Notes on The Skeletal System and Bone Physiology

THE SKELETAL SYSTEM

Bone Tissue

  • Primary structural component of the skeletal system.

Skeleton Overview

  • Axial Skeleton: Comprised of the skull, vertebral column, and rib cage.
  • Appendicular Skeleton: Includes limb bones and the pelvic girdle.
  • Joints: Connection points between bones, allowing for mobility and support.

Hormonal Regulation of Bone Growth

  • Growth Hormone:

  • Most important in stimulating epiphyseal plate activity during infancy and childhood.

  • Thyroid Hormone:

  • Modulates the effect of growth hormone, ensuring balanced growth proportions.

  • Sex Hormones (Testosterone and Estrogens):

  • At puberty, promote growth spurts.

  • Induce closure of epiphyseal plates, ending growth.

  • Note: Hormonal excess or deficiency can lead to abnormal skeletal development.

Bone Remodeling

  • Recycling of Bone Mass:

  • 5-7% of bone mass is recycled weekly.

  • Spongy bone replaced every 3-4 years; compact bone every 10 years.

  • Process:

  • Involves bone deposition (by osteoblasts) and bone resorption (by osteoclasts).

  • Foccuses on periosteum and endosteum surfaces.

  • Remodeling units consist of osteoblasts and osteoclasts that work together during remodeling.

  • Resorption Mechanism:

  • Osteoclasts secrete lysosomal enzymes and protons that digest matrix; dissolve calcium salts and phagocytize demineralized matrix.

Factors Influencing Bone Remodeling

  • Mechanical Stimuli:

  • Bones adapt their structure based on mechanical stress, known as Wolf's law.

  • Stress causes bending: compressing one side while stretching the other.

  • Thicker diaphysis forms where bending stresses are greatest.

  • Biochemical Factors:

  • Mechanical signals and ion concentrations (calcium & phosphate) influence bone matrix deposits.

Hormonal Control of Remodeling

  • Hormones Involved:
  • Parathyroid Hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts, increasing calcium levels.
  • Calcitonin: Released when calcium levels are high; promotes bone deposition but has negligible effects except at high doses.

Essential Minerals and Vitamins for Bone Growth

  • Minerals:

  • Calcium and Phosphorus: Essential for hard extracellular matrix.

  • Magnesium, Fluoride, Manganese: Support bone structure and strength.

  • Vitamins:

  • Vitamin A: Essential for the formation of the bone matrix.

  • Vitamin C: Crucial for collagen synthesis and repair.

  • Vitamin D: Increases calcium absorption in the intestine; deficiencies lead to poor bone mineralization.

  • Vitamins K & B12: Important for synthesis of bone proteins.

Fracture Classification

  • Position of Bone Ends:

  • Nondisplaced: Normal alignment.

  • Displaced: Misalignment.

  • Completeness:

  • Complete: Fully fractured.

  • Incomplete: Partial fracture.

  • Skin Penetration:

  • Open (Compound): Skin is penetrated.

  • Closed (Simple): Skin remains intact.

Fracture Treatment and Repair

  • Reduction:

  • Closed Reduction: Non-surgical aligning.

  • Open Reduction: Involves surgical intervention with hardware.

  • Healing Time: Depends on fracture severity, bone type, and patient age.

Phases of Fracture Repair

  • 1. Reactive Phase: Inflammatory response post-injury.
  • 2. Reparative Phase: Formation of soft (fibrocartilaginous) and hard (bony) callus.
  • Fibrocartilaginous callus connects broken ends; bony callus replaces it in weeks.
  • 3. Remodeling Phase: Refinement of bony callus and reintegration into original bone structure.

Bone's Role in Calcium Homeostasis

  • Storage: Bones hold 99% of the body's calcium.
  • Hormonal Response:
  • PTH increases osteoclast activity, raising blood calcium levels by resorbing bone.
  • Calcitriol enhances intestinal absorption of calcium, essential for maintaining calcium levels.

Aging and Bone Tissue

  • Bone Remodeling Dynamics:

  • Children and adolescents produce more bone than they lose.

  • Post-menopausal women face accelerated bone loss due to lower hormone levels.

  • Exercise: Promotes stronger bones and mineral density.

Homeostatic Imbalances of Bone

  • Osteopenia: Precursor to osteoporosis; lower than normal bone mass.
  • Osteoporosis: Significant bone density loss, particularly in older adults, leading to increased fragility.
  • Paget’s Disease: Excessive, disorganized bone growth leading to weak and painful bones, commonly treated with calcitonin and bisphosphonates.

Summary of Treatment Options

  • Medications to reduce bone resorption (e.g., bisphosphonates).
  • Hormonal replacement therapies to boost bone formation.
  • Importance of Vitamin D for calcium absorption and maintaining bone health.