(CH.6-B) Bone Development, Growth, Remodeling, and Disorders

most important hormone in Ca2+ homeostasis. regulates blood calcium by stimulating osteoclasts to resorb bone, releasing calcium into the blood, and promoting vitamin D activation for intestinal calcium absorption.

Regulated by genetic factors and two control loops: Hormonal controls; Negative feedback loop that controls blood Ca2+ levels. Response to mechanical stress (Wolf’s Law)

Dig depressions or grooves as they break down matrix. Osteoclasts secrete lysosomal enzymes and protons (H+) that digest the bone matrix and convert calcium salts to soluble forms. Osteoclast activation involves PTH (parathyroid

hormone) and immune T cell proteins during an

immune response.

unconfirmed

Wolf's Law states that bones grow or remodel in response to demands

placed on them. like mechanical stress, becoming thicker and stronger where stresses are greatest (Handedness).

Fractures are classified by bone end position (displaced/nondisplaced), completeness of break (complete/incomplete), and skin penetration (open-compound/closed-simple).

1. Hematoma formation 2. Fibrocartilaginous (soft) callus formation (new blood vessels) 3. Bony callus [of spongey bone] formation 4. Bone remodeling (healed fracture)

Osteoporosis is a disease where bone resorption exceeds deposition, leading to decreased bone mass and increased fracture risk, especially in postmenopausal women.

Matrix remains normal, but bone mass declines

• Spongy bone of spine and neck of femur most susceptible

insufficient exercise, poor diet, smoking, hormone imbalances (Hyperthyroidism. Low blood levels of PTH. Diabetes mellitus), and certain medical conditions. Estrogen drops, which forms a role in bone density. Immobility. Males with prostate cancer taking androgen-

suppressing drugs

Calcium and vitamin D supplements, weight-bearing exercise, hormone replacement (Slows bone loss but does not reverse it)

• Controversial because of increased risk of heart

attack, stroke, and breast cancer) and lifestyle changes to maintain bone density.

Excessive and haphazard bone deposit and resorption cause bone to grow fast and develop poorly.

• Called Pagetic bone

• Very high ratio of spongy to compact bone and reduced

mineralization

• Cause unknown: possibly viral

Intramembranous ossification develops bone from fibrous membranes, forming flat bones like the skull and clavicles. (called membrane bones). Begins within fibrous connective tissue membranes formed by mesenchymal cells.

Ost: New bone matrix is deposited by osteoblasts in

the form of this

Osteoid seam: band of unmineralized bone

matrix that marks area of new matrix

Calcification front: abrupt transition zone

between osteoid seam and older mineralized

bone

Secretion by glands and nerve cells. Cell division. 1200 – 1400 grams of calcium in body. Intestinal absorption requires Vitamin D metabolites

PTH: Removes calcium from bone regardless of bone integrity. Promotes activation of vitamin D by kidneys-- to increased absorption of Ca2+ by intestinal mucosa

-Activation of Calcidiol to Calcitriol

Calcitonin: produced cells of thyroid gland in

response to high levels of blood calcium levels. At high doses it can lower blood calcium levels temporarily

Hypocalcemia: low levels of calcium cause

hyperexcitablility. Hypercalcemia: high levels of calcium cause

nonresponsiveness

• Sustained high blood calcium levels can lead to

deposits of calcium salts in blood vessels or

kidneys and formation of kidney stones

Hormone released by adipose tissue

• Role in bone density regulation

• Inhibits osteoblasts in animals

Neurotransmitter regulating mood and sleep

• Most made in gut

• Secreted into blood after eating

• Interferes with osteoblast activity

• Serotonin reuptake inhibitors (e.g., Prozac) cause lower bone density

Mechanical stress causes remodeling by

producing electrical signals when bone is

deformed

• Compressed and stretched regions are oppositely

charged

• Compression/tension changes fluid flows within

canaliculi, which may also stimulate remodeling

Hormonal controls determine whether and

when remodeling occurs in response to

changing blood calcium levels, but

mechanical stress determines where it

occurs

Reduction: the realignment of broken bone ends. Closed reduc.: physician manipulates to correct position. Open reduc.: surgical pins or wires secure ends

-Immobilization of bone by cast or traction is needed for healing

-Time needed for repair depends on bone

broken, and age of patient

Bones are poorly mineralized

• Osteoid is produced, but calcium salts not

adequately deposited

• Results in soft, weak bones

• Pain upon bearing weight

Results in bowed legs and other bone deformities

because bones ends are enlarged and abnormally

long

• Cause: vitamin D deficiency or insufficient dietary

calcium

Most long bones begin ossifying by 8 weeks, with primary ossification centers developed

by week 12. At birth most long bones are ossified (not epiphyses- which persist through childhood/adolescence). At age 25, all bones are completely

ossified, and skeletal growth ceases

Kids: bone formation > resorbtion. Young adults: balance, males more mass. Bone density changes during life, dependent on genetic (Gene for Vitamin D's cellular docking determines mass early in life and osteoporosis risk as age)

Electrical stimulation; Daily ultrasound treatments hasten repair