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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)
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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
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Calcium and Phosphate
• Phosphate is component of DNA, RNA,
ATP, phospholipids, and pH buffers
• Calcium needed in neurons, muscle
contraction, blood clotting and exocytosis
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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
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Carpopedal Spasm
• Hypocalcemia demonstrated by muscle spasm
of hands and feet.
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Hormonal Control of Calcium Balance
• Calcitriol, PTH and calcitonin maintain
normal blood calcium concentration.
7-6
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
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Calcitriol Synthesis and
Action
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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
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Correction for Hypercalcemia
7-10
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
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Correction for Hypocalcemia
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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
7-13