of myofibres

• Myofibres are long, thin, multinucleated cells

• They are packed together in a highly aligned manner with a supporting extracellular matrix

• Contain lots of mitochondria

• Myofibres contain myofibrils

Skeletal Muscle

What surrounds the myofibres?

• A network of collagen surrounds individual myofibres

• The collagen fibres gather together at the ends of the muscle to form tendons that join muscle to bone

Skeletal Muscle

Characteristics of tendons

• Mainly formed from collagen (plus some elastic fibres)

• High tensile strength

  » Made of densely packed collagen fibers

  » Which makes them resistant to stretching

• Variety of shapes and sizes depending on function and location

  » Short and thick (e.g., Achilles tendon in the ankle)

  » Long and thin (e.g., tendons in the fingers)

Skeletal Muscle

Explain how tendons facilitate movement of bone

• Tendons operate using a pulley system

• Tendons are attached to bones via a series of pulleys

• When you flex or bend your fingers, the tendons slide along the bone

• The pulleys act like loops through which the tendons pass, keeping them aligned with the bone

Skeletal Muscle

What happens if pulleys rupture?

• The tendons lose its guiding structure

• So they are unable to slide along the bone

Skeletal Muscle

What cells do tendons contain?

• Tenocyte cells

• Responsible for maintaining tendon structure

• Sparsely populated

• Limited blood supply

Skeletal Muscle

Why are tendons slow to heal?

Tenocyte cells have limited blood supply

Skeletal Muscle

Dinference between tendons and ligaments

Tendons: connect muscle to bone

Ligaments: connect bone to bone

Musculoskeletal System

How do muscles work?

• Muscles can only contract and relax

• This means muscles always work in antagonistic pairs where one muscle contracts (flexor) and the other muscle relaxes (extensor)

Musculoskeletal System

2 main types of muscles

1. Flexors » bend the joint e.g. biceps in the arm

2. Extensors » straighten the joint e.g. triceps in the arm

Musculoskeletal System

Explain how the arm bends and straightens

• Flexion (bending): biceps contract, triceps relax

• Extension (straightening): triceps contract, biceps relax

Musculoskeletal System

Why are there different classes of levers in the body?

Depending on the position of the load, effort and fulcrum

Musculoskeletal System

Give examples of the different classes of levers in the body

Class 1 Lever:

• Head / Neck

• Fulcrum in the middle, effort and load on either side (like a seesaw)

Class 2 Lever:

• Foot / Ankle

• Load is between the effort and the fulcrum

Class 3 Lever:

• Arm / Elbow

• Effort is between load and fulcrum

Skin

2 parts of the skin

• Epidermis » contains flat, thin, dead keratinocytes which are full of keratin

• Dermis » contains collagen, elastic fibres with fibroblasts, capillaries, nerve endings, sweat glands, hair

Blood Vessels (Capillaries)

Structure

• Formed from flat endothelial cells

Blood Vessels (Capillaries)

Function

• Network throughout tissues, providing nutrients and oxygen which diffuse out of blood and into tissues

Blood Vessels (Capillaries)

How much tissue fluid drains into the lymphatic system

10%

What is the difference between tissue repair and tissue regeneration?

Repair: problem is fixed but the tissue cannot go back to its original state

Regeneration: tissue is returned to its original state

How does the skin repair itself e.g. from a cut or wound?

1. Blood from damaged vessels fills the wound and clots to stop bleeding.

2. The clot is comprised mesh of fibrin fibers (formed from circulating fibrinogen) which creates a temporary barrier.

3. White blood cells migrate to the wound site to prevent infection, causing inflammation

4. Fibrin scaffold gets infiltrated with blood vessels and fibroblasts which produce new extracellular matrix proteins

5. Healed skin may have a scarred appearance due to the excess deposition of collagen (fibrosis).

How does the bone repair itself?

1. Blood from broken vessels forms a clot, creating a fibrin scaffold.

2. This scaffold supports the migration of stem cells and osteoblasts to form new bone

3. The fibrin scaffold is gradually replaced with collagen, which acts as a framework for new bone growth.

4. Gradually, the newly formed bone extracellular matrix (ECM) completely replaces the fibrin scaffold

5. The ECM becomes mineralized, giving it strength.

6. Over time, the new bone undergoes continuous natural remodeling, strengthening it.

7. The healed bone can look and function like undamaged bone.

Difference between skin repair and bone repair

• Skin repair often leaves a scar due to fibrosis.

• Bone repair can fully restore structure and function due to continuous remodeling.

Explain how muscles repair themselves

1. When muscle tissue is damaged, a fibrin scaffold forms at the injury site.

2. The fibrin scaffold is quickly populated by capillaries (to supply oxygen and nutrients), and fibroblasts (cells that help form connective tissue).

3. Stem cells present in the muscle tissue multiply and differentiate into myoblasts (muscle precursor cells).

4. These new myoblasts can either:

   • Fuse with the damaged myofibres to repair them.

   • Merge with other myoblasts to form new myofibres, helping to rebuild the muscle.

5. Once the new or repaired myofibres are formed, they integrate into the existing muscle structure.

6. The temporary fibrin scaffold is then replaced by collagen and other extracellular matrix (ECM) molecules

Explain how tendons repair themselves

1. A fibrin scaffold forms at the site of tendon injury. This scaffold supports the growth of tiny capillaries and fibroblasts, which are involved in repairing connective tissue.

2. Over time, the fibrin scaffold is replaced by collagen, the primary structural protein in tendons

3. The collagen gradually is remodelled to become strong and aligned

4. In some cases, fibrosis (excessive scar tissue formation) can occur. This can lead to the development of adhesions, (e.g. between tendon, pulley and bone) which can limit movement and function.

Why is tendon repair a slow process?

Replacing fibrin with collagen takes longer than in muscles because tendons have a poor blood supply, slowing the delivery of necessary nutrients and removal of waste.

How do nerves become damaged?

• If the injury cuts the axon, then the distal part of the axon (on the far side) degenerates

• This means the person loses any motor/sensory function associated with that nerve

How do nerves repair themselves?

1. Debris of degenerated axon are cleared away by macrophages (which cause inflammation)and Schwann cells

2. Schwann cells form a pathway for axon regeneration

3. Axons begin to regrow towards their target tissues

What is the creation of new blood vessels called?

Angiogenesis

When is angiogenesis important?

• Embryonic develop,ent

• Wound healing

• Female reproductive cycle

In what conditions does excessive angiogenesis occur?

• Cancer

• Age-related macular degeneration

• Psoriasis

• Endometriosis

In what conditions is there insufficient angiogenesis?

• Chronic wounds

• Ischaemic heart diseases

What is the predominant ECM protein in most tissues?

Collagen

What is seen in this picture?

Collagen » looks stripy to the eye

What is tropocollagen

• Made of 3 polypeptide alpha chains

• Each chain has a regularly repeating amino acid motif Gly-X-Y

• X and Y can be any amino acid but mostly proline and hydroxyproline