Bone

Question 0

What is trabecular/spongy bone, where is it located, and what is its function?

Answer 0

type of bone on the inner surface bt layers of compact bone, metabolic fn

• allows bones to resist compression
• bone marrow is found in gaps in trabecular bone
• makes bones lights because not solid (thin and porous)

Question 1

What are the main functions of bone?

Answer 1

1) mechanical function
2) bone marrow gives rise to red and white blood cell
3) repository for Ca2+ (release ca from bone and synthesize new bone as a way to store ca)
4) protects organs

Question 2

Compact bone

Answer 2

• solid mineral but microscopically contains cells, bvs, and nerves
  composed of concentric lamellae that form Haversian canals that contain bvs and nerves
• tendons and ligaments attach, mostly mechanical fn, thick and dense

Question 3

What are the different regions of bone called?

Answer 3

epithasis (top of bone) and diaphasis (shaft of bone)

metaphysis (where diaphasis joins the epiphyseal growth plate–> cartilage proliferates at the EGP to elongate bone)

Question 4

What are the different bone surfaces called?

Answer 4

periostium covers the outer surface of the bone with a fibrous material (collagen and other ECM material)
endostium covers the inner surface of bone (endostial surface)

Question 5

What is bone composed of?

Answer 5

Type 1 collagen: 23% (osteoid = type1 collagen + glycosaminoglycans/GAGs)
Hydroxyapatite crystals (calcium and phosphate)- 65%
water 10%
other proteins 2%

Question 6

What are the different types of fibril arrangements ?

Answer 6

1. Lamellar (parallel organization in layer, perpendicular to next layer); can resist tension in multiple directions
2. Woven: less organized, first type of bone laid down in fetal development; laid down first when you have a break as a stop gap measure and is then replaced by lamellar bone

Question 7

What are the types of bone cells?

Answer 7

Osteoblasts, osteoclasts, osteocysts

Question 8

What is the function of osteoblasts?

Answer 8

synthesize bone; synthesize and secrete type I collagen; synthesize proteins that concentrate calcium and phosphate (increase rate that Ca/phosph. accumulates on microfibrils

Question 9

Osteocysts

Answer 9

former osteoblasts that got trapped in the mineralizing bone matrix but are still function

• hypothesized to respond to stress in bone and change the way that bone is synthesized; augment synthesis due to compressive force; digest bone when we need ca and build bone if there is excess ca.
• have projections into the cytoplasm called philapoteia/caniculli that make gap junction between them and other philapoteia/ caniculli on other osteocysts and lets them communicate

Question 10

Osteoclasts

Answer 10

digest bone
multinucleated (2-5 )
secrete H+ onto bone surface that helps digest Ca and Ph crystals ; secrete proteases that digest fibrils

Question 11

What is the function of the sealing zone around osteoclasts?

Answer 11

• prevent H+ and proteases from escaping into rest of bone and also prevents calcium and collagen from escaping (these material are moved via transcytosis to the basal side of the osteoclast)
• made of integrins attached to the bone surface

Question 12

How does bone modeling occur?

Answer 12

osteclasts and osteoblasts work together on opposite surfaces to reshape bone and shift bone orientation in a certain direction, this process allows bone to adapt to changes in compressive forces
- trabecular bone get aligned via bone modeling to orient with compressive force (Also happens in compact bone as you grow and develop)

Question 13

What is the purpose of bone synthesis?

Answer 13

increase length, increase Ca2+ stores

Question 14

What is the purpose of bone reabsorption?

Answer 14

increase ca levels

Question 15

What does the interplay between bone synthesis and reabsorp. achieve?

Answer 15

maintains bone integrity

Question 16

How does modeling change bone size/shape?

Answer 16

makes bone larger and more mechanically robust by increasing diameter of bone without changing density
osteoblasts work on periostial surface to lay down bone; osteoclasts eat bone on endostial surface

Question 17

How does modeling respond to changes in mechanical stress?

Answer 17

increases stiffness; increases thickness on surface where being forced to bend ( increases weight of bone but also strength)

Question 18

How does bone know where to model?

Answer 18

osteocytes sense compression changes through changes in fluid flow across the surface of the osteocytes
- primary (1) cillium “antennae” which are extensions of the cell membrane bend in response to fluid flow changes; triggers signaling cascade that brings in Ca to cell and activate osteocytes by turning on transcription factors

Question 19

What is the primary function of bone remodeling?

Answer 19

replaces old bone with new bone (overtime, general stress creates small fractures in bone that need to be repaired)
osteoclasts and osteoblasts work on the same surface (osteoclasts eat and osteoblasts follow)

Question 20

What are the steps in the bone remodeling process?

Answer 20

1) activation of osteoclasts : not sure how activation of osteoclasts is triggered, they eat up old bone
2) Reversal: time pt at which osteoclasts undergo apoptosis
3) Osteoblasts come in and make new bone (happens in trabecula and compact bone)
- osteoclasts tunnel through compact bone and erode old Haversian systems while forming new canals

Question 21

What is the disease state that occurs when osteoclast activity outpaces osteoblast?

Answer 21

osteoporosis, happens naturally when we age

Question 22

How are osteoclasts activated?

Answer 22

• monocytes are stimulated by monocyte colony synthesizing factor (MCSF) and become preosteoclasts
• reosteoclasts join with osteoblasts or stromal cells via the rank receptors on preosteoclasts that bind to rank ligand on the osteoblast/stromal cell and fusion occurs to create osteoclasts

Question 23

What is the function of osteoprotegenin ?

Answer 23

Osteoprotegenin (OPG) downregulates osteoclast formation by acting as decoy rank receptor that binds to the rank ligands on the osteoblast stromal cells and prevents them from binding to the preosteoclasts
- osteoblasts make OPG and thus regulate osteoclast synthesis

Question 24

What is the role of parathyroid hormone in bone growth?

Answer 24

decreases in calcium trigger parathyroid hormone which works on osteoblasts and cause them to increase their expression of rank ligand and decrease their production of OPG
- UPregulates osteoclast synthesis

Question 25

What is the function of estrogen in bone growth?

Answer 25

downregulates osteoclast activity by making them undergo apoptosis (direct effect )and also increasing OPG and decreasing expression of rank ligand (indirect effect)
- older women have decrease in estrogen production (menopause) so not as effective regulation of osteoclasts in addition to general aging

Question 26

What is the function of bisphosphotate ?

Answer 26

treats osteoclasts that live too long or are overproduced

• bisphos integrates into bone matrix and then osteoclasts eat the bisphos as they eat the bone
• bisphos gets into cystosol via transcytosis and triggers apoptosis of cell

Question 27

What are volkmann canals?

Answer 27

link haversian systems