Bone Physiology and Structure

Bone Physiology

• Bone is a dynamic connective tissue comprising cells and osteoid, an organic matrix that provides structure.
• Osteoblasts are responsible for producing the organic matrix, which includes collagen fibers embedded in a gel-like ground substance.
• Calcium phosphate crystals within the osteoid harden the bone, giving it the strength to support body weight.
• Osteogenesis imperfecta leads to brittle bones due to a lack of collagen, resulting in excessive fragility.
• Rickets is characterized by soft bones due to inorganic salt deficiency.
• Three primary bone cells:
  • Osteoblasts: Secrete osteoid.
  • Osteocytes: Retired osteoblasts residing within the bone they deposited.
  • Osteoclasts: Macrophages that resorb bone tissue.

Bone Structure

• Compact Bone: Dense outer layer consisting of organized osteon units; characterized by concentric lamellae and central canals for blood vessels.
• Trabecular Bone: Also known as spongy bone, found internally, lacks osteons, and contains irregularly arranged lamellae and macroscopic spaces filled with marrow.
• Trabecular bone spaces are either filled with red marrow (blood cell production) or yellow marrow (fat storage).

Calcium Regulation and Hormonal Control

• Approximately 99% of body calcium (Ca2+) resides in the skeleton and teeth, with small amounts in soft tissues and the extracellular fluid (ECF).
• Only free ECF calcium is biologically active and subject to regulation.
• Calcium homeostasis is governed by three main hormones: parathyroid hormone (PTH), calcitonin, and vitamin D.

Regulation of Plasma Calcium

• Normal plasma Ca2+ concentration ranges from 9.2 to 10.4 mg/dL.
• Vital for neuromuscular excitability; adjustments in plasma Ca2+ levels can dramatically affect nerve and muscle functions.
• Ca2+ influences membrane permeability to Na+, affecting excitability.

Bone Remodeling

• Bone resorption (by osteoclasts) and deposition (by osteoblasts) occur simultaneously, allowing continual remodeling.
• Remodeling adjusts bone strength and shape according to stress and prevents brittleness from aging bone.
• Osteoclasts dissolve bone crystals with hydrochloric acid, while osteoblasts fill cavities with new osteoid.

Calcium Balance and PTH Function

• PTH is crucial for maintaining both calcium homeostasis and balance by acting on bone, kidneys, and intestines.
• It releases calcium from bones, increases kidney absorption, and enhances intestinal absorption through vitamin D activation.
• Essential for life; absence of PTH leads to fatal hypocalcemia.

Vitamin D Activation

• Vitamin D is necessary for calcium absorption and is acquired via diet or synthesized from skin exposure to sunlight.
• Requires conversion in the liver and kidneys to become active (1,25-(OH)2-vitamin D3 or calcitriol).
• Vitamin D enhances the absorption of calcium and phosphate from the intestine.

Calcitonin

• Secreted by parafollicular cells in the thyroid gland, acting to lower plasma calcium levels when they rise.

Feedback Mechanism

• PTH secretion is tightly regulated by plasma free Ca2+ levels, typically increasing when Ca2+ is low and decreasing when Ca2+ is high, establishing a negative feedback loop.