MSS: Cellular Structure Of Bone

Question 1

List some functions of bone.

Answer 1

• Support and movement, as an attachment site for muscles
• Protection for internal organs
• Provides a home for bone marrow (RBCs and stem cells)
• Acts as a mineral reservoir (calcium + phosphate) –> has metabolic role
• Collaborates with the endocrine system: is a source of some ‘non-classical’ hormones.

Question 2

Describe bone structure.

Answer 2

Bones come in a variety of shapes and sizes but they share a common structure. The outer layer is called the cortical (compact) bone and deeper inside is the trabecular (spongy, cancellous) bone.

Cortical bone is known as compact. It is found on the surface of bones, and is made of repeated units called oesteons. This consists of a central canal down which capillaries and nerves run, surrounded by lamellar concentric sheets of bone matrix. This results in minute, longitudinal canals that allow for blood vessel penetration. Within the lamellar are microscopic spaces called lacunae (where bone cells live) which are connected by microscopic canals called canuliculi.

Trabecular bone is known as spongy and cancellous. It has a less organised structure, made up of interlocking struts (mesh-like structure). This is found on the inside of the bone which would be in contact with the bone marrow. Trabecullar bones consists of the same cell types but different organisation.

Question 3

What is the composition of bone?

Answer 3

It is 25% protein (organic osteoid matrix), and 75% minerals.

It is also made up of cells, which contribute to the weight.

Question 4

Describe the organic (osteoid) protein matrix.

Answer 4

It is made up of mainly type 1 collagen.

It gives the bone both flexibility and tensile strength (the ability to resist stretching/bending forces).

Question 5

Describe the bone mineral.

Answer 5

It is mainly hydroxy apatite, which is hydrated calcium and phosphate (Ca10(PO4)6(OH)2).
It makes the bone rigid, brittle and gives it a high compressive strength (the ability to resist shortening/longitudinal forces).

Question 6

List the types of bone cells (and their origination).

Answer 6

The three main types of bone cells are:

• osteoblasts
• osteoclasts
• osteocytes

Bone marrow cells:

• Mesenchymal (stromal) stem cells give rise to osteoblasts and osteocytes.
• Haematopoietic stem cells give rise to all blood cells, and osteoclasts (which are the same lineage as macrophages).

Question 7

Describe osteoblasts.

Answer 7

• they are the bone forming cells
• they are derived from mesenchymal stem cells
• they secrete osteoid, the collagen matrix of bone –> high tensile strength
• they promote mineralisation of osteoid (lay down the hydroxyapatite crystals within this organic osteoid) –> high compressive strength.

Question 8

Describe osteoclasts.

Answer 8

• they are bone (digesting/) reabsorbing cells
• they derive from haematopoietic stem cells
• they are large and multinucleate
• they secrete acid to dissolve bone mineral and enzymes to digest the organic matrix
• their life cycle is controlled by apoptosis

Question 9

Describe osteocytes.

Answer 9

• they are mature bone cells -terminally differentiated osteoblasts
• they’re encased in bone mineral matrix (lacunae)
• the osteocytes extend multiple dendrites via minute canals in the bone matrix (canaliculi)
• the Lacunocanalicular system maintains communication between the bone surface and blood vessels
• they’re thought to coordinate osteoblast and osteoclast activity
• may live for decades.

Question 10

What is bone remodelling?

Answer 10

The skeleton is continually renewed throughout life by bone remodelling, a process whereby old bone is broken down (reabsorbed) by osteoclasts in a coordinated cycle of new bone formation by osteoblasts.

The process begins with activation of the osteoclasts. This involves multiple steps of differentiation from haematopoetic stem cells into mature osteoclasts. They then attach to a section of the inner surface of bone and form a seal, in which they secrete acid and digestive enzymes, to reabsorb the bone (hollowing it out and forming cavities in the bone).

The reversal of the cycle occurs with osteoclast apoptosis. There is increased differentiation of the osteoblasts from the mesenchymal stem cells. The new osteoblasts secrete osteoid which becomes mineralised to form new bone.

It is thought the coordination of the process is under the control of osteocytes which finetune the activity between the osteoclasts and osteoblasts. How this is achieved is currently under ongoing research.

The essentials of the process is the same in cortical and trabecullar bones although the details differ.

Question 11

Describe bone remodelling in cortical bone.

not in lecture slides - from year above

Answer 11

Typically, there will be a leading edge where cells differentiate into osteoclasts and start digesting the bone. Behind that, there is osteoblast differentiation that will lay down new bone.
Thus, you have an advancing line of bone reabsorption and formation, which also leaves behind a ‘cement line’ that can be detected histologically.

Question 12

Describe bone remodelling in trabecular bone.

not in lecture slides - from year above

Answer 12

Along the surface of the trabecular struts, there will be an osteoclast eating away at the bone, then osteoblasts subsequently forming new bone.
There are also lining cells on the surface of the bone that detach underneath this, forming a basic multicellular unit.

Question 13

Describe the stages of bone remodelling.

not in lecture slides - from year above

Answer 13

1. ACTIVATION: the promotion of differentiation of new osteoclasts
2. RESORPTION: the duration of osteoclast activity, removing bones and creating pits
3. REVERSAL: the process by which you get osteoclast apoptosis, terminating its activity
4. FORMATION: osteoblast differentiation; formation of new osteoblasts which line the bone surface, forming new osteoid (new bone which subsequently becomes mineralised)

Question 14

What are some ways in which we can control bone remodelling?

Answer 14

• LOAD-BEARING EXERCISE (meaning an individual is active - not necessarily doing strenuous exercise) –> ordinary activity is thought to cause microfractures in the structure ageing bones, this is detected by osteocytes which can then control the cycle of bone remodeling.
• CYTOKINES and other local signals
• ENDOCRINE (systemic factors):
• oestrogen: inhibits osteocyte apoptosis and promotes osteoclast apoptosis - thus promote new bone formation.
• Oestrogen is essential for skeletal health in both sexes. Oestrogen levels fall significantly in post menopausal women - they are at more risk of osteoporosis (disease involving loss of bone mass). Men with an inactivating mutation for the aromatase enzyme develop osteoporosis at a young age. This can be treated with oestrogen therapy.
• Androgens

(aromatase converts androgens to oestrogens)

Question 15

Describe the induction of osteoclast differentiation by RANK ligand.

(bone remodeling - other local signals)

Answer 15

Cytokines and other local signals also control bone remodelling.

RANK (receptor activator of nuclear factor κ-B): a surface receptor on pre-osteoclasts (immune cells), stimulates osteoclast differentiation. (this is the final step of osteoclast differentiation).

RANK-ligand: produced by (mesenchymal stromal cells) pre-osteoblasts and osteocytes; binds to RANK and stimulates osteoclast differentiation. (cell contact required)

OPG (osteoprotegerin): decoy receptor produced by osteocytes; binds to RANK-L, preventing activation of RANK receptor. It is soluble.

The degree of osteoclast production/differentiation occurring is due to the amount of OPG available relative to amount of Rank ligand - hence providing a balance in osteoclast formation. Specifically, the more OPG available then the less likely the Rank receptor is likely to be activated by the rank ligand.

Also therapeutically important in developing drugs based on monoclonal antibodies to rank ligand. Which in effect down regulate activation of the rank receptor and hence down regulate the amount of bone reabsorption. - this is a possible treatment where bone mass is lost such as osteoporosis.

Question 16

Describe the Wnt signalling pathway in osteoblast differentiation.

Answer 16

Wnt is a gene family that encodes for Wnt proteins, which are involved in developmental signalling throughout the animal kingdom. Thus, they are highly conserved genes.

It is a complex signalling pathway that is required for osteoblast differentiation. It is negatively regulated by DKK (dickkopf) and sclerostin (SOST).

The Wnt signalling pathway is activated by a receptor called frizzled. The receptor is activated by interreacting with co-enzyme LRP5/6, the Wnt ligand can then bind to the receptor and activate signalling pathways that lead to changes in gene expression that lead to osteoblast differentiation. This is negatively regulated by DKK (dickkopf) and SOST (sclerostin). They prevent Wnt activation of the frizzled receptor and hence prevent osteoblast differentiation (inhibitory effect).

First discovered in the Drosophila wingless gene experiment (body plane development) in 1987.

Question 17

Describe how osteocytes regulate the process of bone remodelling.

Answer 17

Osteocyte regulation of bone remodeling.

Osteocytes express RANKL and macrophage-colony stimulating factor (M-CSF) to promote, and OPG (osteoprotegerin) and NO to inhibit, osteoclast formation and activity. Osteocytes also regulate bone formation via the secretion of modulators of the Wnt signaling pathway. PGE2, NO, and ATP act to activate Wnt signaling, whereas sclerostin, DKK1, and SFRP1 all inhibit Wnt signaling and subsequent osteoblast activity. Maintenance of this balance between resorption and formation by the osteocyte is essential for bone homeostasis.

There is ongoing research in producing monoclonal antibodies against sclerostin, this can remove the inhibitory affects against osteoblast differentiation and hence bone formation. This can be useful in possibly treating against the loss of bone density and osteoporosis.

Question 18

What are some diseases of the bone?

Answer 18

RARE: mutations that affect key signals (they’re very rare, but have elucidated mechanisms) - e.g. SOST gene, RANK.

The discovery of the SOST gene and the associated sclerostin protein was a result of genetic analysis of 2 families that had very rare conditions which involved increased bone density (sclerosteosis) - following genetic linkage studies the gene was tracked down, its protein product deduced and studied –> turning out to be sclerostin.

LESS RARE: Osteomalacia
the failure of bone mineralisation, so it goes soft, like cartilage. Commonly due to Vit D defiency (causing calcium deficit).

COMMON:
Osteoporosis. Overall decrease in bone mass, measured as bone mineral density. There is the thinning of bone associated with age, and is more common in women than men (because of the dramatic drop of oestrogen following menopause).

Question 19

List some bone diseases caused by mutation.

not in lecture slides - from previous year lectures

Answer 19

• Osteoporosis pseudoglioma
  the inactivation of LRP-5, the Wnt co-receptor
• Sclerosteosis and van Buchem disease
  mutation of the SOSt gene, inactivating the sclerostin protein -> excessive bone formation
• Osteopetrosis:
  a mutation inactivates the RANK-L protein

Question 20

What happens during osteoporosis?

Answer 20

• Increase in bone resorption over formation
• Loss of bone density (there are thresholds that constitute as to what counts as osteoporosis i.e. how much bone mineral density is lost).
• Increased fracture risk
• Common in elderly (progressive decline over the years in bone mass)

Fragility fracture - fracture to a bone where it wouldn’t generally be expected.

(Thinner cortical structure and more gaps in trabecullar structure).

Question 21

Glossary

Answer 21

Hydroxyapatite - The principle mineral component of bone, composed of calcium and phosphate (Ca10(PO4)6(OH)2). Responsible for the hardness of bone.

Osteoblast - Bone cells responsible for forming osteoid and controlling its mineralization.

Osteocyte - Terminally differentiated osteoblast encased within mature bone. Thought to be the master regulators of bone remodelling.

Osteoclast - Large, multi-nucleate cells responsible for reabsorption of bone tissue.

Osteoid - The organic matrix of bone, consisting mainly of collagen and other protein. Secreted by osteoblasts, which line the surface of newly-formed bone.

Osteoid - is initially flexible, like cartilage, but the process of mineralization, also controlled by osteoblasts converts it into the strong and dense mineralized tissue of the skeleton.

Remodelling - The skeleton is continually renewed throughout life by bone remodelling, a process whereby old bone is broken down (reabsorbed) by osteoclasts in a coordinated cycle of new bone formation by osteoblasts.

Question 22

Osteoporosis definition (copy and pasted) from lecturer resource page.

• May be useful for LAQ.

Answer 22

Osteoporosis is a bone disease in which the amount of bone is decreased and the structural integrity of trabecular bone is impaired. Cortical bone becomes more porous and thinner. This makes the bone weaker and more likely to fracture.

In 1994, a committee of the World Health Organization defined osteoporosis based on bone density. The bone density is the weight of mineral per volume of bone. The most common method to measure this is with a bone density machine which results in weight per area of bone (mg/cm2). It has become common to translate this into units called “T-scores” because the different kinds of machines do not measure bone the same way. Details are explained in the bone density section. Accordingly, the categories were:

1. Normal bone (T-score better than -1)
2. Osteopenia (T-score between -1 and -2.5)
3. Osteoporosis (T-score below -2.5)
4. Severe osteoporosis (low T-score and a fragility fracture)
   Many factors lead to fractures, not just bone density. Age, heredity, body weight, diseases, lifestyle, frailty, and amount of trauma all play important roles. The risk of a fracture due to osteoporosis can be estimated using these factors in addition to the bone density.