copyright © 2003 pearson education, inc. publishing as benjamin cummings bone remodeling bone...
TRANSCRIPT
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Remodeling
• Bone remodeling = combination of bone deposition and bone resorption.
• Deposition = taking minerals (Ca2+, Mg2+, Mn2+, phosphate) from the bloodstream and using it to build hard bone tissue.
• Resorption (re + absorption) = breaking down hard bone tissue into the above materials and adding them back to the blood.
• In order for the a bone to maintain its size and strength: rate of resorption = rate of deposition
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Developmental Aspects of Bones
• By age 25, nearly all bones are completely ossified
• In old age, bone resorption predominates
• A single gene that codes for vitamin D binding to cells determines both the tendency to accumulate bone mass early in life, and the risk for osteoporosis later in life
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Long Bone Growth and Remodeling
• Growth in length – cartilage continually grows and is replaced by bone as shown
• Remodeling – bone is resorbed and added by appositional growth as shown
Figure 6.10
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• During infancy and childhood, epiphyseal plate activity is stimulated by growth hormone
• During puberty, by testosterone and estrogens
• Initially promote adolescent growth spurts
• Later induce epiphyseal plate closure, ending longitudinal bone growth
Hormonal Regulation of Bone Growth During Youth
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Remodeling
• Remodeling units – adjacent osteoblasts and osteoclasts deposit and resorb bone at periosteal and endosteal surfaces
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Deposition
• Accomplished by osteoblasts (“bone builders”)
• Occurs where bone is injured or added strength is needed
• Requires a diet rich in protein, vitamins C, D, and A, calcium, phosphorus, magnesium, and manganese
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Resorption
• Accomplished by osteoclasts (“bone crackers”)
• Resorption involves osteoclast secretion of:
• Lysosomal enzymes that digest organic matrix
• Acids that convert calcium salts into soluble forms
• Dissolved matrix is secreted into the interstitial fluid (fluid between cells) and then into the blood
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Importance of Ionic Calcium in the Body
• Calcium is necessary for:
• Transmission of nerve impulses
• Muscle contraction
• Blood coagulation
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Control of Remodeling
• Two control loops regulate bone remodeling
• Hormonal mechanism that maintains calcium homeostasis in the blood (PTH and calcitonin)
• Mechanical and gravitational forces acting to the skeleton (“stresses”)
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Hormonal Mechanism
• Rising blood Ca2+ levels trigger the thyroid to release calcitonin
• Calcitonin stimulates calcium salt deposit in bone
Figure 6.11
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Hormonal Mechanism
• Falling blood Ca2+ levels signal the parathyroid glands to release PTH
• PTH signals osteoclasts to degrade bone matrix and release Ca2+ into the blood
Figure 6.11
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Response to Mechanical Stress
• Wolff’s law – a bone grows or remodels in response to the forces or demands placed upon it
Figure 6.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Response to Mechanical Stress
• Long bones are thickest midway along the shaft (where bending stress is greatest)
• Curved bones are thickest where they are most likely to buckle
Figure 6.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Response to Mechanical Stress
• Trabeculae (bone fibers) form along lines of stress
• Large, bony projections occur where heavy, active muscles attach
Figure 6.12