MSK Module 1B Flashcards
All connective tissue is composed of:
- Cellular component
- Extracellular matrix (with a non-fibrous and fibrous component)
Bone tissue consists of:
- Cellular component (osteoblasts, osteocytes, osteoclasts)
- Matrix component (collagen, proteoglycans, BMPs, glycoproteins, etc.)
Cell types found in bone:
Osteoblasts
Osteoclasts
Osteocytes
What do osteoblasts produce?
- Type 1 collagen
- Non-mineralized bone matrix (osteoid)
Function of osteoblasts
Formation of new bone:
- Produces substances involved
- Facilitates mineralization of osteoid to complete the process
When is an osteoblast considered an osteocyte?
Once the osteoid (surrounding bone matrix) is mineralized
Osteoblasts are located along:
- Trabecular surfaces
- Inner surface of Haversian canal
- Inner surface of periosteum
Osteocytes are formed from:
Osteoblasts
Where are osteocytes located?
Lacunae (small cavity)
Osteocytes make up approx. ___% of cells in mature human skeleton
90
Are osteocytes active or inactive?
Active in monitoring/maintaining bone status
Functions of osteocytes
- Stimulate remodeling process
- Serve as mechanism for mechanical stimulus to bone
- Maintain homeostasis of the mineralized bone
Where are osteoclasts located?
Howship’s lacunae
depressions seen in microscopic view that represent areas of bone resorption
How do osteoclasts resorb bone?
- Secrete acid and lytic enzymes to dissolve surrounding bone
- Elements of bone are resorbed into ossteoclast at base of microvilli
- Releases bony elements into capillaries to be recycled
What happens to an osteocyte after it is resorbed by osteoclasts?
Either degenerates or becomes dormant until needed again
What is bone matrix composed of?
Type 1 collagen (90%) Proteoglycans BMP (bone morphic protein) Glycoproteins Osteocalcin Bone albumin GFs Bone minerals
What is Type 1 collagen’s function within bone?
- Tensile strength of bones
- Weight bearing (compressive) strength
What are proteoglycans? Function?
- Part of bone matrix
- Large polysaccharides attached to protein
- Located b/w collagen fibers
- Plays role in calcification/fluid balance by attracting calcium
Function of bone morphic proteins (BMP)?
- Promote formation of osteoblasts from stem cells
- Promote osteogenesis in osteoblasts
- Promote bone repair
Function of glycoproteins in bone?
- Assist in collagen fiber formation
- May assist in calcification
What is osteocalcin?
- Part of communication b/w osteoblasts and clasts
- Produced by osteoblast
- Promotes osteoclast activity
What does bone albumin do?
- Attracts fluids and maintains fluid balance in bone
- Transports hormones, ions, other metabolites to/from bones
What do growth factors (GFs) do in bone?
Play role in differentiation, activation, growth, and turnover of bone (and other tissue)
What is calcium hydroxyapatite (HAP) and what does it do?
- Bone mineral
- An insoluble crystal that deposits within the collagen fibers
- End stage of calcium crystallization for mineralization
Bone multicellular unit
- Cluster of cells that breakdown an area of bone surface and then fills it with new bone
- Multiple BMU clusters are activated/inactivated at any given point in time and in different locations
6 steps of bone remodeling
- Activation of BMU
- Initiation of osteoclastic activity
- Resorption forms small cavity
- Osteoblast maturation/recruitment
- Osteoid formation
- Maturation of osteoid
What does RANK-L do?
Signals pre-osteoclasts to mature into active osteoclasts
What does osteoprotogerin do?
Inhibits RANK-L
How do estrogen and calcitonin affect bone remodeling?
Inhibit osteoclast activity
How does acidosis affect bone remodeling?
Promotes osteoclast resorption
What are metabolic disorders of bone?
- Osteoporosis
- Osteomalacia
- Paget’s disease
Name the osteochondroses
- Osteonecrosis (avascular)
- Apophysitis (epiphysitis)
Osteopenia
- Low bone mineral density (BMD)
- Between 1 and 2.5 SD below the young adult mean
Osteoporosis
- Severe decrease in BMD
- 2.5 SD or more below young adult mean
- Osteoclast activity is greater than osteoblast activity
Osteomalacia
- “Softening” of the bone
- Insufficient mineralization (no loss of bone)
Osteopetrosis
Increased BMD
Primary osteoporosis
- Unrelated to any underlying disease
- Type 1 (postmenopausal, cancellous bone)
- Type 2 (age related, both cortical and cancellous bone)
Secondary osteoporosis
Secondary to medication or disease
3 phases of bone mass
Growth - 90% of bone density is reached here
Consolidation - remaining 10% of bone density gained
Involution - gradual loss of bone
When is peak bone mass commonly reached?
By 30 years old
Age related rate of bone density loss is greater in males, less in males, or equal between both?
Equal
How much bone mass is loss per year due to age?
Approx. 0.5-1% each year
Rate of bone loss in women is ___ after menopause
Accelerated (approx. 3-5% per year)
Estrogen loss in menopause affects which players in bone remodeling?
- Higher levels of RANK-L (promotes maturation of osteoclasts)
- Reduced levels of OPG (inhibits maturation)
Menopause affects which type of bone most significantly?
Cancellous (trabecular)
*Vertebrae, metaphysis of long bones
Which drugs in particular can cause secondary osteoporosis?
- Corticosteroids
- Immunosuppressants
Why is trabecular bone very sensitive to conditions that alter osteoblast/clast activity?
- Larger surface area and not as large mass to begin with compared to cortical bone
- Experiences greater loss
How does tobacco increase risk for osteoporosis?
Impairs bone progenitor cells (inhibiting osteoblast activity)
How does alcohol increase risk for osteoporosis?
- Impairs osteoblast activity
- Impairs Ca absorption
- Increases renal excretion of Ca
What is the female triad?
- Eating disorder
- Amenorrhea
- Osteoporosis
A decrease in 1 SD of BMD will increase risk of fracture by:
1.5 to 3x
What is the gold standard for diagnosing osteoporosis?
DEXA
Why is x-ray a poor screening tool for osteoporosis?
It only detects bone loss after significant loss has occurred
What is Rickets?
Childhood osteomalacia
What type of fractures are seen with osteomalacia?
“Looser’s zones” or milkman’s pseudofractures
Describe Paget’s disease and epidemiology
- Disease that results in bone deformation
- Excessive resorption is followed by excessive bone formation
- Onset 50+ yo
- Males more than females
Describe osteonecrosis (avascular necrosis)
- Death of bone due to loss of blood supply
- Often no symptoms
- Femoral head is MC site
What is apophysitis and what are common examples?
- A traction apophysitis of the secondary ossification center
- Osgood-Schlatter disease (patella tendon pulling on tibial tuberosity)
- Olecranon (throwing athletes)
- Sever’s disease (Achille’s)
What is osteomyelitis?
Inflammation of bone caused by bacteria or other infectious agents
What is exogenous osteomyelitis?
Trauma exposes bone to bacteria and other foreign material
What is endogenous osteomyelitis?
Infection spreads to bone from pre-existing systemic infection (S. aureus is common)
How does osteomyelitis occur?
- The physical arrangement of bone contains regions that immune defense mechanisms are unable to access
- This makes bones susceptible to infection if bacteria gains access
How does osteomyelitis form in children?
Sequestrum (piece of dead bone) and involucrum (layer of new bone) formation
How does bone respond to a primary benign tumor?
Forms a sclerotic rim in order to contain the tumor
*These do NOT tend to cause pain like malignant tumors
Bone tumor descriptions on imaging
- Geographic (well defined margins)
- Moth eaten (less defined margins)
- Permeative (poor margins)
