Physiology of Bone Repair Flashcards
What are the two main
causes/determinants of bone remodelling?
Calcium homeostasis
Mechanical stress
Which of the following is a symptom/sign of acute hypocalcemia
> Constipation >Fatigue > Low mood >Muscle spasms >Neuromuscular depression
Muscle spasms
Hypocalcaemia induces hyper-excitability
All of the following are symptom/signs of chronic hypocalcaemia EXCEPT Carpopedal spasm Delayed dentition Scoliosis Short stature Enlarged iliac crest
Enlarged iliac crest
Which of the following is a symptom/sign of acute hypercalcemia Chvostek’s sign Rales Joint pains Diarrhoea Arrhythmias
Arrhythmias
Name 5 functions of the skeleton
Maintaining body structure locomotion haematopoiesis ca2+/mineral homeostasis protection of inner organs,
Name three organs that are affected by PTH
Bone, Kidney, Gastrointestinal tract
Name 5 symptoms of chronic hypocalcaemia
Delayed dentition, short stature, pigeon chest, scoliosis, easily fractured bones, etc.
Name 5 symptoms of hypercalcaemia
neuromuscular depression. And calcium deposition
groans (constipation), moans (psychotic noise), bones (bone pain, especially if PTH is elevated), stones (kidney stones), and psychiatric overtones (including depression and confusion).
What is the cause of autosomal dominant osteopetrosis?
bone resorption is reduced compared to mineralisation
This is due to failure of osteoclasts
Osteoclasts cannot secrete acid
Either acid pump or chloride channel is mutated
Name 5 signs of autosomal dominant osteopetrosis AND for each one explain how osteopetrosis leads to this symptom.
Increased bone density (compromised bone resorption due to failure of osteoclasts),
Deafness (nerves compromised when skull foramina filled),
Blindness (skull foramina filled),
Frail bones (bone remodelling is not directional),
Anaemia (bone spaces for haematopoiesis compromised)
How do osteoclasts resorb bone: how do they position themselves, what do they secrete, and why are these two functionally connected?
They create a ruffled border with integrins to surround the region of resorption,
They create a lysosome-like space extracellularly where they secrete acid (to digest hydroxyapatite), and proteases (to digest the matrix proteins such as collagen),
The ruffled border is necessary to prevent dilution of the acid and loss/misapplication of the proteases
Name 5 differences between osteoblasts and osteoclasts with regard to: mitosis, genetic material, size, overall function, and developmental origin
Blasts cannot divide, are mononucleate, smaller than clasts, lay down bone, and develop from mesenchyme.
Clasts are multinucleate, can proliferate, are substantially bigger than blasts, reabsorb bone, and are derived from haematopoietic tissue
Name 5 differences between osteoblasts and osteocytes with regard to: location, shape, size/shape, overall function, and developmental origin
Blasts are on endosteal surface, cigar-shaped, the same size as cytes, they lay down bone and they are mesenchymal.
Cytes are within osteoid, they are stellate (star) shaped with many extended processes, they maintain osteoid, and they are derived directly from osteoblasts that are trapped inside osteoid
Describe PTH in terms of: chemical class, origin, effects at bone, effects at GI tract, effects at kidney
PTH is a peptide hormone, Origin from parathyroid glands, Net effect of resorption, Minor effect on increasing GI calcium absorption, Increases Ca2+ resorption at kidney.
Describe Vitamin D in terms of: chemical class, origin, effects at bone, effects at GI tract, effects at kidney
Vitamin D is a broken steroid,
It is created in skin and is further modified by enzymatic actions derived from the liver, and kidney,
It indirectly stimulates osteoclasts (via effects on osteoblasts),
Strong effect on increasing GI absorption of Ca2+.
Vitamin D’s effects on Ca2+ resorption at Kidney are controversial.
Name 4 classes of molecular constituents in bone matrix
Hydroxyapatite (Ca-phosphate),
Collagen (and other structural proteins such as elastin),
Glycosaminoglycans,
Growth factors,
Name 3 symptoms of acute hypocalcaemia
BAD-CATS: Bleeding, Anaesthesia, Dysphagia, Convulsions, Cardiac arrhythmias, latent tetany, spasms, stridor
What is this entire microscopic structure in bone called
Haversian system, or an individual area is an osteon
Give 3 reasons why you need bone resorption.
Bone resorption is essential for repair of broken bones, remodelling of bones as person grows (ie from child to adult), remodelling of bone for specific stresses, and plasma calcium homeostasis.
How is hydroxyapatite (e.g. calcium phosphate crystals) resorbed when bone is being remodelled? (3)
Hydroxyapatite is brought into solution by
A) acid, which makes calcium phosphate less soluble,
B) phagocytosis of calcium phosphate by osteoclasts, and
C) digestion of proteins etc that nucleate crystallisation by acid hydrolases (eg by Cathepsin K)
What is compressive strength and tensile strength, and what molecules give bone its high values for both?
The compressive strength is resistance to being compressed (ie supporting weight), which is provided in bone by hydroxyapatite. It is very rigid; so when it fails, it cracks.
Tensile strength is resistance to stretching (and shearing), which is provided by collagen, which is flexible.
What are the structural and functional properties of glycosaminoglycans?
GAGs are repeating polysaccharide units. They typically have high negative charges, which means that they repel each other but attract water and cations. This makes them vicous and good as lubricators.
When an osteoclast is degrading bone, what is the name of the depression around itself that forms?
Howship’s lacuna (or just lacuna)
How does the positioning of growth factors in bone matrix contribute to bone turnover?
The demineralisation by osteoclasts exposes and releases growth hormones that stimulate and attract osteoblasts, which will lay down new bone.
Name two types of fetal bone formation, and describe how they differ.
Intramembranous bone formation and Endochondral bone formation.
Endochondral begins with a cartilage structure that is ossified, while intramembranous starts without cartilage.
Name the size, cell shape, nuclear morphology, location, and ONE functional activity of osteocytes
They are embedded in bone matrix. For bone maintenance and detection of environmental & ageing stresses.
Long and thin with extensive branches (that travel through canaliculi), the main cell body inside the lacuna (inside bone spicules) is 20 microns x 5 microns, mononucleate.
Name the size, cell shape, nuclear morphology, location, and ONE functional activity of osteoblasts
20-25 microns, round and regular in shape, mononucleate.
Located on developing bone surfaces.
Formation of new bone, release of signalling substances, produce protein component of acellular matrix,
Regulate bone growth and degradation.
Name the size, cell shape, nuclear morphology, location, and ONE functional activity of osteoclasts
Giant (40 micron) multinucleate cells.
Cube-like, regular often with ruffled border responsible for bone degradation and remodelling bone.
Located in Howship’s lacunae at bone surfaces being resorbed.
Name the precursor cell lineage for osteocytes, osteoblasts and osteoclasts
Osteoblasts come from mesenchymal stem cells,
Osteocytes come from osteoblasts, and
Osteoclasts develop from the same precursors as monocytes (GM-CFUs)
How do osteoclasts resorb bone?
They form a seal with the bone (a ruffled border) with integrin molecules, and into the sealed off region they secrete acids (eg lactate) and enzymes (eg Cathepsin K or TRAP), which solubilise the hydroxyapatite and degrade the collagen and other protein content.
The osteoclasts then phagocytose the products.
Name 5 ways calcium ions are stored in the body and for each, state how accessible that store of calcium is.
Bone, inaccessible (at least without osteoclast action)
Sequestered (e.g. inside cells, inside SR), reasonably inaccessible (without activation of ryanodine receptor, etc)
Bound to proteins (e.g. calbindin), somewhat inaccessible (unless free calcium diminishes). For example, protein bound calcium is not filtered by kidneys.
Complexed with anions (e.g. phosphate), reasonably accessible
Free calcium, immediately accessible
What percentage of the body’s total calcium resides in the extracellular fluid?
< 1 %
Where is PTH made, what chemical class molecule is it, and what is its net effect on plasma calcium levels?
PTH is made in the chief cells of the parathyroid gland, it is a peptide, and it causes an increase in plasma calcium.
Where is Vitamin D (Calictriol) made, what chemical class molecule is it, and what is its net effect on plasma calcium levels?
Calcitriol (1,25-OH cholecalciferol) has its precursor (cholecalciferol) made in skin cells, this is then metabolised by the liver (hepatocytes) to 25-OH cholecalciferol, and it is finally activated in the kidneys (proximal tubule) to be calcitriol; it is a seco-steroid (ie it is steroid-like) and its net effect is to increase plasma calcium.
Where is Calcitonin made, what chemical class molecule is it, and what is its net effect on plasma calcium levels?
Calcitonin is made by C cells of the thyroid, it is a peptide, and it causes a Decrease in plasma calcium.
What are the main effects of PTH on different organs of the body?
Reduces the kidney’s secretion (i.e. it increases reabsorption) of calcium (and increases it secretion of phosphate),
Stimulates bone to mobilise calcium – it is detected by osteoblasts (and osteocytes) which immediately mobilise sequestered calcium as well as releasing cytokines resulting in increased differentiation and activity of osteoclasts. The net effect is to increase plasma calcium.
What are the causes of hypocalcaemia (7)
Parathytoid dysfunction. Insufficient intestinal absorption. Insufficient calcium ingestion (eating disorders). Rickets. Renal failure. Alkalosis. Hyperventilation. Drugs (magnesium, anticonvulsants). Pregnancy and Lactation can draw on calcium, but in healthy women these should be compensated for endocrinologically.
What are the main effects of 1,25 dihydroxycholecalciferol on the body?
Increases intestinal absorption of dietary calcium by increasing calbindin in enterocytes.
It also acts on osteoblasts in bone to release RANKL and thus increase osteoclast differentiation and activity.
(it is a form of Vit D)
Give an example of a bis-phosphonate, what it does, and how it works.
Alendronate
Used in osteoporosis to diminish bone resorption by osteoclasts.
Works by interfering with osteoclast’s metabolism
Mimics pyrophosphate
Interferes with reactions dependent on pyrophosphate
Accumulates specifically in bone because it binds to calcium.
Then ingested by osteoclasts
List the ways that the production of 1,25 dihydroxycholecalciferol is reduced via negative feedback.
25-OH cholecalciferol feeds back on the liver to reduce the production of 25-OH cholecalciferol from cholecalciferol.
Calcium feeds back on the parathyroid gland to reduce the production of PTH, and this reduction reduces the kidney’s activation of 25-OH cholecalciferol to 1,25 (OH)2 cholecalciferol.
Name 4 phases of bone repair that would occur after a fracture, how long would fracture repair take in a healthy young adult.
Reactive: Haematoma / Inflammation Soft Callus Formation Hard Callus formation Remodelling Upper body 2-3 weeks, lower body > 4 weeks
In the repair of a bone fracture, what is the difference between soft callus formation and hard callus formation?
Soft Callus Formation = woven bone or hyaline cartilage join the fractured pieces of bone
Woven bone has disorganised collagen
This process happens first because it is fast
Hard Callus formation = lamellar bone replaces woven bone / cartilage
Lamellar bone has parallel collagen fibres
Slow process, but results in bone that is much stronger
What are pharmacological treatments to reduce osteoporosis and for each list the drug’s nominal mechanism (4)
Bisphosphonates – mimic pyrophosphate in bone leading to osteoclast apoptosis or protein trafficking defects.
Intermittent PTH – increases osteoblast activity and anabolic bone formation
Denosumab – Monoclonal antibody that binds to the RANK ligand (mimicking the action of OPG), thus preventing activation and differentiation of osteoclasts.
Teriparatide – portion of human parathyroid hormone (PTH). Intermittent application activates osteoblasts more than osteoclasts. Encourages osteoblasts to lay down bone
How do you get calcium phosphate crystals to form where bone is needed?
Because calcium and phosphate are at a high concentration, they can crystalise out of solution when
A) the inhibitors to crystalisation are removed and
B) where the proteins (ie collagen) provide nucleation points.
Why doesn’t calcium phosphate fall out of solution in plasma?
Calcium and phosphate in the plasma are at a high enough concentration to form crystals, but do not because there are crystallization inhibitors in the plasma, e.g. pyrophosphate.
Explain why hypocalcaemia can lead to convulsions
Hypocalcaemia makes membranes “more excitable” and “less stable”. Thus, Na+ entry into cells is increased, and the threshold for action potentials is lowered, which explains latent tetany and its signs.
Advanced: If you made a drug to block the activity of each of the following, would it increase (or decrease) bone resorption: OPG, OPGL, PTH, RANK, RANKL
Osteoprogeretin – block would increase resorption
OPGL = RANKL – block would decrease resorption
PTH – block would decrease resorption
RANK – block would decrease resorption
RANK Ligand – block would decrease resorption
Name 5 processes that happen during the reactive phase of bone fracture repair
Blood cells enter wound,
Hematoma forms, (inflammation),
Fibroblasts and blood vessels aggregate to make granulation tissue,
Bone making cells are recruited from periosteum,
Woven bone (or hyaline cartilage) is laid down to join the two broken pieces of bone.
Name 2 temporary structures that are created during the repair phase of bone fracture repair and state briefly how these are created.
The Soft Callus is spongy bone near blood vessels and fibrocartilage further away. It’s made by bone precursor cells (chondroblasts proximally or osteoblasts distally),
Hard callus formation occurs when endochondral ossification takes place (endochondral bone replaces fibrocartilage or woven bone)
What 3 events occur during the remodelling phase of bone fracture repair?
Endochondral bone is replaced by trabecular bone,
It is remodelled to the correct shape, and
Where appropriate it is replaced by compact bone
Why are bisphosphonates specific for affecting bone cells, and have few adverse effects on other cell types?
Bisphosphonate-based drugs’ specificity comes from the two phosphonate groups (and possibly a hydroxyl at R1) that work together to bind to calcium ions.
Bisphosphonate molecules preferentially “stick” to calcium and bind to it.
The largest store of calcium in the human body is in bones, so bisphosphonates accumulate to a high concentration only in bones.
Bisphosphonates, when attached to bone tissue, are “ingested” by osteoclasts.
Also the oral bioavailability of bisphosphonates is low because the drug has highly negative charge (associated with phosphate groups) – it is an acid that donates H+ into solution. Almost all oral drugs used in medicine are weak bases because weak bases are taken up by cells more easily.
Teriparatide increases PTH activity and decreases demineralisation during osteoporosis. Why doesn’t it increase bone destruction by osteoclasts?
Teriparatide is a PTH fragment that acts as a PTH agonist.
Teriparatide increases the PTH-dependent activity of osteoblasts.
Osteoblasts have two opposing effects on mineralisation: 1) osteoblasts lay down bone and 2) they activate osteoclast precursors, which become osteoclasts that demineralise bone.
PTH applied continuously results in bone loss via increased osteoclast activity, but intermittent PTH results in increased osteoblast number and osteoblast mineralisation activity in osteoporosis; the net effect of intermittent PTH is bone strengthening.
