Bone Tissue

Question 1

What are the functions of the bone?

Answer 1

Support
Provides insertion to muscles.
Protection of organs. 
A storehouse for minerals. 
The container for hematopoietic bone marrow.

Question 2

How many bones do human adults have in their body?

Answer 2

206

Question 3

What are the different classifications for bones?

Answer 3

Long bones, short bones, flat bones.

Question 4

Describe the general structue of bone.

Answer 4

Spongy bones in the middle surrounded by compact bone.

Question 5

Describe the appearance of spongy and compact bone.

Answer 5

Spongy bone is characterised by a mainly trabecular presence.

The compact bone appears as a compact layer of tissue that mainly characterises the diaphysis.

Question 6

Describe the bone marrow stored inside bone tissue.

Answer 6

Yellow bone marrow - mainly formed by adipocytes

Red bone marrow - hematopoietic bone marrow.

Question 7

What is the composition of bone tissue?

Answer 7

The tissue is formed by an extracellular matrix and cells.

The matrix is divided into organic and inorganic, where the inorganic is mainly calcium and phosphorus. They get together to form the hydroxyapatite.

The organic part is composed of collagen, proteoglycans and other non-collagenous proteins.

The cells present are osteroprogenitor cells, osteocytes, osteoblasts and osteoclasts.

Question 8

What are the functions of hydroxyapitite in the bone?

Answer 8

It’s the major component of the mineralized extracellular component of the bone.

The hydroxyapatite is responsible for the hardness of bone tissue.

Question 9

What are the functions of collagen and other proteins in the bone?

Answer 9

Collagen type I is found in the extracellular matrix of the bone tissue, it’s organised in a particular way that gives the bone tissue its strength.

There are also proteins responsible for adhesion of bone cells and regulation of bone homeostasis.

Question 10

What would happen to the bone if either the mineral or collagen part was removed?

Answer 10

Mineral - the bone becomes very flexible.

Collagen - the bone becomes very fragile.

Question 11

How can the bone be studied from a histological point of view?

Answer 11

Demineralization in acid solution (EDTA).

Removal of organic components with heat.

Question 12

Describe the method used to study the inorganic part of the bone.

Answer 12

This is obtained by leaving the tissue in water solution for several days, the organic component is degraded to leave the inorganic component.

A specific equipment that contains a ‘sandpaper’ reduces the piece of bone to a very thin size.

This section usually doesn’t need to be stained, we could use the light and air passing through the empty spaces of the cells that were degraded.

Question 13

Describe the appearance of bone after the organic part has been removed.

Answer 13

The black spots are called lacunae, and is where the osteocytes reside before being degraded.

While the fine structures are called canaliculi, which are very tiny channels in which the protrusions of the osteocytes were located.

Therefore this is a very fine way of making contacts between the central canal and the osteocytes present in the osteon.

Question 14

Describe the appearance of bone after the inorganic part has been removed.

Answer 14

The nuclei of the osteocytes can be seen. The presence of the canaliculi can’t be appreciated in this image.

Although the CT proper in which blood vessels and nerves are contained can be observed, as well as the edges of the osteocytes.

Question 15

Describe the non-lamellar structure of bone.

Answer 15

Immature bone is non-lamellar and is formed by woven collagen fibers.

Question 16

Describe the non-lamellar (bundle or woven) structure of bone.

Answer 16

Immature bone is non-lamellar and is formed by woven collagen fibers.

Question 17

Describe the lamellar structure of bone.

Answer 17

Immature bone is replaced by lamellar bone, so the organization of a mature bone tissue is a lamellar organisation.

On the surface of each lamella, small cavities are carved out called lacunae, which contains osteocytes (surrounded by tissues). A non-calcified extracellular matrix surrounds the osteocytes.

Each osteocyte communicates with a nearby osteocytes through the net of canaliculi. They are very important as they interconnect all lacunae with blood vessels.

Question 18

Why are the canals present in lamellar bone?

Answer 18

The extracellular matrix of the bone tissue is mineralized (non-hydrated), and can not have any diffusion process occuring in the extracellular matrix.

There needs to be an internal network of canals that allows the passage of nutrients from the blood vessels to each of the osteocytes in the lacuna.

Question 19

Describe the haversian system in a compact lamellar bone.

Answer 19

Lamellae forms cyclinders that are arranges inside one another.

The cavity delimited by the innermost lamella is called Haversian canal and contains blood vessels and nerves.

The set of lamellae surrounding a canal is called Haversian system (osteon).

Question 20

What structure do the collagen fibers form on each lamella?

Answer 20

The collagen fibers (type I) form the supporting scaffold for the mineralized matrix of the lamellae.

Arranged in parallel but their orientation is antiparallel.

Direction changes in adjacent lamellae with an inclination of 90o, this creates an alternating pattern.

This helps the bone withstand tortion stress.

Question 21

How do osteocytes contact nearby osteocytes?

Answer 21

Osteocytes have tiny protrusions which enter inside the canaliculi and make contact with nearby osteocytes.

There are formations of gap junctions in between.

Question 22

What are perforating channels (Volkmann’s canals)?

Answer 22

At haversian canals, there are blood vessels passing through.

Perforating canals (Volkmann’s canals) connect blood vessels contained at the peripheral of the bone tissue with ones inside the compact bone.

Question 23

What are interstitial systems?

Answer 23

In between complete osteons, there is lamellar bone that doesn’t form a real osteon.

These are called interstitial systems, which is proof that the bone is not static and undergoes remodeling through our lives.

Question 24

What are circumstantial lamellae?

Answer 24

Layers of lamellae which runs for the entire circumference of the bone tissue not forming true osteons - circumferential lamellae.

Question 25

Describe the structure of spongy bone as a lamellar bone.

Answer 25

Spongy bone is lamellar bone.

Though the lamellae in spongy bones are not organised to form osteons but instead forms trabeculae.

Question 26

Describe the periosteum.

Answer 26

Outer fibrous sheath of dense irregular CT covering of the bone except articular surfaces.
2 layers:
Outer fibrous layer
Inner cellular (osteoprogenic) layer - where osteoprogenitor cells are contained.
Anchored to bone tissue due to Sharpey’s fibers.

Question 27

Describe the periosteum.

Answer 27

Outer fibrous sheath of dense irregular CT covering of the bone except articular surfaces.

2 layers:
Outer fibrous layer
Inner cellular (osteoprogenic) layer - where osteoprogenitor cells are contained.
Anchored to bone tissue due to Sharpey’s fibers.

Question 28

Describe the endosteum.

Answer 28

Covers inner surface.

Contains bone lining cells - osteoprogenitor cells very important for maintenance of hematopoietic stem cells.

Question 29

Where do osteroprogenitor cells reside?

Answer 29

Osteoprogenitor cells reside at the peripheral.

Question 30

What is the function of osteoblasts?

Answer 30

Osteoblasts are responsible for deposition of the extracellular matrix, they remain entrapped in the deposited extracellular matrix.

Question 31

What is the function of osteoblasts?

Answer 31

Osteoblasts are responsible for deposition of the extracellular matrix, they remain entrapped in the deposited extracellular matrix.

Stimulate production of osteoclasts.

Probably contributes to maintenance of ‘niche’ of hemopoietic stem cells.

Question 32

What are the functions of osteocytes?

Answer 32

Involved in bone turnover.
The osteocyte network is involved in ion exchange (mineralization).
Role in functional adaption of the bone.

Question 33

What happens if there’s a mutation of the Runx2/Cbfa1 gene?

Answer 33

If there is a mutation in this master gene, a pathological condition may result in which there is an impaired ossification. E.g. in cleidocranial dysplasia, the clavicular bones don’t form.

Question 34

Describe the origin and formation of osteoblasts.

Answer 34

Formes from mesenchymal cells or osteoprogenitor cells of the periosteum and the endosteum in the bone marrow.

Numerous growth factors, such as BMPs, regulate the formation of osteoblasts.

Question 35

How do osteoclasts degrade the mineralized matrix?

Answer 35

Sitting in an area completely isolated from the rest.

By acidifying the area, the mineralized component is completely degraded.

Question 36

How can osteoclasts regulate calcium levels?

Answer 36

Through the ability to reabsorb the mineralized component of bone, they are responsible for regulating the level of calcium in blood.

So if calcium levels are low, PTH (Parathormone) is released and indirectly stimulates formation of osteoclasts.

Question 37

What histochemical method can be used to identify osteoclasts?

Answer 37

They can be identified by cytochemical staining, e.g. TRAP staining (Tartrate Resistant Alkaline Phosphatase).

Question 38

How can osteoclasts regulate calcium levels?

Answer 38

Through the ability to reabsorb the mineralized component of bone, they are responsible for regulating the level of calcium in blood.

So if calcium levels are low, PTH (Parathormone) is released and indirectly stimulates formation of osteoclasts, which breaks down bone to release calcium into the bloodstream.

Question 39

What is the function of calcitonin?

Answer 39

Calcitonin (released by thyroid) acts directly on osteoclasts, inhibiting their action.

Question 40

How does PTH stimulate formation of osteoclasts?

Answer 40

The action of the PTH is indirect as the receptor for PTH is contained by osteoblasts.

They respond to PTH by expressing RANK ligand, which binds to its receptor (RANK).

This is expressed by macrophages and mesenchymal cells, together with another signal CSF1, they induce cell fusion - formation of osteoclasts.

Question 41

What causes osteopetrosis?

Answer 41

If there is impairment of CSF factor, osteoclasts are not formed to reabsorb the mineralised component of the bone.

Consequently a continuous increase in size and thickness of bones, severe condition which leads to death.

Question 42

Describe the condition of osteopetrosis.

Answer 42

A rare hereditary bone disorder characterised by the increase in bone density due a defect in bone remodelling caused by failure of normal osteoclast functions.