Bone Flashcards
Cortical bone
- Forms the external surface
- Prominent in tubular long bone
- Provides strength
- Made of tightly packed osteons (Haversian systems)
Trabecular bone
- Lies beneath the cortex
- Predominates in vertebral bodies, flat bones of the pelvis and skull
- Much less organized than cortical bone
- Consisting of interconnecting trabeculae, larger blood vessels and bone marrow
Epiphysis
- Separated from the metaphysis by the physis (growth plate)
Diaphysis
- Joins the two metaphysis and provides length
Apophyses
- AKA tibial tuberosity are the site of new bone formation like the epiphyses but do not contribute to bone length
- They lay down new bone in response to traction
Periosteum
- Covers bone except for articular surfaces
- Fibrous outer layer is site of attachment of muscles
- Inner layer contains osteoblasts that generate new bone
Where does linear growth occur?
Physis
Where does circumferential growth occur?
Periosteal deposition
When does the growth of the appendicular skeleton cease?
Ossification of the iliac apophyses
When does the height of the vertebral bodies cease?
Until the 3rd decade
How can skeletal bone age be determined?
- Can be determined by secondary ossification and physeal closure (earlier in girls)
What factors influence growth at the physis?
- Thyroxine
- GH
- IGF-1
(these three work together to cause peak bone mass during puberty) - Testosterone (anabolic effect)
- Estrogens suppress the growth rate by increasing the calcification of the matrix (prerequisite to epiphyseal closure)
Biochemical composition of bone
- 70% minerals and 30% organic constituents
- Hydroxyapatite (mostly Ca and Phos) accounts for 95% of minerals with Mag
- Organic component is 98% matrix, mostly type I collagen and non collagenous protein ( osteocalcin, fibronectin, osteonectin, and osteopontin)
What makes up the other 2% of the organic component?
Bone cells
What do bone cells do?
- Responsible for formation, resorption and maintenance of the remodeling cycle
Osteoclasts
- Derived from mononuclear cells and resorb bone
Osteoblasts
- Form osteoid and osteoid matrix
Osteocytes
- Differentiated osteoblasts and maintain the integrity of bone through a network of canaliculi
Alkaline Phosphatase
- Enzyme secreted by osteoblasts, but also other cells (liver, gut, kidneys)
- In children, 50 percent from bone
- Released during osteoblastic activity
Osteocalcin
- Synthesized by osteoblasts and chondrocytes and deposited in extracelullar bone matrix
- Sensitive and specific marker of bone formation
Acid phosphatase (TRAP)
- Present in osteoclast and released during osteoclastic activity
- Not bone specific
Urinary calcium to Cr ratio
- Marker of bone resorption
- Elevated values indicate hypercalcemia and increased bone resorption
What is the role of parathyroid hormone?
- Maintains Ca concentration in the blood
- Stimulates osteoclastic activity causing bone resorption (and increased serum calcium)
- Stimulates conversion 25 (OH)D3 to 1, 25 (OH)2D3
Calcitriol
- Stimulates intestinal absorption of calcium, elevating serum calcium concentration
- Receptors present on intestinal cells
Factors affecting bone synthesis
- Calcium intake
- Latitude
- Exposure to sunlight
- Ingestion of vitamin D
- Metabolic state
- Nutritional status
- Hormones
Peak bone mass
- Level of bone mass achieved at the end of skeletal maturation
- Usually reached in late adolescence during second decade
Intrinsic factors that effect bone mass
- Hereditary (Race, genes for vitamin D receptors, IL-6, calcitonin, TGF B, estrogen receptor A, osteocalcin etc)
- Sex
- Hormones
Extrinsic factors that affect skeletal bone mass
- Calcium
- General state of health
How do steroids impact calcium?
- Decrease GI calcium absorption and increase urinary excretion
What are the benefits of weight bearing?
- Enhance bone mineral accrual in children, esp in early puberty
Dual energy XR absorptiometry (DEXA)
- Gold standard
- Low radiation, high speed and accuracy
- Gives areal BMD (not true volumetric density)
- Should be interpreted with age matched controls and corrections for height, weight and geography
Quantitative computerized tomography
- Axial skeleton only
- True volumetric density can be measured
Tendons
- Major component is Type I collagen
- Attach muscle to bone
Ligament
- Function is to joint bone to bone, stabilize the joint, limit mobility
Entheses
- Attach tendon, ligament, fascia or capsule to bone
Two types of entheses
- Fibrous
- Fibrocartilaginous
Fibrous
- Found at attachment of tendon to metaphysis or diaphysis composed of dense fibrous tissue, probably not important in rheum
Fibrocartilaginous
- Present on epiphyses of long bones and small bones of hands and feet
- Achilles, knee, foot
Muscle bundle (Fascicle)
- Group of muscle fibers ensheathed by a perimysium
What is the basic unit of muscle?
Fiber–> made of myofibrils
What is the basic contractile unit?
- Sarcomere
- Made of thin (actin) and thick (myosin) myofilaments
- Gives muscle striated appearance
Epimysium
- Surrounds a skeletal muscle
Perimysium
- Surrounds individual fascicles
Alpha motor neurons
- Cause action potential that stimulates calcium diffusion in the myoplasm and binding to thin-filament regulatory proteins
Type I muscle fibers
- Slow/red
- Narrower
- Poorly defined myofibrils
- Irregular in size
- Thick Z bands
- Rich in mitochondria and oxidative enzymes
- Poor in phosphorlyases
- Sustained contractions
Type II muscle fibers
- Fewer mitochondria and poor in oxidative enzymes
- Rich in phosphorylases and glycogen
- Acute strength
Z band (disk)
Helps to define the sarcomere
I band
- Surrounds the Z disk
- Zone of thin filaments (actin) not superimposed by thick filaments
A band
- Contains the entire length of single thick filament
M Line
Inside the H zone of thick filaments not superimposed by thin filaments
- Formed by cross connective elements of the cytoskeleton
What is the arterial supply of the long bone?
- Arises from nutrient artery that penetrates the diaphysis and terminates in end arteries at epiphyseal plate
What is the arterial supply of the epiphyses?
Juxta-articular arteries which also supply the synovium
When does arterial communication between the metaphyseal and epiphyseal synovial circulations occurs?
- Not until growth has ceased
Components of ECM of hyaline cartilage
Collagen fibers, water, proteins and proteoglycans
3 compartments of ECM matrix of hyaline cartilage
Inner rim: Aggrecan rich, surrounds chondrocytes, no crosslinking
Middle rim: Aggrecan bound to hyaluronan
Outer rim: fine collagen fibrils
Three major classes of collagen
- FIbril forming
- Non fibril forming
- Fibril associated collagens with interrupted triple helices (FACITS)
Type I collagen
Most connective tissues, bone, skin, tendons
Type II collagen
Cartilage, intervertebral disk, vitreous humor
Type III
Most connective tissues, particularly skin, lung, blood vessel
What is a proteoglycans?
Macromolecules consisting of a protein core with attached GAG
Keratan sulfate
Serum concentration of this reflects cartilage metabolism
Where can you find elastin?
- Walls of blood vessels
- Ligaments
What are the properties of elastin
- Lack tensile strength of collagens but can stretch than return to original length
- Produced by fibroblasts and smooth muscle cells
Laminin
Major constituent of the basement together with Type IV collagen
Fibronectin
Dimeric glycoprotein that acts as attachment protein in ECM
