Name 3 types of cartilage
What are the four main types of tissue in the body?
[Each has subdivisions]
What are the major functions of connective tissue?
Binding and structural support
Describe the features of fibrous connective tissue.
Where is it found?
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
Describe the features of elastic connective tissue.
Where is it found?
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
Describe the features of hyaline cartilage
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
Describe the features of fibrocartilage
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.
Describe the features of elastic cartilage
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.
What type of cartilage makes up the IV discs?
What type of cartilage forng long bones and costal cartilage?
Where would you find elastic cartilage?
lobe of the ear, epiglottis, tunica media of blood vessels (arteries)
What are the functions of bone?
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
Describe the microscopic structure of bone
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
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
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
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.
Describe the structure of compact bone
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
Describe the structure of the haversian canal system
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.
Describe the structure of trabeular bone
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.
Factors that delay the healing of fractures
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
Describe the features of cartilage
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.
Factors that improve the rate of union of the bone
Stability of the fracture
Adequate blood supply
Apposition of bone ends
What are the two processes of bone development?
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
What is the difference between primary and secondary ossification centres?
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.
What are the four zones in the epiphyseal growth plate?
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
What stimulates the epiphyseal growth plate?
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
Why can a fracture at the epiphysis cause stunted growth?
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.
Types of bone
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.
Briefly describe how bone resorption is controlled
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.
Describe the process of bone remodelling
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.
Types of bone marrow
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.
How does PTH increase serum calcium levels?
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
What is the embryonic origin of bone?
Epiphyseal growth place
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.
What are the steps in healing of a fracture?
1. Formation of a haematoma
2. Granulation tissue
3. Callus formation
4. Woven bone
5. Lamellar bone
Describe the process of bone healing
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
Describe fracture healing in trabecular bone
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.
Blood and nerve supply to bone
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.
Description of fracture union
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.