7-1

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)

7-2

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

7-3

Calcium and Phosphate

• Phosphate is component of DNA, RNA, ATP, phospholipids, and pH buffers

• Calcium needed in neurons, muscle contraction, blood clotting and exocytosis

7-4

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

7-5

Carpopedal Spasm

• Hypocalcemia demonstrated by muscle spasm of hands and feet.

7-6

Hormonal Control of Calcium Balance

• Calcitriol, PTH and calcitonin maintain normal blood calcium concentration.

7-7

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

7-8

Calcitriol Synthesis and Action

7-9

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

7-10

Correction for Hypercalcemia

7-11

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

7-12

Correction for Hypocalcemia

7-13

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