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) 7-1

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 7-2

Calcium and Phosphate

•

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

•

Calcium needed in neurons, muscle contraction, blood clotting and exocytosis 7-3

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 7-4

Carpopedal Spasm

•

Hypocalcemia demonstrated by muscle spasm of hands and feet.

7-5

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 7-7

Calcitriol Synthesis and Action

7-8

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 7-9

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 7-11

Correction for Hypocalcemia

7-12

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