BONE HOMEOSTASIS
Bone Tissue Lecture Notes
Calcium Homeostasis
Day-to-Day Control of Calcium Regulation:
Involves several key hormones:
PTH (Parathyroid Hormone):
Stimulates osteoclastic activity, leading to an increase in serum calcium levels.
Critical for maintaining calcium homeostasis.
Works by acting on the bones, kidneys, and intestines.
Increased production of cyclic AMP in response to low blood calcium (Ca²+) levels.
Calcitonin (Thyrocalcitonin):
Stimulates osteoblastic activity, thereby lowering serum calcium levels.
Functions alongside hGH (human Growth Hormone) and sex hormones that have a lesser effect on calcium regulation.
Vitamin D:
Essential for the absorption of Ca²+ and PO₄³⁻ ions from the small intestine.
Necessary for the reabsorption of these ions from the kidneys, supporting overall calcium homeostasis.
Hormonal Control:
The regulation of serum Ca²+ levels and the process of mineralizing bone is tightly controlled and balanced by hormonal interactions.
Fracture and Repair
Fracture Classification:
Fractures can be categorized based on different criteria:
Anatomical Appearance:
Partial: Bone is not broken all the way through.
Complete: Fracture traverses through the entire bone.
Closed (Simple): Skin remains intact.
Open (Compound): Fracture punctures the skin.
Greenstick: A small linear break in the bone cortex, similar to breaking a green twig.
Impacted: The distal part of the bone is pushed into the proximal part.
Comminuted: Bone is shattered into several pieces.
Spiral, Transverse, Displaced: Describe specific fracture patterns based on the direction and placement of the fracture.
Disease or Mechanism of Injury:
Pathological Fracture: Results from cancerous processes or chronic diseases.
Compression Fracture: Caused by extreme forces, such as trauma.
Stress Fracture: Result of repetitive stress or activities, such as running.
Eponymous Fractures:
Named after individuals; includes:
Colles' Fracture: A fracture of the distal radius, often involving displacement.
Pott's Fracture: A fracture of the distal fibula, often accompanied by injury to the ankle.
Fracture Repair Process:
Phase 1: Formation of fracture hematoma
Occurs 6-8 hours post-injury.
Results from blood vessels breaking in the periosteum and osteons.
Phase 2-3: Formation of callus
Involves phagocytes cleaning up cellular debris.
Fibroblasts deposit collagen to form a fibro-cartilaginous callus which takes a few weeks to months to develop.
Phase 4: Formation of bony callus
Osteoblasts form new spongy bone over the fibro-cartilaginous callus.
Phase 5: Remodeling
Takes several months.
Spongy bone is replaced by compact bone, with visible signs of the fracture often disappearing but some evidence remaining.
Exercise and Bone Tissue
Mechanical Stress Effect on Bones:
Under mechanical stress (e.g., from muscle pull and gravity), bone tissue strengthens through the deposition of mineral salts and collagen fibers produced by osteoblasts.
Conversely, unstressed bones weaken as mineral deposits decrease.
Real-World Implication:
Astronauts in microgravity conditions suffer rapid loss of bone density due to the absence of mechanical stress.
Aging and Bone Tissue
Effects of Aging on Bone Mass:
A decrease in bone mass is observed as sex hormone levels decline during middle age, particularly in women after menopause.
This leads to an imbalance where bone resorption by osteoclasts surpasses bone deposition by osteoblasts.
Impact on Females:
Women typically have smaller bones than men, experiencing a more pronounced effect of aging on bone density.
Two Principal Effects of Aging on Bone Tissue:
Loss of Bone Mass:
The calcium loss from bones is a common symptom associated with osteoporosis.
Brittleness:
Collagen fibers are responsible for tensile strength in bone tissues.
As protein synthesis declines with age, tensile strength reduces, making bones more susceptible to fractures.
Osteoporosis:
A condition characterized by bone resorption outpacing deposition, often due to calcium depletion and inadequate intake.