MSS: Cellular Structure of Bone

Question 1

List some functions of bone.

Answer 1

Structure - give structure and shape to the body

Mechanical - sites for muscle attachment - if bones were missing, would not be able to move

Protective - vital organs and bone marrow e.g. skull protects brain. Bone marrow makes red blood cells, white blood cells, platelets

Metabolic - reserve of calcium and other minerals. Bone is also an endocrine organ and has a role in metabolism.

Question 2

Describe bone composition.

Answer 2

Partly inorganic and partly organic.

Inorganic - makes up 65% of bone

• consists of crystals of calcium hydroxyapatite (Ca10(PO4)6(OH)2)
• inorganic component is storehouse for 99% of Calcium in the body
• and 85% of the Phosphorus, 65% Sodium, Magnesium

Organic - makes up 35% of bone
- bone cells and extracellular protein matrix (mainly different collagens e.g. collagen type 1 as well as other proteins)

Question 3

List the different anatomical bone types.

Answer 3

• Flat e.g. sternum, skull - protective function of specific
  organs
• Long - limb bones e.g. femur, humerus, ulnar, radius - allow us to make large movements
• Short/cuboid e.g. carpals in wrists, tarsals in feet - provide stability in joins and allows us to make smaller, more precise movements
• Irregular - specific complex shapes that allow us to protect organs e.g. pelvis protecting reproductive organs, vertebrae protecting spinal chord
• Sesamoid - embedded in muscles or tendons. Reduce stresses and strains on muscles. Most very small and found in feet and hands - one large on is the patella (kneecap in leg)

Question 4

Describe the macroscopic structure of bone.

Answer 4

Trabecular bone - also called cancellous/ spongy bone. Thin ‘honeycomb-like’ bone - highly metabolically active and also adds strength to the bone

• Cortical bone - also called compact bone, thick bone that makes up the shaft of the bone

Question 5

Describe the microscopic structure of bone.

Answer 5

• Lamellar bone - mature bone

- Woven bone - immature bone - occurs during initial bone development

Question 6

What are some differences between cortical and trabecular bone?

Answer 6

Cortical (thick bones that makes up the external tube):

• long bones (predominant type of bone found in long bones)
• 80% of skeleton
• Appendicular (bones of appendages e.g. arms/legs)
• 80-90% calcified
• Mainly has structural, mechanical, and protective function

Trabecular (spongy bone criss-cross on internal diameter):

• vertebrae and pelvis
• 20% of skeleton
• axial (bones of body’s long axis e.g. vertebrae)
• 15-25% calcified
• mainly metabolic (highly metabolically active due to large surface area)
• large surface area (due to criss-cross nature)

Question 7

Describe the structure of a bone.

Answer 7

• Cortical/compact bone on the outside of the tube
• Trabecular/spongy bone in the middle.
• Crossing the Medullary cavity - the space enclosed by the cortical bone.
• The main shaft in the middle of the bone is called the Diaphysis.
• The ends of the bones are called the Epiphysis.
• Epiphysis and diaphysis are separated by growth plate
• Region below the growth plate separating the epiphysis and diaphysis is called the Metaphysis.

Question 8

How is bone development completed?

Answer 8

Bones develop throughout growth and into adulthood

• Growth plate fusion and ossification completes development
• Clavicles growth plates fuse at around 20 years old

Question 9

What are the two different processes by which bone develops?

Answer 9

Intramembranous ossification (inside the membranous tissue)

• Direct differentiation of osteoblasts (bone forming cells) from mesenchyme precursors in the mesenchymal connective tissue and begin to lay down layers of bone
• Flat bones typically go through this process

Endochondral ossification (within the cartilage)
- Bones form from a pre-existing cartilage model
- Long bones mainly go through this process e.g. femur/limb bones
(This process is what allows us to have rapid linear growth during childhood and adolescence)

Question 10

What happens in endochondral ossification?

Answer 10

Early foetal development - cartilage scaffold of long bones forms within limb buds.

By birth - scaffold has expanded in size, has an issue with vasculature within the cartilage model. Centre of the cartilage model becomes hypoxic. To correct this, blood vessels then invade the scaffold and bring the precursors of the bone cells which convert the centre of the cartilage scaffold into bone. Known as the primary ossification centre. Ossification then expands out of the ossification centre in the diaphysis, moving gradually towards the epiphysis of the bone.

Childhood - bone continues to increase in size, a secondary ossification centre will form in the epiphyses. This remains separate from the primary ossification centre via a cartilage structure known as the growth plate. This allows rapid linear growth and increase height.

Growth then finally finishes and bone development is complete when the cartilage growth plates finally becomes ossified. This fuses the primary and secondary ossification centres together.

Question 11

Describe the distinct zones in the chondrocytes.

Answer 11

Reserve zone - though to contain the stem cell population in growth plates. Proliferate very slowly and maintain their population. Close to the blood supply in the secondary ossification centre.

Proliferative zone - chondrocytes highly proliferative and form distinct column-like structures as they divide. Cells move further away from the epiphysis and the blood supply. Chondrocytes undergo hypertrophic
differentiation.

Hypertrophic zone - chondrocytes expand in size and produce a specific collagen called collagen 10. The further into the hypertrophic zone the cell is, the larger it will be as well as it is further from the blood supply. Eventually they apoptose,

Calcified cartilage zone - left behind after the apoptosis

Ossification zone - calcified cartilage is then converted into bone

Question 12

List three specialised bone cells?

Answer 12

Osteocytes - Mature bone cells. Make up over 90% of all cells found in bone tissue. They are a mechanosensory network embedded in mature bone.

Osteoclasts - Bone breaking cells. Multi nuclear cells that resorb/remove (break down) bone.

Osteoblasts - Immature bone cells. Produce new bone via the secretion of osteoid

Question 13

Describe osteocytes.

Answer 13

• Embedded in lacunae in mature bone
• Connected together via ‘dendritic-like’ processes moving through canalicular channels
• Form a mechanosensory network throughout bone
• Network can detect pressures and strains going through bone and signal if it needs other cells to come in and make any necessary changes to the tissue

Question 14

Describe osteoclasts.

Answer 14

• Bone breaking cells
• Initially formed from fusion of monocyte macrophage precursors.
• Osteoclasts seal off a portion of bone beneath them from the rest of the environment via an actin ring
• They then secrete acids and enzymes onto the bone to ‘resorb’ the sealed off portion of bone

Question 15

Describe osteoblasts.

Answer 15

• Form new bone
• Secretes osteoid - contains organic component of new bone
• Osteoid mineralised over time, incorporating the inorganic portion of bone tissue to become mature bone
• Some osteoblasts are embedded in new bone (osteoids) during this process and differentiate into mature bone cells known as osteocytes. This is how the osteocyte network is maintained in newly forming bone.

Question 16

Describe the bone remodelling cycle.

Answer 16

• As you move around, your bones are exposed to stresses and strains which can cause micro fractures to form (typically within older bone tissue)
• These are detected by the osteocyte mechanosensory network which signals for osteoclasts to come in to differentiate at the site of damage and resorb away the old damaged bone. When they are done, they undergo fission and dedifferentiate back into the individual mononuclear cells.
• The osteoblasts then arrive and secrete osteoid to produce an amount of new bone equal to the amount removed by the oestoclasts.
• The bone becomes mineralised and the bone becomes repaired

Question 17

List key endocrine and paracrine factors in bone remodelling regulation.

Answer 17

Key Endocrine factors

• Estrogen
• Thyroid hormone
• PTH

Key Paracrine factors

• RANKL (RANK Ligand)
• Various Wnt signalling factors

Question 18

What is the RANK receptor?

Answer 18

• Found on the surface of osteoclasts and their precursors

- Activation required for osteoclast differentiation and survival

Question 19

What is RANK Ligand?

Answer 19

• Master regulator of osteoclasts
• In the absence of RANK Ligand you wont have any differentiation of oestoclasts and soon as RANK Ligand is removed there will be apoptosis of osteoclasts.
• RANK Ligand is produced by cells of the oestoclasts lineage - including both oestoblasts and osteocytes and can be secreted or found bound to the cell membrane
• Binds to the RANK receptor. When this happens a signalling cascade through the nuclear factor K beta pathway results in the differentiation, fusion, maturation and activation of oesteoclast precursors into mature oesteoclasts.

Question 20

What is OPG (osteoprotegerin)?

Answer 20

• Bone protecting molecule
• Cells of osteoblast lineage also produce this substance
• Decoy receptor for RANKL (RANK Ligand)
• Inhibits the differentiation of osteoclasts by binding up the available RANK Ligands before it can interact with the RANK receptor on the osteoclast precursors

Question 21

Describe Wnt signalling in bones.

Answer 21

• Wnt stimulates the differentiation of osteoblasts from their mesenchymal precursors
• Wnt ligands bind onto a cell membrane receptor called the Fz receptor. It is a seven transmembrane spanning protein with similarities to a G-protein
  coupled receptor
• Before it can be activated by a Wnt ligand, frizzled must be in a complex with a co-receptor
• When Wnt binds to the complex of LRP5/6 and Fz, it canonically sets off a signalling cascade that results in the translocation of beta catenin into the cell nucleus
• This causes a change in gene expression and leads to osteoblast differentiation.

Question 22

Describe Wnt antagonism.

Answer 22

• Sclerostin and Dkk-1 bind to LRP5/6 and prevent them from interacting with the frizzled receptor.
• Therefore when these two are present Wnt cant have an affect
• So beta catenin does not enter the nucleus
• Osteoblast differentiation is inhibited

Question 23

How do osteocytes carry out regulation?

Answer 23

• Regulate osteoclast differentiation and activity by producing RANK Ligand as well as inhibiting it by producing OPG. Balance of RANKL and OPG production by osteocytes controls rate of osteoclast differentiation and activity.
• Osteocytes also produce Sclerostin and DKK1 so can differentiation of osteoblasts.

Question 24

What is Osteopetrosis?

Answer 24

• Excess bone formation. There are a few conditions that can cause it.

LRP5 activating mutations:
- Wnt signalling pathway is continuously activated, resulting in increased osteoblast differentiation throughout life. Results in unusual facial features and
difficulty swimming.

van Buchem’s disease and SOST:
- Loss of function in the SOST gene that encodes Sclerostin produced by the osteocytes. So no inhibition of the Wnt signalling within the osteoblasts and so there is increased osteoblast differentiation and bone formation throughout life. Progressive and results in trapped nerves, palsies and headaches due to increased intracranial pressure.

Question 25

What is Osteoporosis?

Answer 25

• Osteoporosis - ‘porous bones’
• Defined as having a bone mass greater than 2.5 SD below average peak bone mass
• Estimated that it will effect 1:2 women and 1:5 men.
• Causes 500,000 low impact bone fractures every year in the UK
• Can be primary (menopause, aging), or secondary (drugs, disease, lifestyle e.g smoking/alcohol)
• During menopause, the loss of oestrogen causes a temporary increase in osteoclast activity, resulting in a sudden loss of bone that lasts until the body adapts to the new oestrogen levels. So there is a sudden loss in bone mass in women during menopause.