Skeletal (Bones) System

Bone functions

storage depot for Ca2+

important for structure (sit up straight/stand)

protects vital organs

lever system for mobility

production of WBC and RBC (bones are primary site for RBC formation)

Red marrow

hematopoietic stem cells: can differentiate into RBC, WBC, osteoclasts

Yellow Marrow

Mesenchymal stem cells: can differentiate into osteoblasts, osteocytes, chondrocytes, adipocytes

Osteoclasts

bone degradation

Osteoblasts

bone formation (build bone)

Osteocytes

bone cells

adipocytes

fat cells

Compact bone

surrounds the entire bone, mineralized portion making bone hard

Where is red marrow found?

spongy bone (epiphysis)

Where is yellow marrow found?

bone marrow, diaphysis

What do compact and spongy bone do?

contribute to compressive and tensile strength, compact bone better in terms of both types of strength

What are the requirements for bone growth and calcium storage?

Cartilage → Osteoid → hydroxyapatite crystals = Ca10(PO4)6(OH)2

What are babies bones mostly made of?

initially bones are primarily made of cartilage so that babies can be pushed out of birth canal

What do chondrocytes do>

produce cartilage starting in the diaphysis pushing up to epiphysis in order to lengthen bone in both directions

later die, leaving behind cartilage

Osteoblasts function

after chondrocyte death, osteoblasts create osteoid and osteoid then binds to cartilage

osteoblasts also produce hydroxyapatite which binds to osteoid

(more blasts than blasts)

Osteocyte function

control activity of osteoblasts and osteoclasts

Canaliculi

helps direct blood to cells keeping them alive even though they are in mineralized portion

Growth hormone function

stimulates formation/activity of chondrocytes and osteoblasts

Where is growth hormone made and stored?

Made in pituitary, hypothalamus stimulates secretion

Growth hormone cascade of events

hypothalamus released GHRH stimulating the pituitary to release growth hormone

epiphyseal closure (growth/epiphyseal plate)

chondrocytes die and don’t make cartilage anymore so the bones can no longer elongate

dictated by genetics

Remodeling

bones have stopped growing but can still change composition

Osteon (Haversian Canal/System)

compact bone

osteocytes, osteoblasts, osteoclasts

Osteoporosis prevalence

for every 1 man, 3 women with osteoporosis

Why are women at greater risk than men for osteoporosis?

bone loss starts at age 30-35 at 1% a year for women and increases to 3% at age 50-55, in men bone loss starts at age 40-45 at 1% a year

Why does menopause affect bone density?

Lack of estrogen, when estrogen is low sclerostin is high

Sclerostin

made by osteocytes, lowers osteoblasts activity (clast>blasts)

What do doctors do when on the verge of osteoporosis?

prescribe estrogen replacement

Risk factors for osteoporosis

smoking (earlier menopause), asian, decreased Ca2+ consumption, heredity, testicular cancer where testes removed

Osteoporosis prevention

strength training to make bones more dense and increase osteoblast activity