Osteoblasts synthesize type I collagen and produce alkaline phosphatase, osteocalcin, and bone sialoprotein. They are found on the surface of bone and have an eccentrically located nucleus and a prominent Golgi apparatus.

Osteoblast synthetic activity is increased by physical activity, parathyroid hormone and growth factors, and is decreased by inactivity and steroids. They are responsive to parathyroid hormone and express vitamin D and estrogen receptors.

Osteocytes are differentiated (mature) osteoblasts that lie in small lacunae within the mineralized bone.

Osteocytes maintain contact with other osteocytes and osteoblasts through cellular processes, which travel through canaliculi (not visible). The cellular processes terminate at gap junctions.

Osteoclasts are derived from histiocytes and produce tartrate-resistant acid phosphatase. They have receptors for calcitonin and regulate calcium homeostasis.

Osteoclasts are multinucleated cells that resorb bone and attach to bone through the vitronectin receptor. They form cavities in bone called Howship’s lacunae and secrete carbonic anhydrase II, which causes acidification of the extracellular space and mineral dissolution.

Osteoid represents secreted type I collagen. On hematoxylin eosin (H&E) sections, osteoid appears as irregularly shaped islands of pink material. Within the newly formed osteoid, there are numerous osteocytes ( arrowhead ), giving it a “starry-sky” appearance.

Woven bone represents areas of rapidly forming new bone and is seen in variety of conditions, including embryogenesis, fracture healing, infection, and tumors. The newly formed mineral stains a dark purple ( arrowhead ) and is easily recognizable.

Polarized light clearly demonstrates the disorganized, interlacing, nonparallel collagen fibers.

Trabecular bone, also called cancellous or medullary bone , forms a complex intramedullary network and regulates mineral metabolism. On H&E sections, the bone is reddish-pink, the intervening marrow consisting of fat and hematopoietic cells.

Over time, the osteoblasts and osteoclasts transform the disorganized collagen bundles into parallel lines of collagen, which increases tensile strength. This polarized light examination highlights the lamellar or parallel nature of trabecular bone.

Cortical bone, also called compact bone , provides structural stability. It makes up 80% of skeletal bone by weight.