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30.2C: Bone Tissue and the Effects of Aging

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  • Page ID
    8417
  • As individuals age, bone resorption can outpace bone replacement, which can lead to osteoporosis and fractures.

     

    LEARNING OBJECTIVES

     

    Analyze osteoclastic activity

     

    KEY TAKEAWAYS

    Key Points

     

    • Bone resorption is the process by which osteoclasts break down bone and release its minerals, resulting in a transfer of calcium from bone to blood.
    • Bone resorption is highly regulated. It can be stimulated or inhibited by signals from other parts of the body depending on the demand for calcium.
    • As people age, the rate of bone resorption far exceeds the rate of bone formation. Thus, bones can weaken, leading to conditions like osteoporosis.
    • Osteoporosis can be prevented with lifestyle changes such as proper nutrition and exercise.
    • The risks of osteoporosis can also be reduced through the prevention of falls. Fall-prevention advice includes exercise to tone deambulatory muscles, proprioception-improvement exercises, and equilibrium therapies.

     

    Key Terms

     

    • osteon: Any of the central canals and surrounding bony layers found in compact bone.
    • periosteum: A membrane surrounding a bone.

     

    EXAMPLES

     

    Joints undergo substantial wear and tear. The longer one lives, the more a joint is used. Gardeners and flooring installers put a lot of stress on their knees. Use of knee pads can relieve some of that stress.

    Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone to blood.

    The Role of Osteoclasts

    The osteoclasts are multi-nucleated cells that contain numerous mitochondria and lysosomes. These cells are responsible for the resorption of bone and are generally present on the outer layer of bone, just beneath the periosteum. Attachment of the osteoclast to the osteon begins the process. The osteoclast then induces an infolding of its cell membrane and secretes collagenase and other enzymes important in the resorption process. High levels of calcium, magnesium, phosphate, and collagen products are released into the extracellular fluid as the osteoclasts tunnel into the mineralized bone. Osteoclasts are also prominent in the tissue destruction commonly found in psoriatic arthritis and other rheumatology-related disorders.

    Regulation of Bone Tissue

    image

    Osteoclast: Osteoclast, displaying many nuclei within its “foamy” cytoplasm above a bone’s surface.

    Bone resorption is highly constructible, stimulated or inhibited by signals from other parts of the body depending on the demand for calcium.

    Calcium-sensing membrane receptors in the parathyroid gland monitor calcium levels in the extracellular fluid. Low levels of calcium stimulate the release of parathyroid hormone (PTH) from chief cells of the parathyroid gland. In addition to its effects on the kidney and the intestine, PTH also increases the number and activity of osteoclasts to release calcium from bone, thus stimulating bone resorption. High levels of calcium in the blood, on the other hand, lead to decreased PTH release from the parathyroid gland. This decreases the number and activity of osteoclasts, resulting in less bone resorption.

    Aging

    As people get older, the rate of resorption tends to exceed the rate of replacement, leading to conditions like osteoporosis. Bone resorption can also be the result of disuse and the lack of stimulus for bone maintenance. For instance, astronauts undergo a certain amount of bone resorption due to the lack of gravity providing the proper stimulus for bone maintenance. In addition, certain medical conditions such as hormone imbalances can cause bone resorption to increase, leading to increased susceptibility to fractures.

    Osteoporosis risks can be reduced with lifestyle changes and sometimes medication. Lifestyle change includes diet, exercise, and fall-prevention measures. Medication includes calcium, vitamin D, and bisphosphonates. Fall-prevention advice includes exercise to tone deambulatory muscles, proprioception-improvement exercises, and equilibrium therapies. With its anabolic effect, exercise may simultaneously stop or reverse osteoporosis, a component of frailty syndrome.

     

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