Cartilage and Bones Flashcards
What 3 functions is cartilage structurally designed to do?
-withstand tension & compression
-provides low friction surface at joints and
to provide support to soft tissue.
-Provide a framework for long bone osteogenesis (during development).
Functional difference between chondrocytes and chondroblasts
- Chondroblasts produce new matrix until skeleton stops growing. They are ‘immature/undifferentiated’
- Chondrocytes are found in groups in cavities called lacunae. They are ‘mature cells’ and are less active.
5 differences between cartilage and bone
Cartlage is:
- More flexible
- Avascular (less blood supply, thus heals very slowly)
- Less organized structure
- No nerve fibers
- Composed of up to 80% water
How are nutrients provided to cartilage?
Via diffusion through the perichondrium
What is the perichondrium?
A fibrous connective tissue sheath that contains:
- Type I collagen fibers
- Vascular supply for collagen
- Chondrogenic cells
- perichondrium is NOT found in all cartilage types
Main difference between 3 different types of cartilage
- cell distribution and number
- type of fiber in the matrix
What two main fibers are found in cartilage?
1) collagen fibers - stronger than steel fibers of the same size
- Thin fibers - hyaline cartilage
- Thick fibers - fibrocartilage
2) elastic fibers
- elastic cartilage
3 main components of cartilage
Cells, fibers, and ground substance
What does ground substance of cartilage consist of, and what is the main function?
- contains water, glycosaminoglycans, glycoproteins, and proteoglycans
- proteoglycans are aggregates of chondroitin sulfate and hyaluronic acid
- functions to generate electrostatic repulsion and resist compression
What is caused by the loss of chondroitin sulfate in cartilage ground substance?
osteoarthritis
Explain proteoglycan repulsion
occurs between negative fixed charges on adjacent glycosaminoglycan molecules
Main functions of hyaline cartilage
- absorbs mild compression, provides support, flexibility, and resilience
- weakest of the 3 types of cartilage
- most widespread type
simple composition of hyaline cartilage
- has perichondrium
- has spherical chondrocytes
- type II collagen fibers
- no nerves or blood vessels
4 hyaline cartilage locations
- articular cartilages - end of long bones
- costal cartilage - ribs/sternum
- respiratory cartilage
- nasal cartilage
What is ossification?
The process of hyaline cartilage turning into bone
-chondroblasts die and are replaced by osteoblasts in ossification centers
Characteristics of fibrocartilage
- strongest type of cartilage
- contains both type I and type II collagen
- lacks perichondrium
- highly compressible
- located at sites subject to pressure & stretch
Characteristics of elastic cartilage
- resembles hyaline cartilage, but more stringy
- contains perichondrium
- network of elastic fibers within matrix in addition to type II collagen fibers
- found in external ear and epiglottis
5 functions of bone
- support
- protection
- assisting movement
- mineral homeostasis
- blood cell production
What 3 hormones regulate calcium homeostasis?
- parathyroid hormone
- 1,25-dihydroxycholecalciferol
- calcitonin
Characteristics of parathyroid hormone
- secreted by chief cells of parathyroid glands
- mobilizes calcium from bone
- increases urinary phosphate excretion
characteristics of 1,25-dihydroxycholecalciferol
- formed from vit D in skin via sun, and from liver and kidneys
- increases calcium absorption from intestine
- increases calcium in bone
characteristics of calcitonin
- calcium lowering hormone secreted by parafollicular cells in thyroid gland
- inhibits bone resorbtion
Interaction of PTH and calcitonin
- PTH increases blood levels of calcium when plasma levels get too low
- calcitonin decreases blood levels of calcium when plasma levels get too high
Regulatory feedback between vit D and PTH
- PTH helps activate vit D to 1,25-dihydroxycholecalciferol in kidney
- 1,25-dihydroxycholecalciferol can then downregulate PTH release
What happens when calcium is too high in plasma? Too low?
- Too high: calcitonin is secreted from parafollicular cells of thyroid to inhibit calcium reabsorbtion from bone
- Too low: parathyroid chief cells release PTH to stimulate the reabsorbtion of calcium from bone into plasma, reabsorbtion in kidney, and activation of vit D
What is the function of activated vit D with respect to calcium?
it increases calcium reuptake in the gut
*calcitriol feedback inhibits PTH release
What cells secrete calcitonin, and from where?
parafolluclar cells of the thyroid
Where are G-protein coupled receptors for calcitonin found?
bones (on osteoclasts) and kidneys
In what 4 ways does calcitonin lower plasma calcium levels?
- inhibits calcium absorption by the intestines
- inhibits osteoclast activity in the bones
- stimulates osteoblastic activity in bones
- inhibits renal tubular cell reabsorption of calcium, allowing it to be excreted in the urine
What 4 ‘other’ hormones impact calcium metabolism?
- glucocorticoids
- growth hormone
- estrogens
- insulin
Effect of glucocorticoids on bone
-lowers plasma calcium levels by inhibiting osteoclast formation and activity
Effect of growth hormone on bone
- increases calcium excretion in urine
- increases intestinal absorption of calcium
Effect of estrogens on bone
-prevents osteoporosis by inhibiting stimulatory effects of cytokines on osteoclasts
Effect of insulin on bone
- increases bone formation
* significant bone loss in untreated diabetes
Rickets/Osteomalacia
-Rickets (children) and osteomalacia (adults) is caused by a defective bone matrix due to vit D and/or calcium deficiency
What is osteopetrosis?
- osteoclasts are defective and unable to absorb bone, so osteoblasts operate unopposed
- bone density is increased and growth becomes distorted
What is osteoporosis?
- relative excess of osteoclast function results in loss of bone matrix and high risk of bone fractures
- treatment using bisphosphonates to inhibit osteoclasts
What disfiguring bone condition is associated with bisphosphonate?
Osteonecrosis of the jaw (ONJ)
Compare bone & cartilage
BONE/CARTILAGE
osteocytes/chondrocytes
periosteum/perichondrium
vascularized/avascular
How does bone grow?
- Osteocytes are unable to divide
- All bone growth is by apposition - deposition of bone on preexisting surfaces
Main protein found in bone matrix
- type I collagen makes up over 90% of bone matrix
- type I collage is as strong as steel
What are lamellae?
- what adult bone is composed of
- 2 types of architecture: compact (cortical) bone and trabecular (spongy) bone
Osteoclasts, osteoblasts, and osteocytes
- Osteoclasts - bone resorption
- Osteoblasts - bone formation
- Osteocytes - mature bone cells
Phosphorous homeostasis
- 85% of the body’s phosphorous is stored in bone
- kidney regulates human phosphate homeostasis via excretion and reabsorption
