Bone Chemistry and Histology

Question 1

What cells are found in Haversian systems?

Answer 1

osteocytes

Question 2

What are the main components of bone?

Answer 2

The organic matrix is compsed primarily of the protein collagen which provides flexibility. 10% of adult bone mass is collagen

The mineral component is composed of hydroxyapatite, which is an insoluble salt of calcium and phosphorus. About 65% of adult bone mass is hydroxyapatite.

* Bone also contains small amounts of magnesium, sodium, and bicarbonate.
* Water comprises approximately 25% of adult bone mass.

Question 3

Describe osteoclasts

Answer 3

large cells with many nuclei that derive from monocytes and are formed from fusion of precursors. After resorbing bone, osteoclasts undergo apoptosis

Question 4

Describe osteoblasts

Answer 4

Cuboidal and columnar shaped with a central nucleus located on the bone surface. They come from bone marrow precursor cells. These precursors are capable of turning into either osteoblasts or fat cells, and various factors determine which kind of cells will be made. One of the factors is called Cbfa 1, which will cause the cell to differentiate into an osteoblast.

Question 5

How do osteoblasts communicate?

Answer 5

Gap junctions with neighboring osteoblasts allow cells to communicate with each other

Question 6

What receptors do osteoblasts have?

Answer 6

They have receptors for hormones such as vitamin D, estrogen, and parathyroid hormone.

Question 7

What is PHEX?

Answer 7

a protein secreted by osteoblasts that helps to regulate the amount of phosphate excreted by the kidney.

Question 8

What happens to osteoblasts when they finish making bone?

Answer 8

When the team of osteoblasts has finished making new bone, some become surrounded with matrix and differentiate into osteocytes. Others will remain on the surface of the new bone and differentiate into lining cells. The rest undergo apoptosis (cell suicide) and disintegrate

Question 9

What are osteocytes?

Answer 9

These live inside the bone and have long branches which allow them to contact each other as well as the lining cells on the bone surface.
* . . . are in a perfect position to sense any mechanical strain on the bone.
* . . . can secrete growth factors which activate the lining cells or stimulate the osteoblasts.
* Their exact role is still under investigation, but probably the osteocytes direct bone remodeling to accomodate mechanical strain and repair fatigue damage.

Question 10

What are lining cells?

Answer 10

former osteoblasts which have become flat and pancake-shaped.
* . . . line the entire surface of the bone.

Question 11

What are the roles of lining cells?

Answer 11

These are responsible for immediate release of calcium from the bone if the blood calcium is too low.

• protect the bone from chemicals in the blood which dissolve crystals (such as pyrophosphate).
• have receptors for hormones and factors that initiate bone remodeling.

Question 12

Answer 12

This diagram summarizes the origins and fates of the bone cells. Mesenchymal refers to cells which were deep within the embryo during early development; some of them remain in the bone marrow but do not form blood cells. The hematopoietic cells form the liquid part of the bone marrow, and some of them circulate with the blood.

Question 13

What are Bone morphogenetic proteins (BMPs)?

Answer 13

BMPs are produced in the bone or bone marrow. They bind to BMP receptors that are on mesenchymal stem cells within the bone marrow. This causes the cells to produce Cbfa1, a transcription factor that causes cells to differentiate into mature osteoblasts. Without Cbfa 1, the cells would turn into fat cells instead!

Question 14

What do Insulin-like growth factors (IGFs) do?

Answer 14

These growth factors are produced by osteoblastic cells in response to several bone active hormones, such as PTH and estrogens, or BMPs. IGFs accumulate in the bone matrix and are released during the process of bone remodeling by osteoclasts. IGFs stimulate osteoblastic cell replication – in other words, they cause the osteoblasts to divide, forming new cells. They may also induce differentiation.

Question 15

What do Interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF) in terms of bone?

Answer 15

These factors are produced by osteoblastic cells in response to systemic hormones or other cytokines.

IL-6 can cause:
* Bone marrow stem cells to differentiate into pre-osteoclasts
* Changes in proliferation and differentiation of osteoblasts
* Inhibition of apoptosis of osteoblasts

Question 16

What is the role of RANKL (RANK-ligand)?

Answer 16

This is a cytokine that stays on the surface of osteoblast-related cells. The cells make RANKL in response to systemic hormones (such as 1,25dihydroxyvitamin D3) and cytokines (such as IL-6). Cell contact between RANKL-expressing osteoblastic cells and RANK-expressing osteoclast precursors induces osteoclast development.

Question 17

Apoptosis of bone

Answer 17

The mature bone is always remodeling: the old bone is resorbed and replaced with new bone. A team of osteoblasts and osteoclasts move along the bone, dissolving and rebuilding. What happens to the cells when they have finished rebuilding an area of bone? The osteoclasts and most of the osteoblasts undergo apoptosis. They are not killed. There is no lack of oxygen or nutrients. There are no toxic materials. Instead, there are genes in the cell which can be activated, causing the cell to disintegrate. These genes (of course) are carefully regulated within the cell. The factors that regulate apoptosis are currently under investigation. Some are related to estrogens, or to interleukins. Medications which could modify apoptosis have the potential for treating or preventing osteoporosis.

Question 18

Osteoclasts

Answer 18

• Osteoclasts are multinucleate giant cells formed by differentiation and fusion of monocytes. Differentiation involves cell-cell interaction with lining cells/pre-osteoblasts through a RANK-RANKL interaction.
• They resorb a region of bone matrix sealed off below the cell body by releasing acid and proteases into the sealed off space.
• Demand for calcium stimulates bone resorption to release Ca2+ from mineralized matrix. Bone acts like a bank for deposits and withdrawals of calcium.

Question 19

•Differentiated osteoblasts make _____ a collagenous matrix containing bone-specific proteins.

Answer 19

osteoid

Question 20

Osteoblasts that become entrapped in the matrix are called _______ that can communicate with each other and with osteoblasts through cell processes in canaliculi.

Answer 20

osteocytes

Question 21

How do long bones grow in children?

Answer 21

from a growth plate, composed of cartilage, near the end of the bone. After new bone is formed under the growth plate, the wide sides are resorbed by osteoclasts (because the diaphysis narrower). These stages are accompanied by periosteal expansion, which increases the size of the sides of bone

Question 22

How does growth in flat bones occur?

Answer 22

Intramembranous ossification: The growing skull, mandible (jaw) and clavicle (collar bone) form directly from osteoblasts, and do not have a cartilage stage.

A layer of cells which can differentiate into the osteoblasts lines the bone, with a special concentration between the bones of the skull. These precursor cells turn into osteoblasts which secrete collagen in a haphazard pattern called woven bone. Later osteoclasts resorb (dissolve) the woven bone and new osteoblasts form the mature, lamellar (layered) bone.

Question 23

How does bone respond to stress (1 of 2)?

Answer 23

1) The osteocytes near the crack undergo apoptosis
2) Other osteocytes detect the strain and secrete factors, including growth factors, prostaglandins, and NO
3) The lining cells pull away from the bone matrix and form a canopy which merge with the blood vessles (steps here are not well understood). Meanwhile, stromal cells are now released from sclerostin inhibition and/or exposed to other factors like IL-1 and generate pre-osteoblasts and secrete M-CSF which helps generate pre-osteoclasts
4) The pre-osteoblasts proliferate and secrete more factors, such as WnT, ILs and BMPs
5) The pre-osteoblasts then start to express RANK-L
6) Pre-osteoclasts bind to RANKL receptors and enlarge and fuse into mature osteoclasts

Question 24

How does bone respond to stress (2 of 2)?

Answer 24

1) The osteoclasts then bind to bone matrix with integrins and secrete acid and cathepsin K to resorb the bone. This takes about 2 weeks. Bone-derived growth factors IGF adn TGF-B are released
2) Eventually the osteoclast undergoes apoptosis.
3) Pre-osteoblasts mature into osteoblasts which stop making RANKL and secrete OPG to block differentiation of pre-osteoclasts
4) The osteoblasts proliferate and line the resorbed cavity
5) the osteoblasts secrete osteoid and then mineralize it to fill in the cavity in 3-4 months. The matrix also contains IGF and TGF-B. Some osteoblasts turn into osteocytes some into lining cells, and rest undergo apoptosis
6) The canopy regresses and the osteocytes reform a network with each other and the lining cells
7) The new matrix will accumulate mineral and icnrease in density for about 3 yrs

Question 25

What is a BMU?

Answer 25

BMU means “Basic Multicellular Unit” and also has been called “Bone Remodeling Unit”. Remodeling actually happens in 3-D, spreading over an area on the surface of the bone

Question 26

How does fracture repair occur?

Answer 26

1) blood clot forms immediately after the fracture
2) fibroblasts arrive from the periosteum and blood
3) some fibroblasts become osteoblasts, which make collagen (similar to formation of a scar)
4) After about 2 weeks, the collagen becomes mineralized which results in a ‘callus’
5) The callus gradually remodels and eventually is replaced

Question 27

Answer 27

The first x-ray was taken right after a fracture. The second was 2 months later, showing some callus. Notice that the leg is now in a cast, so the entire bone looks a little more dense and fuzzy. The last x-ray was 4 months after the fracture, showing a good callus. The bone now can bear weight, but it will take many months to remodel the area and complete the repair.

Question 28

Endochondral bone formation

Answer 28

• Bone forms on a cartilage scaffold in the process of endochondral bone formation. The cartilage becomes calcified and then is replaced by bone matrix and a vascular bed at the growing ends of bone.
• Components of cartilage like type II collagen and aggrecan are replaced by bone specific proteins like osteocalcin and a highly crosslinked type I collagen (bone collagen crosslinks are a commonly used marker for clinical bone turnover measurement).

Question 29

What are the effects of estrogen on bone?

Answer 29

inhibit osteoclasts and osteoclast precursors

decrease IL-6

may enhance osteoblast survival

Question 30

Effects of PTH?

Answer 30

stimulates osteoblast activity

increases RANK-L expression on pre-osteoclasts

Question 31

What are the effects of VitD on bone?

Answer 31

promotes differentiation of osteoblasts and osteoclast precursors

Question 32

What are the effects of calcitonin on bone?

Answer 32

inhibits osteoclast function

Question 33

What are the effects of cortisol on bone?

Answer 33

promotes osteoblast apoptosis and increases bone resorption

decreases GI calcium absorption

increases renal calcium excretion

decreases sex steroid production

Question 34

What are the effects of VitA?

Answer 34

directly stimulates osteoclasts

Question 35

What are the effects of thyroid hormone?

Answer 35

stimulates osteoclasts

Question 36

What are the effects of GH/IGF-1?

Answer 36

increase osteoblast function

Question 37

Osteoporosis

Answer 37

The osteoporotic fractures occur throughout the skeleton. The most typcial fractures are of the spine, wrist and hip. Osteoporosis is not painful until the bone actually breaks. Fractures in hips and wrists are always painful, but about 2/3 of the spine fractures do not cause pain. The spine (vertebral) fractures cause height loss, curvature of the spine, disfiguring posture, and muscle aching. In severe cases the ribs can touch the pelvic bones. The lungs can’t expand normally and heartburn is more frequent.

Question 38

Osteoporosis is more common in what ethnicities?

Answer 38

caucasians and asians

Question 39

What is Paget’s disease?

Answer 39

A disorder of bone remodeling which is focal (located in discrete areas in the bones and not throughout the entire skeleton). The abnormal remodeling results in overgrowth or deformity of bone.

Question 40

Epidemiology of Paget’s disease

Answer 40

Paget’s disease usually occurs in adults who are older than 55 years old, and there are slightly more men than women with the disease. The disease does not spread, but remains in the same bones as patients grow older. It it most commonly found in the skull, spine, pelvis, femur, and tibia.

Whereas some patients have only one bone with Paget’s disease, others have Paget’s in many bones.