Bio 201 . module 2 Bone tissue part 2

Physiology of Osseous Tissue

• Mature bone is dynamic and interacts with its environment.
• It is involved in its own maintenance, growth and remodeling.
• Bone continuously exchanges minerals with tissue fluid.
• Disturbance of calcium homeostasis impacts the function of other organ systems, particularly the nervous and muscular systems.

Mineral Deposition and Resorption

• Mineral deposition (mineralization) is the process of calcium phosphate and other ions crystallizing from blood plasma to form bone tissue.
• Osteoblasts produce collagen fibers that mineralize and harden the matrix.
• Calcium and phosphate ions, specifically hydroxyapatite, are deposited along these fibers.
• For crystal formation to occur, calcium and phosphate ion concentration must surpass a critical value, known as the solubility product.
• Most tissues have inhibitors to prevent calcification, but osteoblasts neutralize these inhibitors allowing salts to precipitate in the bone matrix.
• Initial crystal formations (seed crystals) attract more calcium and phosphate from solution.
• Abnormal calcification (ectopic ossification) may occur in various tissues, including lungs, brain, eyes, muscles, tendons, and arteries, leading to conditions like arteriosclerosis.
• Calculus refers to calcified masses in soft organs, such as the lungs.
• Mineral resorption is the dissolving of bone and the release of minerals back into the bloodstream.
• Osteoclasts are responsible for this process, utilizing a ruffled border.
• Hydrogen pumps secrete hydrogen ions into the space between the osteoclast and bone surface, followed by chloride ions.
• Hydrochloric acid (pH 4) dissolves bone minerals.
• Acid phosphatase enzymes digest collagen.
• Orthodontic appliances move teeth by dissolving bone ahead of the tooth where pressure is highest, and depositing bone behind the tooth where pressure is lower.

Calcium Homeostasis

• Calcium and phosphate are essential for various biological processes beyond bone structure.
• Phosphate plays a role in DNA, RNA, ATP, phospholipids, and pH buffers.
• Calcium is essential for neuron communication, muscle contractions, blood clotting, and exocytosis.
• The skeleton serves as a reservoir for these minerals, depositing and withdrawing them as needed by other functions.
• An adult body contains approximately 1,100 grams of calcium, with 99% located in the skeleton.
• This calcium exists in two forms: easily exchangeable calcium ions and a more stable hydroxyapatite reserve.
• Approximately 18% of the adult skeleton is exchanged with blood each year.
• Normal blood plasma calcium concentration ranges from 9.2 to 10.4 mg/dL, with 45% as Ca2+ diffusible across capillary walls.
• The remaining calcium is bound to plasma proteins.

Hypocalcemia

• Hypocalcemia is a deficiency of blood calcium.
• Causes include excessive excitability of the nervous system leading to muscle spasms or tetany (inability to relax).
• Hypocalcemia increases sodium channel responsiveness, leading to increased sodium influx and nerve and muscle cell excitation.
• Tetany typically begins when plasma calcium levels fall to 6 mg/dL.
• One sign of hypocalcemia is strong spasmodic flexion of the wrist and thumb, known as Trousseau's sign, often induced by a blood pressure cuff.
• Chvostek's sign, a facial spasm, is also associated with hypocalcemia, but it is not a definitive indicator.

Calcitriol

• Calcitriol is a potent form of vitamin D produced sequentially in the skin, liver, and kidneys.
• The process begins with epidermal keratinocytes converting 7-dehydrocholesterol, a steroid, to previtamin D3 using UV radiation.
• The liver adds a hydroxyl group, converting it to calcidiol.
• The kidneys add a second hydroxyl group, producing calcitriol (the most active form of vitamin D).
• Calcitriol, also found in fortified milk, functions as a hormone that increases blood calcium concentration through several mechanisms.
• It enhances calcium absorption by the small intestine.
• It promotes calcium resorption from the skeleton.
• It elevates kidney reabsorption of calcium ions, reducing urinary loss.
• Adequate vitamin D is crucial for bone deposition and proper calcium and phosphate utilization.
• Lack of vitamin D can lead to rickets in children and osteomalacia in adults, characterized by abnormal bone softness.

Calcium Homeostasis

• The daily dietary calcium requirement is approximately 1,000 mg.
• Calcitriol, calcitonin, and PTH (parathyroid hormone) work together to maintain normal blood calcium concentration.
• Calcitriol increases calcium absorption in the digestive tract and facilitates calcium resorption from bone.
• PTH, secreted by parathyroid glands, stimulates calcium resorption from bone and promotes calcium reabsorption in the kidneys.
• Calcitonin, secreted by C cells of the thyroid gland, inhibits osteoclasts and stimulates osteoblasts, lowering blood calcium concentration.

Calcitonin

• Calcitonin is secreted by C cells of the thyroid gland when blood calcium levels rise.
• It lowers blood calcium concentration in two ways:
  • Inhibits osteoclasts, reducing calcium liberation from bones.
  • Stimulates osteoblasts, increasing calcium deposition into the skeleton.
• Calcitonin plays a significant role in children due to their faster bone remodeling and higher calcium release.
• Estrogen has a stronger effect on bone growth than testosterone, explaining the faster growth and earlier full height attainment in girls.
• Males grow for a longer period and achieve greater height.
• Anabolic steroids can prematurely close the epiphyseal plate, leading to stunted adult stature.
• Excessive cola consumption (more than 3, 12-ounce servings) is linked to bone loss, particularly in women.

Bone Disorders

• Orthopedics, originating as the treatment of skeletal deformities in children, now encompasses preventing and correcting injuries and disorders related to bones, joints, and muscles.
• It includes designing prosthetic joints and limbs and treating athletic injuries.

Fractures and their Repairs

• A stress fracture is caused by repetitive or abnormal trauma to a bone, frequently occurring during falls, athletic activities, or military combat.
• A pathological fracture is a break in a bone weakened by a pre-existing disease, such as bone cancer or osteoporosis.
• Fractures are classified based on structural characteristics including the direction of the fracture line, whether the break penetrates the skin, and the number of bone fragments.
• Simple (closed) fractures do not penetrate the skin, while compound (open) fractures involve the skin.
• A comminuted fracture has multiple bone fragments.
• A greenstick fracture involves the bending of a bone, usually in children.
• A spiral fracture occurs when bone twists and breaks.
• A transverse fracture is straight across the bone.
• An oblique factor is angled across the bone.