ECM and Bone Cells

Extracellular Matrix of Bone

has inorganic matrix and organic matrix

Inorganic Matrix

About 65% of bones total weight

Consists mostly of calcium salts and phosphorus as part of a large mineral called Hydroxyapatite Crystals which gives bone its strength and ability to resist compression

Bicarbonate 3(HCO ,) potassium, magnesium, and sodium salts are also in the inorganic matrix

Organic Matrix (Osteiod)

About 35% of bones total weight

Consists of protein fibers (mostly collagen), proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins such as Osteocalcin

Collagen helps bone resist torsion (twisting) and tensile (pulling or stretching) forces and aligns with hydroxyapatite crystals to enhance bone hardness

Osteocalcin binds to calcium ions and hydroxyapatite crystals to organize bone’s inorganic matrix

Glycosaminoglycans and proteoglycans draw water out of the blood vessels and cells to help the ECM resist compression

Glycoproteins bind to both hydroxyapatite crystals and bone cells to bind them together

Bone Cells

osteoblasts, osteocytes, osteoclasts

Osteoblasts

bone builders, mature into osteocytes

derived from osteogenic cells

Cuboidal to columnar cells in the inner periosteum and endosteum that perform the process of Bone Deposition, by secreting organic matrix and assisting in formation of the inorganic matrix

Osteocytes

mature osteoblasts

maintain bone

surround themselves with matrix and become trapped in a cavity called a Lacuna

They secrete chemicals to maintain the ECM; Recruit osteoblasts to build areas of bone under tension

Osteoclasts

break down bone (releases calcium when the body needs it, and for maintaining blood calcium levels)

Large, multinucleated cells with Ruffled Borders

Derived from fusion of cells formed in the bone marrow

Reside in internal or external surfaces of bone

Responsible for process of Bone Resorption, which is breaking down bone ECM

-Secrete hydrogen ions that dissolve the inorganic matrix

-Secrete enzymes that break down organic matrix

Minerals, amino acids, and sugars released from bone enter the osteoclast for delivery to the blood

Osteopetrosis

aka Marble Bone Disease

occurs when defective osteoclasts do not properly degrade bone

Bone mass increases, but the bone is weak and brittle and appears stonelike (petra = “rock”

there is infantile osteopetrosis and adult osteopetrosis

Infantile Osteopetrosis

Predominantly inherited, more severe form

Skull openings fail to enlarge trapping nerves resulting in blindness and deafness

Medullary cavities in all bones fail to enlarge, decreases red bone marrow which can prove fatal

Drugs are used to stimulate osteoclasts and bone marrow

Adult Osteopetrosis

Inherited

Symptoms include bone pain, recurrent fractures, nerve trapping, and joint pain