Osteoclast

• An osteoclast is a type of bone cell that resorbs bone tissue.
• This function is critical in the maintenance and repair of compact bones in the mammalian skeleton.

• Kolliker discovered osteoclasts in 1873.

• An osteoclast is a large multinucleated cell and human osteoclasts on bone typically have five nuclei and are about 150-200 μm in diameter.
• In bone, osteoclasts are found in pits in the bone surface, which are called resorption bays, or Howship's Lacunae.

• Osteoclasts are characterized by a cytoplasm with a homogeneous, "foamy" appearance.
• This appearance is due to a high concentration of vesicles and vacuoles.
• These vacuoles include lysosomes filled with acid phophatase.
• This permits characterization of Osteoclasts by their staining for high expression of tartrate resistant acid phosphatase (TRAP) and cathepsin K.

• Osteoclast rough endoplasmic reticulum is sparse, and the Golgi complex is extensive.
• Osteoclasts, large irregular cells with multiple nuclei, fill much of their cytoplasm with mitochondria to supply the energy required for these cells to resorb bone.

• They usually lie directly against the bone matrix on endosteal, periosteal, and Haversian system bone surfaces, but unlike osteocytes, and presumably osteoblasts, they can move from one site of bone resorption to another.

• Osteoclasts appear to form by fusion of multiple bone-marrow-derived mononuclear cells.
• When they have finished their bone resorbing activity, they may divide to reform multiple mononuclear cells.

• One of the most distinctive features of osteoclasts is the complex folding of their cytoplasmic membrane where it lies against the bone matrix at sites of bone resorption.

• This ruffled or brushed border appears to play a critical role in bone resorption, possibly by increasing the surface area of the cell relative to the bone and creating a sharply localized environment that rapidly degrades bone matrix.

• The fluid between the brush border and the bone matrix probably has a high concentration of hydrogen ions and proteolytic enzymes: the acidic environment could demineralize bone matrix, and the enzymes could degrade the organic bone matrix.

• In cancellous bone, osteoclasts resorbing the bone surface create a characteristic depression called a Howship's lacuna.
• In cortical bone, several osteoclasts lead the osteonal cutting cones that remodel dense cortical bone.