13 - Bone Cell Biology Flashcards
What are the four functions of bone?
- Infrastructure
- Bone marrow
- Reservoir of Ca2+ and phosphate
- Specialized connective: realize that bone is calcified ECM
What are the components of bone as connective tissue (CT)?
Cells:
- Osteoblasts
- Osteocytes
- Osteoclasts
Matrix
Osteoclasts are activated by ________ and inhibitied by ________.
Osteoclasts are activated by PTH (parathyroid) causing release of calcium, and inhibited by calcitonin (thyroid).
What percentage of bone is inorganic and what are the components? Why is this significant?
70% is inorganic
Calcium and phosphorous - hydroxyapatite (this is why bone is hard)
99% of the body’s calcium is stored in bone
What percentage of bone is organic (osteoid) and what are the components?
~30%
Type 1 collagen confers acid(eosin)ophilia (stains red)
Proteoglycans: less than in cartilage
Glycoproteins: promote hydroxyapatite - osteocalcin is a bone specific glycoprotein that promotes mineralization
How does bone differ from hyaline cartilage based on mineral, water, collagen, and neuronal/vascular structures?
Mineral: Bone 70%, HC none
Water: Bone 25%, HC 75%
Collagen: Bone type I, HC type II
Neuronal and vascular structures: Bone present, HC none
What are osteoblasts and where do they come from?
Specialized fibroblasts.
Differentiate from mesenchymal stem cells (MSCs) in the presence of growth factors such as bone morphogenetic proteins (BMPs)
What do groups of osteoblasts make? How does bone formation occur?
Osteoid - type I collagen and glycoproteins (single osteoblasts can’t do this).
Bone formation: completed by deposition of Ca2+ in the osteoid (note, without hydroxyapatite there’s no weight bearing).
Knockout of what prevents bone development?
Runx2 (aka Cbfa1), the bone ‘master gene’
Note: Runx2 and osteocalcin are osteoblast-speficic and differentiate osteoblasts from fibroblasts.
Where are osteocytes located? How much space do they take up and what do they bind to?
- Occupy lacunae between lamellae of bone matrix; there’s one osteocyte in each lacuna (cartilage has more than one per lacunae).
- Osteocytes make up 90% of all bone cells.
- Cytoplasmic “dendrites” penetrate the matrix and bind to other osteocytes via gap junctions.
What is the function of osteocytes? How long do they live?
Mechano-sensation (cell to cell signals) - regulating bone remodeling
Secrete sclerostin, which inhibits Wnt signaling in osteoblasts - stopping bone growth.
Long lived - T1/2 = 25 years
What are the cytoplasmic processes of osteocytes called?
Canaliculi.
What is the function of osteoclasts? How do they develop?
Destroy bone matrix for remodeling.
Multinuclear cells that form from macrophage-like cells that fuse together.
Where are osteoclasts located?
Reside in hollowed-out areas of matrix termed “Howship’s lacunae”.
Have ruffled borders that attach ECM, forming a microenvironment for bone resorption.
How are osteoclasts regulated by hormones? Describe the biochemistry invovled in bone resorption.
Calcitonin from the thyroid inhibits osteoclasts, while PTH from the parathyroid activated osteoclasts.
- Lysosomes in osteoclasts secrete cathepsin-K creating a microenvironment that destroys the bony matrix
- CO2 > H2CO3 (carbonic acid) > HCO3- (bicarb) + H+
- This creates an environment optimized for bone resorption
What is this a picture of?
An osteoclast located in howship’s lacuna with 3-4 nuclei (normal, may have 7-10)
An osteoblast nearby (these are smaller).
What is the structure of bone?
Bone is lined by outer and inner layers of connective tissue respectively termed periosteum and endosteum.
Where are osteoblasts, osteocytes, and osteoclasts located in bone?
Osteoblasts are located in the periosteum, with smaller numbers in the endosteum
Osteocytes are in the lacunae of the bony matrix, which is between the periosteum and the and endosteum
Osteoclasts are mostly attached to the bony matrix on the endosteal side
Describe compact vs spongy bone? What percentage of long bone does each makeup?
Compact (aka cortical bone) is dense with no cavitation and makes up 80% of long bone.
Spongy (aka cancellous aka trabecular bone) is cavitated and makes up 20% of lone bone.
What are flat bones? What is an example?
2 plates of compact bone surrounding diplöe of spongy bone.
Exp: calvaria of the skill
What are the components of long bones?
Diaphysis (shaft): compact, with spongy bone lining marrow
Epiphyses (ends): caps of compact bone around spongy bone
Long bones are both ______ and ______.
Cortical and trabecular.
Trabecular bone is not as organized as cortical bone.
What is an osteon in a long bone? What other structures are present?
Osteon: cylinder with concentric lamellae = the unit of bone structure
Lamellae have lacunnae that:
- harbor osteocytes
- are connected via canaliculi
The innermost lamella of long bones surround what?
The Haversian canal - which contains blood vessels, nerves, and lymphatics and travel lengthwise in the bone.
Haversion canals are connectes by Volkmann’s canals which carry the same structures but travel horizontally.
How do bones develop? What are the two possible ways?
From primary bones (immature, woven) to secondary bone (lamellar; mature)
- Intramembranous: osteoblasts deposit osteoid into loose framework of reticular CT (not really a membrane)
- Endochondral: osteoblasts deposit osteoid into cartilage
Describe endochondral bone development?
Bone forms on hyalin cartilage.
At the diaphysis osteoblasts invade calcified cartilage and secrete osteoid > ossification.
At the epiphyses same process; articular cartilage remains at ends of bone, while epiphyseal plate cartillage remains for growth in length.
- zones of epiphyseal growth
How do long bones get long?
Sex steroid hormones > pituitary > growth hormone (GH;somatotropin) > liver > insulin-like growth factor-1 (IGF-1) > epiphyseal plate
What are the ‘zones’ of epiphyseal growth
Zone of cartilage proliferation: activated by IGF-1
Zone of cartilage hypertrophy: 20% of fractures
Zone of cartilage calcification (collagen X not 1)
Zone of ossification: eosinophilia due to deposition of collagen 1 by osteoblasts
What are the four steps of fracture repair?
- Macrophages remove debris
- Fibroblasts and chondroblasts secrete a fibrocartilaginous callus
- Osteoblasts replace fibrocartilaginous callus with bony callus
- Primary bone is replaced by lamellar secondary bone
What do difficult fractures require? How common is this?
Grafting
~500,000/yr in the US
Some don’t heal; patients lack good bone.
What is in pipline for bone tissue engineering? What are some present issues?
- Bone morphogenetic proteins: BMP-2 and BMP-7: but not regulated enough and cause too much bone growth
- Adult stem cells (MSCs) > osteoblasts
- Growth factors or cells, or both, are implanted within biodegradable ‘sponges’ made of collagen 1.
We get a new skeleton every ______ years. ______ excavat bone, which is then replaced by activated _______.
We get a new skeleton every ~10 years.
Osteoclasts ecavate bone, which is then replaced by activated osteoblasts.
What are two disease of bone remodeling?
Osteopetrosis: dense heavy bone; osteoclasts lack ruffled border.
Osteoporosis: resorption by osteoclasts outpaces osteogenesis = hollow fragile bones. Can cause broken wrists, hips, spine.
How common is osteoporosis? Who gets it?
~28 million americans, 2/3 women.
Post-menopausal women lose ~2% of bone mass annually.
Treatable.
How can you prevent, screen, and treat osteoporosis?
Prevent: diatary Ca2+, vitD (to improve VitC absorption), weight bearing exercise.
Screen: bone mineral density (BMD; grams/cm^2) compare to “young normal” subjects.
Therapeutic targets: osteoclasts and osteoblasts.
What role does parathyroid hormone (PTH) play in osteoClast production?
It induces stromal cells in the bone marrow to secrete: RANKL, OPG, and M-CSF. These act on monocytes.
What effect do RANKL, OPG, and M-CSF (secreted by stromal cells in the BM) have on osteoClast production?
- M-CSF (macrophage colony stimulating factor): induces monocyte/mcrophage proliferation.
- RANK-L (receptor for activator of nuclear factor-kb ligand): induces differentiation into osteoclasts
- OPG (osteoprotegerin): antagonizes RANK-L by binding its receptor; ie OPG inhibits osteoclast production
What inhibits osteoBlasts? What induces them?
Inhibited by leptin (obesity > risk for osteoporosis)
Induced by BMP
Induced by PTH
What inhibits osteoClasts? What induces them?
Inhibited by calcitonin, osteoprotegerin
Induced by RANK-L
Induced by PTH
*calcitonin is not used clinically; this is just an example of targets that could be used to manage osteoporosis.
What is the “PTH paradox”? What is the resolution of this paradox?
That both osteoblasts and osteoclasts are induced by PTH.
Spikes of PTH levels attained by injection favor osteoBlast production, whereas constant PTH levels favor osteoClasts.
How can anabolic drugs help manage osteoporosis? What is an example?
They are pro-osteoblast.
PTH 1-34 (teriparatide) : resolution of PTH paradox:
- spikes of PTH levels attained by injection favor osteoBlast production (whereas constant PTH levels favor osteoClasts)
When are anabolic drugs (pro-osteoblast) clinically indicated?
At T scores worse than -2.5
This is because anti-resorptive drugs don’t work at this score.
How can anti-resorptive drugs help manage osteoporosis? What are some examples?
They are anti-osteoClast.
- SERMs: selective estrogen receptor modulators - relozifene
- Bisphosphonates: ibandronate - Boniva
- mAbs that bind RANKL - Denosumab
What drugs concepts are in the pipline for treating osteoporosis?
Drugs with more specificity:
- Osteoclast inhibitors (OPGs)
- Drugs that inhibit binding of osteoclasts to matrix
- Anabolic agents such as Runx2
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