• Function: Secretes factors that activate osteoclasts (RANK-ligand).
Osteocytes • Function: reabsorbs bone matrix. Homeostatic maintenance of bone density and blood concentrations of calcium and phosphate ions. • 10-20% of osteoblasts incorporate into bone matrix to become osteocytes. • Account for 90% of cells in mature bone. • Osteoblast to mature osteocyte ~3 days. • Osteocyte half-life ~25 years. • Two types of Osteocytes o Nascent osteocyte • Remain in direct contact with overlying bone lining cells and osteoblasts. • Cell volume decreased ~30% • Extend pseudopodia toward previously embedded cells. • Develop gap junctions between cell processes. o Mature osteocyte • Completely embedded within bone matrix, within lacunae. • Projections through canaliculi • Cell volume decreased ~70% • Organelles simplified. • Osteocytes linked through gap junctions.
Osteoclasts • Function: Bone dissolving cells found on the bone surface. • Giant (30 – 100 μm), multinucleated from hematopoietic precursors. • Precursors circulate in the blood and bone marrow. • Mature osteoclasts formed from fusion of 10-20 precursors. • Polarized, ruffled border to increase surface area for resorption. • Motile, only reabsorb when not moving. • Form tight junctions between bone surface and basement membrane to form a sealed compartment. • After reabsorbing bone, undergoes apoptosis. • Osteocalsts often reside in pits called resorption bays that they have etched into the bone surface.
Bone matrix Inorganic Component • 60 – 70% • Function: Mechanical stability and Storage • Stores: o 99% of body’s calcium o 85% of phosphorus o 80% of carbonates o 60% of magnesium o 35% of sodium Organic Component • 30 – 40% • Function: Provides tensile strength • Collagen ~90% • Minor components: o Fibronectin o Osteonectin o Proteoglycans
Ted Talk • Molly Stevens: A new way to grow bone • http:// _grow_bone • http:// - bone-disease-born-1558277
growth and development
Bone Cells Osteoblasts: Bone forming cells Osteocytes: Former osteoblasts trapped in bone matrix. 90% of cells in mature bone Bone-lining cells Osteoclasts: Resorb bone, release calcium and phosphates
• Bone remodeling occurs at discrete packets throughout the skeleton. • Bone Remodeling Steps 1. Stress in the bone is sensed by osteocytes, hormones, or cytokines. 2. Lining cells in periosteum is activated. 3. Lining cells secrete RankL. 4. RankL binds to pre-osteoclasts. 5. Pre-osteoclasts fuse to form multi-nucleated osteoclasts.
• Steps continued: 6. Osteoclasts dissolve bone. • Form resorption pit. • Remove break down products. • Resorption lasts ~3 weeks. 7. Osteoclasts undergo apoptosis.
- Spring '12
- Bone marrow