Mineral Deposition • Mineralization is crystallization process • osteoblasts produce collagen fibers spiraled the length of the osteon • minerals cover the fibers and harden the matrix • ions (calcium and phosphate and from blood plasma) are deposited along the fibers • ion concentration must reach the solubility product for crystal formation to occur • Abnormal calcification (ectopic) • may occur in lungs, brain, eyes, muscles, tendons or arteries (arteriosclerosis)
Mineral Resorption from Bone • Bone dissolved and minerals released into blood • performed by osteoclasts • hydrochloric acid (pH 4) dissolves bone minerals • enzyme (acid phosphatase) digests the collagen • Dental braces reposition teeth and remodel bone • create more pressure on one side of the tooth • stimulates osteoclasts to remove bone • decreased pressure stimulates osteoblasts
Calcium and Phosphate • Phosphate is component of DNA, RNA, ATP, phospholipids, and pH buffers • Calcium needed in neurons, muscle contraction, blood clotting and exocytosis
Ion Imbalances • Changes in phosphate levels = little effect • Changes in calcium can be serious • hypocalcemia is deficiency of blood calcium • causes excitability of nervous system if too low • muscle spasms, tremors or tetany ~6 mg/dL • laryngospasm and suffocation ~4 mg/dL • with less calcium, sodium channels open more easily, sodium enters cell and excites neuron • hypercalcemia is excess of blood calcium • binding to cell surface makes sodium channels less likely to open, depressing nervous system • muscle weakness and sluggish reflexes, cardiac arrest ~12 mg/dL • Calcium phosphate homeostasis depends on calcitriol, calcitonin and PTH regulation
Carpopedal Spasm • Hypocalcemia demonstrated by muscle spasm of hands and feet.
Hormonal Control of Calcium Balance • Calcitriol, PTH and calcitonin maintain normal blood calcium concentration.
Calcitriol (Activated Vitamin D) • Produced by the following process • UV radiation and epidermal keratinocytes convert precursor to vitamin D3 • liver converts it to calcidiol • kidney converts that to calcitriol (vitamin D) • Calcitriol behaves as a hormone that raises blood calcium concentration • increases intestinal absorption and absorption from the skeleton • increases stem cell differentiation into osteoclasts • promotes urinary reabsorption of calcium ions • Abnormal softness (rickets) in children and (osteomalacia) in adults without vitamin D
Calcitonin • Secreted (C cells of thyroid gland) when calcium concentration rises too high • Functions • reduces osteoclast activity as much as 70% • increases the number and activity of osteoblasts • Reduces bone loss in osteoporosis
Parathyroid Hormone • Glands on posterior surface of thyroid • Released with low calcium blood levels • Function = raise calcium blood level • causes osteoblasts to release osteoclast-stimulating factor increasing osteoclast population • promotes calcium resorption by the kidneys • promotes calcitriol synthesis in the kidneys • inhibits collagen synthesis and bone deposition by osteoblasts
Other Factors Affecting Bone • Hormones, vitamins and growth factors • Growth rapid at puberty • hormones stimulate osteogenic cells, chondrocytes and matrix deposition in growth plate • girls grow faster than boys and reach full height earlier (estrogen stronger effect) • males grow for a longer time and taller • Growth stops (epiphyseal plate “closes”) • teenage use of anabolic steroids = premature closure of growth plate and short adult stature