Cartilage and Bone: Development, Growth, Repair

Question 1

Name 3 types of cartilage

Answer 1

Hyaline

Elastic

Fibrocartilage

Question 2

What are the four main types of tissue in the body?

Answer 2

Epithelium

Connective tissue

Muscle tissue

Nervous tissue

[Each has subdivisions]

Question 3

What are the major functions of connective tissue?

Answer 3

Binding and structural support

Protection

Transport

Insulation

Question 4

Describe the features of fibrous connective tissue.

Where is it found?

Answer 4

Fibrous tissue is made up mainly of closely packed bundles of collagen fibres with little matrix.

Forms ligaments which bind bones together. Forms the periosteum of bone. Forms fascia and tendons

Question 5

Describe the features of elastic connective tissue.

Where is it found?

Answer 5

Matrix of elastic connective tissue consists of elastic fibres secreted by fibroblasts. Capable of considerable extension and recoil.Form a weblike mesh within lacunae. Covered by perichondrium.

Found in organs where stretching or alteration of shape is required - large blood vessel walls, trachea, bronchi, lungs

Question 6

Describe the features of hyaline cartilage

Answer 6

Smooth tissue, clear, glassy tissue.

Made up of type II collagen

Chondrocytes are in small groups with cell nests, the matrix is solid and smooth. Usually covered by perichondrium

Provides flexibility, support and smooth surfaces for movement at the joints.

Forms the cartilage framework during endochondral ossification

Question 7

Describe the features of fibrocartilage

Answer 7

Dense white mass of collagen fibres with widely dispersed cells arranged in parralel.

Type I collagen

Tough flexible supporting tissue. Resists compression and absorbs shock in some joints.

Question 8

Describe the features of elastic cartilage

Answer 8

Consists of yellow elastic fibres lying in a solid matrix. Chondrocytes lie between the fibres

Type II collagen and elastin

Provides support and maintains shape.

Question 9

What type of cartilage makes up the IV discs?

Answer 9

Fibrocartilage

Question 10

What type of cartilage forng long bones and costal cartilage?

Answer 10

Hyaline cartilage

Question 11

Where would you find elastic cartilage?

Answer 11

lobe of the ear, epiglottis, tunica media of blood vessels (arteries)

Question 12

What are the functions of bone?

Answer 12

Support: provides the framework of the body

Protection: forms boundaries to protect viscera

Movement: forms joints moved by muscles

Haematopoiesis: cells produced in the bone marrow

Mineral homeostasis: storage of calcium phosphate, maintains calcium levels

Question 13

Describe the microscopic structure of bone

Answer 13

Bone is a type of conenctive tissue

Composed of an inorganic matrix (hydroxyapatite) which gives the bone rigidity

Osteoid, composed of type I collagen gives bone flexibility and resilience

Question 14

Osteoblasts

Answer 14

Derived from osteoprogenitor cells.

Found in deeper layers of the periosteum, centres of ossification in immature bone, epiphyseal growth plates and at the site of fractures.

Synthesize organic bone matrix (osteoid) which contains type 1 colagen, proteoglycans and glycoproteins

Alkaline phosphatase and osteocalcin is secreted to aid mineralisation of the ECM.

Become trapped within the newly formed bone and become osteocytes

Question 15

Osteocytes

Answer 15

Mature bone derived from osteoblasts.

Encased in lacunae within the bone matrix, and are interconnected by dendritic processes that pass through canaliculi in the bone.

Monitor and maintain bone tissue by releasing signals in response to stress that directs osteoblasts to form bone. Also plays a role in calcium homeostasis

Question 16

Ostoclasts

Answer 16

Derived from monocyte precursors.

Multinucleated cells which lie on bone surfaces and release enzymes and acid which resorb bone. This formes resporption craters (Howship’s lacunae)

Found under the periosteum to maintain the shape of bones during drowth and remove excess callus formed in the healing of fractures

Found i the walls of the medullary cavity during growth and canalise the callus during healing.

Fine balance of osteoclastic and osteoblastic activity requierd for normal bone structure and function.

Question 17

Describe the structure of compact bone

Answer 17

Makes up about 80% of the bone mass. Has a series of Haversian systems (osteons). These are made up concentric lamellae of bone around a central canal that contains blood vessels.

Osteons aligned in the direction force is applied to the bone which gives the bone strength.

Space between Haversian systems is filled with bony lamellae (interstitial lamellae).

Outer circumfrential lamellae extends around the bone

Question 18

Describe the structure of the haversian canal system

Answer 18

Central canal contains nerves, lymphatics and blood vessels. Each canal is linked with its neighbouring canal by Volkmans canals which run transversely across the bone.

Lamellae: cyllindrical plates of bone arranged around each central canal are oriented perpendicularly.

Small cavities lie between lamellae (lacunae) which communicate with each other by canaliculi - these are tiny channels that allow the circulation of interstitial fluid in the bone.

Interstitial lamellae lie between haversian canals, and are the result of bone remodelling.

Question 19

Describe the structure of trabeular bone

Answer 19

Trabecullar (spongy) bone consists of a framework of lamellae and osteocytes interconnected by cannaliculi.

Imparts strength to the bone while adding minimum weight. Trabeculae develop along the bone’s line of stress

Spaces between the trabeulae contain red marrow.

Question 20

Factors that delay the healing of fractures

Answer 20

Tissue fragments/Splinters of dead bone not removed by phagocytosis

Deficient blood supply. Delays growth of granulation tissue, decreases the number of osteoblasts, favours chondrocyte development from osteoprogenitor cells.

Poor alignment. Can result in formation of a large irregular callus that heals slowly

Continued mobility. Results in fibrous granulation tissue followed by fibrous union of the fracture

Healing also affected by: age, infection, malnutrition, drugs

Question 21

Describe the features of cartilage

Answer 21

Supportive connective tissue with a flexible matrix

Chondroblasts (precursor cells) secrete an matrix of collagen (sometimes with elastin) and surround themselves within it until they become trapped in lacunae (small cavities), and become chondrocytes.

Cartilage is avascular, nutrition and waste removal is dependent on simple diffusion through the matrix.

ECM contains collaged and elastin fibres, and ground substance which contains water soluble molecules e.g. proteoglycans

A fibrocellular covering (perichondrium) covers the surface of cartilage, and is a source of new chondroblasts.

Question 22

Factors that improve the rate of union of the bone

Answer 22

Stability of the fracture

Adequate blood supply

Apposition of bone ends

Question 23

What are the two processes of bone development?

Answer 23

Endochondral ossification: hyaline cartilage forms an initial model of the future bone from mesenchymal cells that differentiate into chondroblasts. Once the hyaline model has formed, osteoblasts replace the cartilage with bone matrix which is then ossified. Mostly seen in long bones of the arms and legs

Intramembranous ossification: bone forms directly in the condensed mesenchymal cells. Tends to be found in flat bones of the skull, lower jaw and scapula

Question 24

What is the difference between primary and secondary ossification centres?

Answer 24

Primary ossification forms the diaphysis of long bones. It is composed on an outer core of compact bone lined with spongy bone surrounding a medullary cavity filled with bone marrow.

Secondary ossification centres form the epiphyses at the ends of long bones. Spongy bone remains in the interior of the epiphyses and bone grows from the centre to the outer surface.

Question 25

What are the four zones in the epiphyseal growth plate?

Answer 25

zone 1: resting cartilage. Close to the epiphysis. Contains small chondryocytes that anchor the growth plate to the bone, provide nutrients for the developing cartilage cells and store materials (lipids glycogen proteoglycans) required for growth.

zone 2: proliferating cartilage. chondrocytes are dividing to replace those dying at the diaphyseal side of the growth plate. They produce bone matrix and are responsible for longitudinal grwoth

zone 3: hypertrophic cartilage. Chondrocytes hypertrophy, lyse and deteriorate, promoting calcification of the matrix. Ths is necessary for invasion of metaphyseal blood vessels and the formation of bone. No growht occurs at this layer

zone 4: calcified cartilge. Composed of chondrocytes surrounded by a calcified matrix. This is removed by osteoclasts and replaced with bone by osteoblasts

Question 26

What stimulates the epiphyseal growth plate?

Answer 26

Growth hormone.

Stimulates zone 2 (proliferating cartilage). When the levels of Gh begin ot decline, chondrocytes stop dividing so the growth plate gets thinner and begins to close

Question 27

Why can a fracture at the epiphysis cause stunted growth?

Answer 27

If a fracture damages the epiphysea growth plate while it is still open, the fractured bone will be shorter than normal because the growth plate is an avascular structure and damage accelerates closure of the plate, so growth is reduced.

If the rate of bone formation is reduced then the affected bonew ill be shorter and may cause misalignment of joint surfaces, casing shorter stature.

Question 28

Types of bone

Answer 28

Mature lamellar bone: present in the adult. Has orderly layered arrangement of collagen fibres which makes it strong

Immature woven bone: found mainly in the foetus and in fracture healing or excessively rapid bone remodelling. Can be produced quickly however collagen fibres are disorganised and it is weaker than lamellar bone.

Question 29

Briefly describe how bone resorption is controlled

Answer 29

Bone remodelling is controlled by a balance of osteoclast and osteoblast activity. This is regulated by hormones (PTH, oestrogen), cytokines and growth factors.

Control of osteoclast activity involves RANK, RANKL and OPG.

RANK is a transmembrane receptor expressed on osteoclast precursors. RANKL is expressed on osteoblasts and marrow stromal cells, and OPG is a decoy receptor made by osteoblasts that blocks RANKL.

When RANK binds to RANKL signalling activates transcription of NFKB which is essential for osteoclast survival.

The OPG:RANKL ratio affects bone formation and resorption. Levels controlled by systemic factors.

Question 30

Describe the process of bone remodelling

Answer 30

Osteoclast precursors are stimulated to differentiate by PTH, ILs and OB activity.

Resorption of bone by OCs matches the formation of new bone by OBs. OCs attach to the surface of old bone that is about to be resorbed and secretes collagenases and lysosomal enzymes and acid that degrade and dissolve the underlying bone into proteins and minerals. These are taken up by OCs and secreted into the extracellular space.

Osteoblast precursors are stimulate to differentiate by cytokines and hormones. Mature OBs migrate to the space prepared by OCs. They synthesise Type 1 collagen, osteocalcin and proteoglycans. The osteoid matrix is coated in calcium salts and hardens. As OBs become embedded in the mineralising matrix they become osteocytes.

Whole process takes 160-200 days.

Question 31

Types of bone marrow

Answer 31

Red marrow: sites of active haematopoiesis. Found in medullar cavity of vertebrae, ribs, sternum and pelvis

Yellow marrow: more adipose tissue, less haematopoeisis. Retains ability to revert to red marrow if required.

Question 32

How does PTH increase serum calcium levels?

Answer 32

Increases bone resoprtion by OCs to release calcium

Increases production of active vitamin D to increase calcium absorption from the GIT

Increases calcium reabsorption by the kidney

Question 33

What is the embryonic origin of bone?

Answer 33

Paraxial mesoderm

Question 34

Epiphyseal growth place

Answer 34

Cartilage located between the primary and secondary ossification centres (metaphysis and epiphysis) that allows long bones to growth in length.

N.B. Bones grow in width by appositional growth.

Question 35

What are the steps in healing of a fracture?

Answer 35

1. Formation of a haematoma
2. Granulation tissue
3. Callus formation
4. Woven bone
5. Lamellar bone
6. Remodelling

Question 36

Describe the process of bone healing

Answer 36

1. Haematoma. Blood vessels that cross the fracture line are damaged and blood leaks out into the fracture site. Clot forms after 6-8hrs
2. Inflammatory reaction: Blood supply to bone cells on either side of the fracture is disrupted and they die. Dead cels induce macrophages and OCs to remove dead bone and cause localised swelling and inflammation
3. Granulation tissue: blood capillaries grow into the haematoma and organise into granulation tissue (procallus)
4. Callus: fibroblasts and osteoprogenitor cells from the periosteum and red marrow start to invade to procallus. Collagen is produced by fibroblasts, which become chondroblasts and connect the two ends of the fracture. Fibrocartilage produced by chondroblasts replaces the procallus.
5. Bony callus: Cartilagenous components of the callus are replaced by bone through endochondral ossification. Osteogenic cells differentiate into OBs and begin to form new woven bone. The trabeculae join the living tissue on either side of the fracture. (transformation takes ~ 3 months)
6. Lamellar bone: The woven bone lattice is rearranged into lamellar bone.
7. Remodelling: Excessive callus resorbed by OCs and medullary cavity re-established

Question 37

Describe fracture healing in trabecular bone

Answer 37

Trabecullar bone is a sponge-like lattice with a surrounding shell of cortical bone.

It has a rich supply so there is little necrosis of the bone.

Fractures form endosteal callus rather than periosteal. Osteogenic cells from the endosterum produces woven bone which is replaced by lamellar bone via internal remodeling.

Question 38

Blood and nerve supply to bone

Answer 38

Periosteal arteries are accompanied by nerves and the enter the diaphysis through Volkmann’s canals. Supply periosteum and compact bone

Nutrient artery passes thought compact bone near the centre of the diaphysis and enters the medullary cavity. Divides into proximal and distal branches which supply the inner layers of compact bone and spongy bone.

Metaphyseal and epiphyseal arteries supply the ends of bone and arise from arteries tha supply the joint.

Nerves accompany the blood vessels of bone. Periosteum has a rich supply of sensory nerves which transmit pain.

Question 39

Description of fracture union

Answer 39

Malunion - fracture heals in an unsatisfactory position

Delayed union: fracture healing takes longer than expected

Non-union: fracture fails to unite and a fibrous union is formed. Causes pseudoarthritis.