5.2 Connective and Skeletal tissue Flashcards
What makes up the connective tissue that underlies epithelia?
Cells, fibres and macromolecules:
- Macrophages
- Fibroblasts
- Mast cells
- Collagen fibres
- Fibrillin fibres
- Elastin fibres
- Hyaluronan/hyaluronic acid
- Proteoglycans
- Glycoproteins
- Glycosaminoglycans (GAGs)
What other structures may also be present in connective tissue?
Capillaries
What is the general function of resident cells within the extracellular matrix/connective tissue?
They synthesise more of the extracellular matrix (e.g. fibroblasts)
What is the name for cells that are not resident but can still be present within the ECM/connective tissue?
Migratory cells, each has its own specific function
What is the general purpose of the fibres?
Tensile properties, anchorage and support of the tissue
What are the mechanical functions of connective tissue?
- Supporting matrix for tissues above and below
- Transmission of any forces
- Facilitation of movement (more motile and adaptable than other tissues)
What are the metabolic functions of connective tissue?
- Allow a route for exchange between blood and tissues
- Can act as a fat storage (adipose tissue is considered a part of this tissue)
What are the exact defence and repair functions of connective tissue?
- Insulation
- Cushioning and padding
- Protection from injury and infection, allows repair
What are the growth and morphogenesis related functions of connective tissue?
- Control cell behaviour
- Proliferation and growth
- Growth factor gradients
What are the different types of connective tissue?
- Loose connective tissue (areolar)
- Two types of dense connective tissue
- > dense regular connective tissue
- > dense irregular connective tissue
- Elastic tissue
- Adipose (fat) tissue
- > White adipose
- > Brown adipose (different functions and frequencies)
What are the features of loose/areolar connective tissue?
- Component of most parts of the body
- Delicate, flexible, well-vascularised, NOT very resistant to stress
- Supports structures that are under pressure but low friction
- A good shock absorber, relatively ‘squishable’
Where can loose/areolar connective tissue be found?
Filling space:
- Between muscle cells
- Supporting epithelial tissue (under almost all epithelial layers)
- Sheaths lymphatic and blood vessels
- In the hypodermis of skin (along with dense connective tissue)
What cells are present within loose/areolar connective tissue?
Numerous fibroblasts and macrophages
What is the function of fibroblasts?
They lay down components of the extra-cellular matrix (ECM)
What is the function of macrophages?
Large white blood cells that carry out phagocytosis, finding foreign material and ingesting it as part of the immune response
What proportion of and what fibres are present in loose/areolar connective tissue?
Moderate amounts of:
- Collagen
- Elastin
- Reticular (type III collagen)
What are some features and the function of dense connective tissue?
- Provides resistance and protection
- Fewer cells, predominance of collagen fibres
- Less flexible, more resistant to stress
What defines dense regular connective tissue?
Collagen fibres are aligned with linear orientation of fibroblasts along lines of prolonged stress in one particular direction
What is an example of dense regular connective tissue?
Tendons that attach striated muscles to bone - very high tensile stress due to bundles, allows muscles to remain attached to the bones
What defines dense irregular connective tissue?
Collagen and elastin fibres are collected in bundles with no predominant orientation, the 3D network forms a resistance to stress in all directions
Where can dense irregular connective tissue be found?
In the superficial dermis of the skin, associated with loose connective tissue in the same area
What is the function of dense irregular connective tissue in the skin?
Anchors the epithelium
What is the function of loose/areolar connective tissue in the skin?
Lies deeper than the dense, allowing skin to move over underlying muscle. Deposits of adipose tissue can also be found within/throughout this layer
What connective tissues are found within the skin?
- Dense irregular connective tissue
- Loose/areolar connective tissue
- Adipose connective tissue (there are areas of fat within the loose layer)
What are adipocytes?
Cells that are involved in fat storage, and come in two types; white and brown
What are the features of ‘white’ adipose tissue?
- Tissue is mostly made up of mature adipocytes which have a single, large droplet of triglyceride fat and very little cytoplasm (all organelles are pushed up against the cell membrane)
- Immature cells have smaller lipid droplets
What is the function of white adipose tissue?
- Energy storage
- Insulation
- Padding
What are the features of brown adipocytes?
- More cytoplasm
- Many mitochondria
- Multiple small fat droplets
- Appears brown due to good blood supply
This type of tissue is seen in young/new-born children and some hibernating animals
What is the function of brown adipose tissue?
Heat generation (they are innervated sympathetically)
What are the features of elastic tissue?
- Bundles of thick parallel elastic fibres
- Fibres of elastin are associated with a glycoprotein
- This tissue can stretch but will then return to its original shape
Where can elastic tissues be found?
- Lining arteries (such as the aorta)
- In the (true) vocal cords
What is the basal lamina?
A layer of the basement membrane that is secreted by the epithelial layer (so therefore remains in contact with it)
What are some features of the basement membrane?
- Specialised sheets of extracellular matrix proteins and glycosaminoglycans (GAGs)
- Associated with epithelial layers, muscle cells, glands and blood vessels
What is the basement membrane?
A thin, highly specialised layer of the extracellular matrix that lies between/connects more external tissue to the connective tissues lying underneath
What are the functions of the basement membrane?
- Cell adhesion
- Acts as a diffusion barrier
- Regulates cell organisation
- What are some methods through which the basement membrane can be isolated under a microscope?
- Immunohistochemistry, where specific labelled antibodies are used to stain target molecules and tissues (immunological staining)
- Use of Periodic acid-Schiff (PAS) staining, allows complex carbohydrates to be viewed
What are some of the protein components of the basement membrane?
- Laminin (connects cells to connective tissue)
- Type IV collagen (only exists in basement membrane structures)
What are the layers of the basement membrane?
- Lamina lucida
- Lamina densa
(- Basal lamina) - Fibroreticular lamina
How are cells anchored to the basement membrane?
- Hemidesmosomes (intermediate filaments)
- Actin-linked cell-matrix adhesion anchors
How does the basement membrane act as a diffusion barrier?
- Can act as a molecular sieve or permeability barrier
- Pore size depends on GAGs
Example: In kidney, prevents protein loss from filtered blood
How does the basement membrane regulate cell growth, organisation and differentiation?
Through cell surface receptors and the presence of the ECM
What are the migratory cells of connective tissue?
- Macrophages
- Mast cells
- Leukocytes
What are the resident cells of connective tissue?
Fibroblasts
What are some features of fibroblasts?
- Elongated oval/fusiform nuclei
- Cytoplasm elongates along the lines of the fibres that they produce
- Can synthesise collagen, which provides the strength in the ECM
- Will lay down their fibres (e.g. collagen) in a particular direction
What molecules can fibroblasts secrete?
All make up the ECM:
- Collagen
- Elastin
- Glycosaminoglycans (GAGs)
- Proteoglycans
- Glycoproteins
- Growth factors (these regulate nearby tissues)
- How does collagen appear in the cornea?
- Regular layers
- Stacked orthogonally (‘criss cross’)
- Gives translucency to the tissue
- Corneal fibroblasts are also known as keratocytes
What are the features of macrophages?
- Lots of electron dense, membrane bound lysosomes
- Large nuclei
- Ruffled edges
- Derived from blood monocytes
What is the function of a macrophage?
- Tissue phagocytes
- Engulf dead cells and invading organisms
- Both initiate and down-regulate the inflammatory response
- Recruit polymorphonuclear (lobed nucleus, cytoplasm contains granules) leukocytes to kill pathogens
- Where do macrophages have specialised roles?
- Liver (Kupffer cells, protect hepatocytes and engulf large molecules)
- CNS (microglia, clear cellular debris and dead neurons)
- Skin (Langerhans cells, immune sentinels, can migrate to the lymph system)
What are the features of mast cells?
- Contain many secretory granules filled with many different bioactive molecules
- Round nucleus
- Have IgE receptors on the cell surface
- Also able to phagocytose bacteria
- What can the granules within a mast cell contain?
- Histamine
- Serotonin
- Tumour necrosis factor
- Proteases
- Heparin
- Prostaglandins
What are the functions of mast cells?
- Activation of IgE receptors causes activation of mast cell and release of granules
- Once bacteria are phagocytosed, they release granules that will cause vasodilation, increase capillary permeability, breakdown the ECM and contract smooth muscle
- Able to attract other immune cells to its location
Involved in both the immune and the inflammatory response
What are leukocytes?
A sub-group of white blood cells, made up of 3 types:
- Basophils (release pharmacological compounds, e.g. histamines)
- Eosinophils (involved in allergic and vasoactive reactions, control mast cells and inflammation)
- Neutrophils (polymorph, involved in phagocytosis)
- How do polymorphs (e.g. neutrophils) migrate to an area of inflammation?
ICAM (intercellular adhesion molecule) binds to integrin on neutrophil membrane (also binds to eosinophils in asthma, lymphocytes in immunological disease and to viruses such as rhinovirus)
- What is integrin?
A glycoprotein expressed on some cell surfaces that can bind to ECM components
From what cell type are adipose cells derived from?
Fibroblast precursor cells - the fat cells develop as lipid droplets accumulate, fuse and enlarge
What is different about the action of mitochondria in brown adipose tissue?
When stimulated, they release heat rather than synthesis ATP - no cycling of protons, heat is released preferentially
By what nervous system is brown adipose tissue stimulated?
Autonomic innervation, sympathetic neurons
What are some features of the extracellular matrix (ECM)?
- More plentiful than the cells that it surrounds
- Determines a tissue’s physical properties
- MORE than a passive scaffold for physical support, allows active and complex regulation of the cells within it
What are the main components of the extracellular matrix (ECM)?
- Fibrous proteins (e.g. collagen)
- Glycosaminoglycans (GAGs, polysaccharide chains, highly hydrated, gel-like ‘ground’ substance)
What are the major connective tissue fibres?
- Collagen (multiple types, most abundant protein in the body, makes up 25% of body protein mass)
- Reticular fibres (very thin type III collagen)
- Elastic fibres (elastin and fibrillin)
- How many collagen genes are there in the human genome?
42
- How many different collagen proteins have been described?
27
- What are some examples of the different types of collagen, their individual properties and where they are found?
- Type I, fibrillar/fibril-forming. Found in bone, skin, ligaments, cornea, internal organs. Accounts for 90% of body collagen. Mutant phenotypes result in severe bone defects and fractures
- Type IV, network-forming, creates a sheet-like network. Found in the basal lamina. Mutant phenotypes result in kidney defects/diseases (glomerulonephritis) and deafness
- Type VII, network forming, but instead forms anchoring fibrils. Found beneath stratified squamous epithelia, mutant phenotypes result in skin blistering
- Type XVII, transmembrane, polymerised form is non-fibrillar. Found in hemidesmosomes, mutant phenotype causes skin blistering
What are some features of collagen molecules/fibres?
- String of repeated glycine-containing motifs
- Three collagen chains self-assemble to form collagen fibres
- Glycine is the only amino acid small enough to fit within the crowded interior of the triple helix
- Repeated motif of Gly-X-Y-Gly-X-Y- (X is typically proline, Y is typically hydroxyproline)
- Hydroxyproline allows formation of more hydrogen bonds
- Collagen molecules assemble end-to-end but slightly staggered, to form a fibril
- Fibrils self-assemble to form a banded collagen fibre
- What happens if there is a mutation in a single glycine codon?
Causes sharp angulation in the pro-collagen molecules, this reduces the ability of the molecule to work
How long is the average collagen fibre?
300nm
How much of a distance is between adjacent collagen molecules?
35nm
How much overlap is there between adjacent molecules?
67nm
What form of collagen is secreted from cells?
Procollagen - an immature form with blunt ends, as the full mature form is too potent to produce immediately within the cell. Instead, it is secreted and modified extracellularly (propeptides cleaved). The fibrils and fibres and then self-assembled
What are some specialisations of fibroblast cells?
- Developed rough ER and Golgi apparatus
- Many transfer and secretory vesicles
- Wide network of microtubule arrays
- Contains all the enzymes and factors necessary for fibre (e.g. collagen) production and secretion
What type of fibre are reticular fibres?
Type III collagen
What are the features and function of reticular fibres?
- Short, thin tubes
- Form the delicate meshwork holding tissue elements together
- Highly evident in haemopoietic tissue (tissue that generates blood cells), e.g. lymph nodes, spleen and bone marrow
- Stain positively using a silver stain
What are some features of elastin fibres?
- Made of an elastin core (coiled polypeptide chains of tropoelastin)
- These are strongly crosslinked to form a 3D meshwork
- On the surface are microfibrils of a glycoprotein known as fibrillin
- Elastin fibres are able to branch
- Able to stretch and then return to their initial position, giving elasticity and resilience to tissues, reversibly stretch and recoil
- Elastin molecules covalently cross-link to form fibres or sheets
- What is a disease caused by a mutation that weakens fibrillin microfibrils and what are the symptoms?
Marfan syndrome: Elastin fibres are weakened, causing:
- Flexible joints
- Heart valve and aorta defects (not enough elasticity in the aorta, so repeated pressure may cause it to split)
- The patient will be tall and thin in stature (President Abraham Lincoln has been theorised to suffer from this condition)
What form do elastin fibres have in a relaxed state?
Random coil structure
Where are elastic fibres found?
- Lungs
- Skin
- Blood vessel walls
What are glycosaminoglycans (GAGs)?
Very long unbranched polysaccharide chains that can be covalently linked to proteins
What charge do GAGs have?
Highly anionic/negatively charged, making them highly hydrophilic
What functions does the charge of GAG molecules allow?
- Negative charge attracts Na+ ions and water, as positively charged
- > this causes swelling/turgor, which can resist compression e.g. in joints
- Form porous hydrated gels
What are proteoglycans?
Proteins that are covalently attached to many repeating units of carbohydrates - in the ECM, these will be GAGs
What is the difference between a proteoglycan and a glycoprotein?
- A proteoglycan has (often many) repeat units of a carbohydrate covalently attached to the main protein
- A glycoprotein only has a short, non-repeating glycan chain covalently attached to the protein
What are the four main types of GAG?
- Hyaluronic acid
- Chondroitin sulphate/Dermatan sulphate
- Heparan sulphate/Heparin
- Keratan sulphate
Which GAG isn’t protein linked or sulphated?
Hyaluronic acid
What are the function of proteoglycans?
- They form gels of varying pore size (huge range in size of actual molecule), act as molecular sieves
- Resist mechanical compression forces, act as a lubricant
What are the functions of connective tissue glycoproteins?
To connect together the different structural systems, linking them together or to the cell itself - most bind to integrins
What are two examples of glycoproteins in connective tissue?
- Fibronectin
- Laminin
- Why are RDG (arg-gly-asp) peptides, found in connective tissue glycoproteins, a useful treatment to prevent metastasis?
This region of the glycoprotein binds to integrins, securing the structural matrices to the cell and improving stability/cell-ECM junctions, this will prevent cells from splitting off and metastasising in cancer
What is the function of integrin receptors?
Lie in the plasma membrane and link the cytoskeleton to extracellular glycoproteins and collagen - holds cells on to the connective tissue
How are cells able to move through the ECM?
Through progressive binding of integrins to extracellular matric proteins
What are the four types of extracellular signalling molecules?
Long range:
- Chemoattraction (e.g. netrins), positive response
- Chemorepulsion (e.g. semaphorins), negative response
Short range:
- Contact attraction (e.g. Eph ligands), positive response
- Contact repulsion (e.g. cadherins), negative response
Is connective tissue static?
No, it is dynamic!
What is the structure and function of a ligament?
- Dense regular connective tissue
- Hold bones in place, provide stability, prevent dislocation
What are the structure and function of aponeuroses?
- Dense regular fibrous connective tissue, in delicate sheets (although similar structure to tendons and ligaments)
- Provide an attachment point for muscles to attach to bone, much like tendons, as well as binding muscles together, enveloping muscles and organs and binding muscles to other tissues
What is the structure and function of fascia?
- Dense regular fibrous connective tissue
Three types: - Superficial fascia, associates with skin, soft medium for nerves and vessels, conserves temperature
- Deep fascia, associated with bones, muscles, nerves and blood vessels, supports and protects soft tissues and acts as a barrier against infection from elsewhere
- Visceral fascia, associated with the visceral organs, suspends and protects these internal organs
What type of connective tissues are cartilage and bone?
Both are dense connective tissues
What is the definition of a connective tissue?
Cells embedded in an extracellular matrix
What are the three types of cartilage?
- Hyaline
- Fibrocartilage
- Elastic
What is the difference between the matrix of bone and of cartilage?
In bone, the matrix is rigid
In cartilage, the matrix is somewhat more flexible
What are the different types of bone?
- Lamellar (mature)
- > Compact/cortical
- > spongy/trabecular/cancellous
- Woven (immature)
Where can hyaline cartilage be found?
- Template for the formation of a bony skeleton during development
- Line the articulating surfaces in synovial joints
- Provide a flexible skeleton for parts of the rib cage, trachea, bronchi and nose
Where can fibrocartilage be found?
- Intervertebral discs
- Intra-articular discs in synovial joints
- Tendon attachments to bone, ‘enthesis’
Where can elastic cartilage be found?
- External ear
- Epiglottis
- Vocal cords
Where can compact bone be found?
- Outer ‘shell’ of all bone
- Form strong cylinders around the shafts of long bones
Where can trabecular/cancellous bone be found?
- Internal supporting plates/struts of bones, criss-cross pattern
- Common in epiphyses
What defines lamellar/Haversian bone and where can it be found?
- Defined by the organisational structure of ‘osteons’
- Can be found in the shafts of bones
Where can woven bone be found?
Temporary form of bone, so is found during:
- Development
- Repair
Which method of imaging can be used to view hyaline cartilage?
MRI scans - it CANNOT be viewed using radiograph or CT scans
What is the main function of hyaline cartilage?
- To reduce friction at synovial joints by lining the articulating surfaces within the joint capsule
- Surface chondrocytes secrete synovial fluid, which again reduces friction and also pressure
What are the two ways by which cartilage grows?
Appositional growth - new cartilage is added to the surface of older cartilage by chondroblasts from the deep layer of the perichondrium, which take on more chrondrocytic activity. Grows in thickness.
Interstitial growth - new cartilage is formed within older cartilage by chondrocytes that divide and form a new matrix. Grows in length, can only occur in bones for as long as the epiphyseal plate is open.
What is the perichondrium?
The outside layer of undifferentiated cells (but will be instructed to differentiate into chondroblasts and then chondrocytes as they ae moved down the structure) in a cartilaginous structure, envelops the rest of the cartilage when it is not at a joint
What is a lacuna?
The hole in the extracellular matrix where chondrocyte cells are sitting and secreting/laying down more of the extracellular matrix
Where do the chondrocytes lie in cartilage and what is a unique feature about them?
The lie deeply embedded into the extracellular matrix, and are one of the few cells that are able to divide (although not all do) once fully differentiated, so groups of one, two or potentially 4 can be seen in the same lacunae deep in the ECM
What is the general structure of cartilage?
Chondrocytes embedded in a very well hydrated matrix
How do nutrients permeate/reach cells throughput cartilage?
- No vessels or capillaries within the structure
- This means that nutrients have to reach the entire tissue through diffusion
It has been proposed that a failure to provide these nutrients could result in arthritis, but that has yet to be proven
What are the key molecules in hyaline cartilage?
- Type II collagen
- Proteoglycans with glycosaminoglycans (GAGs), e.g. chondroitin sulphate, keratan sulphate, hyaluronic acid
What features of the matrix aid its function?
- Charged and well hydrated
- > Allows for good shock absorption
- Collagen fibrils add tensile strength
Draw out the structure of a large proteoglycan (pg4 of Clive’s cartilage and bone lecture)
DO IT!!
What is the role of cartilage in long bone growth?
The cartilage ossifies on one side (diaphysis/shaft side) and forms new cartilage on the other (epiphysis/head side), allowing the elongation of the bone
What are the names of the different stages of cartilage going from the epiphyseal to the diaphyseal plates?
- Resting zone (progenitor cells, irregularly arranged, attaches epiphyseal bone here)
- Proliferative zone (rapid replication, form stacks in parallel to growth of bone, become chondrocytes)
- Hypertrophic zone (cells increase massively in size)
- Ossification zone (closest to diaphyseal plate, chondrocytes die and are ossified, diaphysis grows in length)
How does fibrocartilage composition differ to that of hyaline cartilage?
Fibrocartilage has a lot more type I collagen
What is the structure of an intervertebral disc?
Inner nucleus pulposus, outer fibrocartilage layer (known as annulus fibrosus)
- Layer is thinner at the bine surface
- Layer is thicker at the periphery
Function: provides the connection to the bone
- What can happen if the nucleus pulposus is able to break out of the fibrocartilage/annulus pulposus?
Intervertebral disc herniation:
- Nucleus pulposus ‘leaks’ and is squeezed out from between vertebrae, meaning that it could then impinge upon nerve roots emerging from the spine, causing inflammation and pain
What is the identifying feature of elastic cartilage?
Matrix is permeated with elastin fibres
What stain can be used to view elastin fibres?
Orcein stain
What are the functions of the skeleton?
- Support
- Protection of other organs (skull, ribcage)
- Levers of motion
- > These three functions are allowed due to bone’s ability to be resistant to tensile and compressive forces
- Reservoir of calcium
- Repository of bone marrow (store of stem cells)
What are the main cell types?
- Osteoblasts
- Osteocytes
- Osteoclasts
What are the two types of bone formation during development?
- Intramembranous ossification
- Endochondral ossification
Give an example of where intramembranous ossification occurs?
Primarily in the skull
What is the process of intramembranous ossification?
- Osteogenic progenitor (less potent than stem cells) cells differentiate in mesenchyme (multipotent stromal stem cells)
- Progenitor cells differentiate into osteoblasts, initial formation of bone spicule (small cells laid down in initial formation of the new bone matrix)
- Spicules enlarge, osteocytes are trapped in the matrix and invasion of blood vessels occurs
- Spicules fuse to form a plate of bone, osteoclastic remodelling starts (change from weaker woven bone to more permanent structures)
What is the process of endochondral ossification?
- Chondroblasts differentiate from the mesenchyme, laying down ECM to form cartilaginous ‘ghost’ of the bone, early perichondrium is also formed
- Osteocytes and osteoblasts lay down ossified layers in the diaphysis, forming the bony collar. Periosteum also develops
- Blood vessels invade, primary ossification in diaphyseal plate starts, epiphyseal plate is still developing
- Trabeculae form in the diaphysis
- Compact and trabecular bone are formed from woven bone during ossification
- Epiphysis starts to ossify at a much later point in development than the diaphysis
- Growth plates allow elongation of long bones due to osteoblasts are instructed to lay down woven bone at the edge of ossified tissue
What are osteons?
- Circular structures in adult (lamellar/Haversian) bone, Haversian canal in the centre contains a blood vessel
What is the periosteum?
A dense layer of vascular connective tissue that envelopes the bones at everywhere except for the joints
What is a feature of trabecular bone?
Components are aligned to be strong but lightweight
How are osteocytes linked to each other and surface osteoblasts?
Through fine processes/cell projections known as canaliculi and through gap junctions (allows transfer of nutrients and signalling molecules/communication)
How are collagen fibres arranged in bone?
Helices will be at different angles in different lamellae/layers, arrange themselves to oppose/resist stress in the tissue
How are calcium hydroxyapatite molecules arranged in bone?
The crystals are attached to the collagen fibres, calcium salts exist in platelets or rods
What types of strength do collagen and calcium hydroxyapatite crystals give to the bone tissue?
- Collagen: tensile strength
- Calcium hydroxyapatite: compressive strength
Is bone a static tissue?
No, it is dynamic - the adult structure formed is a result of extensive remodelling (e.g. formation of osteons) and involves three types of cells (osteoblasts, osteoclasts and osteocytes)
What precursor cells are osteoblasts derived from?
Fibroblast-like precursor cells (that also give rise to adipocytes, myocytes and chondrocytes)
What are the osteoblast derived matrix glycoproteins?
- Osteocalcin (local regulatory protein, bone specific)
- Osteonectin (binds calcium, hydroxyapatite and collagen)
- Osteopontin (RGD/arg-gly-asp sequences, binds osteoclasts)
- Bone morphogenic protein (induces new cartilage and bone formation)
Why do glycoproteins have a useful effect on osteoblasts?
Because osteoblasts express integrins, so allow binding
What is the function of osteoblasts?
They secrete the components of bone ECM (i.e. glycoproteins) that then allow for mineralisation and crystallisation, form woven bone. Will be induced in instances of bone growth or repair
What two lipid-soluble vitamins are needed for osteoblast gene expression?
Vitamins D and K
- How was the identification of a bone inducer discovered?
- Use fact that osteocalcin is specific to bone and sequence promoter
- Search database for similar sequences that are known to bind transcription factors
- Identify the protein coded, Cbfa-1 is rich in bone tissue
- Make Cbfa-1 knockout mouse
- Mouse is unable to induce osteoblast action so is bone with no bones, only a cartilage model of the skeleton
What is the function of an osteocyte?
- Control immediate area of bone tissue
- Force sensors
- Signal osteoblasts when repair is necessary
What are some features of osteocytes?
- Embedded in the ECM/bone tissue
- Have gap junction-linked processes that form a communication and force-sensing network in canaliculi throughout the bone
- These projections are also connected to osteoblasts, so can instruct them to repair or strengthen bone where necessary
What is the function of an osteoclast?
They secrete acid and proteases from their ruffled edge, which seals ‘Howship’s lacunae’ into which the acid and enzymes are secreted
What are some features of osteoclasts?
- Multinucleate macrophages
- Derived from monocyte blood-borne precursor cells
How are osteoclasts able to attach to tissue/the ECM?
- Integrins within the osteoclasts bind to the RDG (arg-gly-asp) sequences in the osteopontin matrix protein
Explain the process of how an osteoclast functions.
- Integrins in the osteoclast membrane seal it onto the area of bone which needs to be broken down (Howship’s lacuna)
- Ruffled border develops on the cell membrane once osteoclast has attached to the contact zone
- Osteoclast starts to produce and secrete hydrogen ions/protons via a proton pump
- Carbonic anhydrase produces more proteins through the production of carbonic acid
- The bicarbonate produced is exchanged for chloride through the action of a membrane exchanger
- A chlorine channel allows chloride to pass into the lacuna to produce hydrochloric acid
- Proteases are secreted to break down collagen/other proteins
- Calcium and signalling peptides are released from the degraded bone by proteases -> this signalling process can stimulate osteoblasts to fill in the holes created
What stimulates osteoblasts to fill in the holes created by osteoclasts?
The signalling peptides and calcium released by the proteases working at Howship’s lacunae
What controls osteoclast action?
They are controlled by osteoblast products:
- ODF (osteoclast differentiation factor) aka RANK-ligand, stimulates osteoclast differentiation
- RANK is the stimulating receptor on osteoclasts
- OPG (osteoprotegerin), also produced by osteoblasts, inhibits action of RANK-L through acting as a decoy receptor, inhibiting bone breakdown
- Balance of RANK-L and OPG determines bone turnover, has regulatory action - if this goes wrong (in either direction), can result in various conditions, such as arthritis, osteoporosis, Paget’s disease and bone metastases
What is the general process of bone remodelling?
- Stimulus occurs to remodel, this is received by osteocytes and periosteal osteoblasts
- Osteoclasts are instructed to remove unwanted bone, after this they die
- Osteoblasts are then recruited and from new bone
- Osteoblasts then become inactive bone-lining cells
What method does bone use to release products?
It acts as an endocrine tissue (no ducts, secretes directly from tissue)
What organ does FGF23 (secreted by bone) affect?
The kidneys, increases phosphate excretion
What tissue does sclerostin (secreted by bone) affect?
Cartilage, has been shown to decrease osteoarthritis progression
Also affects FGF23 and therefore the kidneys
What tissues and organs does osteocalcin (secreted by bone) affect?
- Pancreas, affects insulin secretion, increases insulin sensitivity, DLK-1 secretion, beta cell proliferation and beta cell mass
- Brain, increases neurotransmitter production and cognitive function
- Muscles, increases insulin sensitivity
- Testes, increases testosterone production and secretion
- Adipose tissue, increases beige adipogenesis
What controls interstitial growth in long bones/production of the cartilage matrix?
- Growth hormone
- Insulin-like growth factor-1 (IGF-1)
- Their respective receptors
- What happens if growth factors are continually expressed?
Gigantism! Growth will not stop in an organism, resulting in them becoming larger than usually expected - however the rest of the body is often unable to keep up, resulting in heart failure
- What happens in growth factor receptor-knockout mice?
They are born considerably smaller than normal, but are otherwise fully formed
- What is achondroplasia?
The most common form of dwarfism, due to a mutation in the FGF3 receptor causing it to be upregulated, chondroblast differentiation is over-promoted, resulting in very short long bones (proliferation was blocked)
Appositional growth still possible, so bones are often quite stocky
- What happens in FGFR3 knockout mice?
Fibroblast growth factor receptor 3 instructs immature chondroblasts to mature into chondrocytes, so without this receptor, the cartilage growth and ossification will just continue, resulting in overlong bones
What are the stem cells for chondrocytes?
Chondroblasts
What is bone a store of?
Calcium and phosphate
How are bones adapted to be light but strong?
Through the presence of cancellous (aka trabecular or spongy) bone, as the many holes in the structure allow for a decreased weight, but the criss-cross alignment of fibres within the trabecular structure and the alignment overall resist strength and allow the tissue to be strong
Contains many spicules and trabeculae which add strength, give the impression of a sponge due to many holes in the tissue
There is also compact bone, which is made of a series of closely packed osteons/Haversian canals, with the many lamellae providing a lot of strength
Where can red bone marrow be found?
Mainly in cavities within the flat bones and in the cancellous/spongy material at the ends of long bones
What is the function of red bone marrow?
Contains haemopoietic stem cells that are able to differentiate into two more types of stem cell (myeloid and lymphoid stem cells) that can then differentiate to form red blood cells, white blood cells and platelets. This tissue can also remove cells from circulation.
Part of the lymphatic system, stored in cavities within bone.
Where can yellow bone marrow be found?
In the medullary cavity in the shafts of long bones, often surrounded by a few layers of red bone marrow
What is the function of yellow bone marrow?
Contains stem cells that differentiate to form cartilage, bone and fat cells. Also acts as an important fat/adipocyte storage.
Part of the lymphatic system, storied in cavities within bone.
How does bone affect calcium homeostasis?
It stores calcium, so can be triggered to release or take up some (by calcitonin and calcitriol respectively) if there are insufficiencies elsewhere (other factors are diet and the effect of the parathyroid hormone on the kidneys)
- What is Paget’s disease?
A mutation affecting osteoclast function
- Increased number and size of osteoclast
- Contain viral nuclear inclusions (aggregates of stable molecules within the nucleus, suggests potential viral input)
- Express viral antigens
- Contain viral transcripts
- Increased activity of osteoclasts causes increased activity of osteoblasts also, meaning a lot of woven bone is laid down
- Has some genetic component
- Results in excessive remodelling and increases the thickness of bones (extremely noticeable in the skull)
What is osteogenesis imperfecta?
- In 90% of cases, dominant mutation in the type I collagen gene
- This causes very weakened, easily broken bones
- Frequent breaks result in gross deformities
- Other 10% shown to be caused by recessive mutation in CRTAP protein (non-enzymatic member of prolyl-3-hydroxylase family)
What is Ehlers-Danlos syndrome?
- Mutations in various collagen genes (many different types), for example type V collagen, which regulates the formation of type I collagen fibrils
- Results in either easily broken or stretchy skin and highly elastic/hypermobile joints (seen fairly frequently in violinists)
What is osteoporosis?
A condition that weakens bones due to an imbalance in osteoclast-osteoblast action (either hyperactivity of osteoclasts or hypoactivity of osteoblasts)
Makes fractures more likely and far more common, including hip and vertebral fractures after falling from standing height
What are some factors that can cause osteoporosis?
Environmental:
- Nutrition
- Smoking
- Exercise
- Drugs (e.g. steroids affect vitamin D uptake so can cause osteoporosis)
Genetic:
- Gonadal
Aging:
- Menopause in women (sporadic, shows hormone effect)
What is a potential treatment for osteoporosis?
Using oestrogen-type therapies after menopause
In which sex is osteoporosis more common?
Females (due to drop in hormones after menopause)
- How can a decrease in oestrogen lead to osteoporosis?
Osteoclast apoptosis is regulated by oestrogen, so if the concentration of the hormone decreases then fewer osteoclasts may undergo apoptosis, resulting in more osteoclasts being present and breaking down bone in an unbalanced matter/faster than osteoblasts can replace it, weakening the structure
- What is and what causes rickets?
- Rickets is a failure to absorb calcium
- Results in weak, flexible bones that are deficient in calcium
- Caused by a lack of or inability to process vitamin D3
Summarise bone remodelling.
- Osteoblasts lay down woven (immature and irregularly structured) bone
- Osteoclasts can instruct osteoblasts to release factors and trigger osteoclast action
- Osteoclasts form Howship’s lacunae, break down and absorb the bone tissue
- Osteoblasts are then instructed to lay down lamellar/mature bone structures
