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.

Explore the dynamic nature of bone as a connective tissue, including its cells and structure. This quiz covers the roles of osteoblasts, osteocytes, and osteoclasts, as well as conditions like osteogenesis imperfecta and rickets. Test your knowledge on compact and trabecular bone!