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Bone Growth and Remodeling

  • Processes: Bone Growth and Remodeling

    • Bone growth occurs at the epiphyseal plate (length) and underneath the periosteum (width).

    • Remodeling: Breakdown and rebuilding of bone tissue to maintain bone health and respond to stressors.

    • Examples include:

      • Weightlifting: Increased stress leads to stronger bones.

      • Orthodontia: Braces apply stress, causing remodeling of tooth sockets.

Hormonal Regulation of Bone

  • Calcium Homeostasis

    • The skeleton functions as a calcium bank, storing or releasing calcium based on the body's needs.

    • Key hormones involved:

      • Growth Hormone: Stimulates overall growth, including bone; derived from the pituitary gland.

        • Acts through insulin-like growth factors to stimulate osteoblasts.

      • Thyroxine (Thyroid Hormone): Similar effects to growth hormone, promotes osteoblast activity and protein synthesis.

      • Calcitriol (Vitamin D3): Increases calcium absorption in the gut and reabsorption in the kidneys.

      • Sex Hormones (Estrogen and Testosterone): Promote bone growth and regulate the closure of epiphyseal plates during puberty.

        • Increases in estrogen and testosterone lead to growth spurts in teenagers.

Hormones Influencing Bone Health

  • Parathyroid Hormone (PTH)

    • Released from parathyroid glands in response to low plasma calcium levels.

    • Actions:

      • Increases osteoclast activity (bone resorption) and decreases osteoblast activity.

      • Raises plasma calcium by releasing calcium from bones and decreasing urinary loss of calcium.

  • Calcitonin

    • Released from parafollicular cells in the thyroid in response to high plasma calcium levels.

    • Actions:

      • Inhibits osteoclast activity and increases osteoblast activity, promoting bone formation and decreasing plasma calcium levels.

Negative Feedback Loops

  • Calcitonin Counteracts High Calcium: Reduces plasma calcium by promoting bone deposition.

  • PTH Counteracts Low Calcium: Increases plasma calcium by promoting bone resorption and being less excreted in urine.

Health Implications

  • Osteoporosis: A condition characterized by decreased bone density and strength, often seen in postmenopausal women due to hormonal changes.

    • An imbalance where osteoclast activity exceeds osteoblast activity leads to weakened bones, particularly in areas like the femur and vertebrae.

Examples of Hormonal Disorders

  • Pituitary Gigantism: Excess growth hormone leading to abnormal height and proportions.

  • Pituitary Dwarfism: Insufficient growth hormone during development results in proportional short stature.

  • Acromegaly: Excess growth hormone post-epiphyseal closure causes changes in facial features, not height.

Visual Examples of Disorders

  • Photographs of individuals affected by hormonal disorders:

    • Pituitary Gigantism: Tall stature and long limbs.

    • Pituitary Dwarfism: Proportional body measurement but smaller size.

    • Acromegaly: Enlarged facial structures, such as a pronounced jaw and larger hands/feet, exemplified by athletes like Michael Phelps.