| Part type : | part |
| On both sides : | ??? |
| Digitisation completed : | false |
Structure
Osteoclasts are large (100 (m in diameter), multinucleate (up to 50 nuclei), nondividing, motile cells that reabsorb excess or inferior bone matrix in a process known as osteolysis. They form and are located in depressions known as Howship's lacunae, which represent areas of bone resorption. Osteoclasts display four distinct regions when they are viewed in electron micrographs. 1) The osteoclast membrane adjacent to the bone forms a ruffled border of deeply infolded membranes. This is the site of active bone resorption. 2) The clear zone surrounding the ruffled border contains microfilaments that help osteoclasts maintain contact with the bone surface. 3) The vesicular zone contains exocytotic vesicles that transport lysosomal enzymes to Howship's lacunae and endocytotic vesicles that transport degraded bone products from Howship's lacunae to the cell's interior. 4) The basal zone contains most of the cell organelles. It is located on the opposite side of the cell to the ruffled border. Osteoclasts are derived from blood monocytes. Cells of the monocyte-macrophage line are attracted towards bare bone surfaces where they can form osteoclasts by fusing with one another or by fusing with preexisting macrophage-like or osteoclast-like cells. Thus, instead of belonging to the bone cell lineage and being derived from osteoblasts as was originally believed, osteoclasts represent an extension of the monocyte cell line of differentiation.
Function
Osteoclasts are large (100 (m in diameter), multinucleate (up to 50 nuclei), nondividing, motile cells that reabsorb surplus or inferior bone matrix in a process known as osteolysis. They are required for the remodeling that occurs during bone growth and repair, and also for the removal of substandard or weakened matrix. They form and are located in depressions known as Howship's lacunae, which represent areas of bone resorption. During bone resorption, osteoclasts secrete acid. This lowering of pH causes the decalcification of the bone's surface layer. Acid hydrolases, collagenases and other proteolytic enzymes secreted by the osteoclasts then break down the organic portion of the bone matrix. The organic and inorganic degradation products are reabsorbed by the osteoclasts and sequentially released into the capillaries where they are recycled to other locations. During the growth and continual remodeling of bone, osteoclasts and osteoblasts work together in the balance of resorption and formation. The relative placement of osteoblasts and osteocytes determines the shape of bones as they grow. Osteoclasts are derived from blood monocytes. Cells of the monocyte-macrophage line are attracted towards bare bone surfaces where they can form osteoclasts by fusing with one another or by fusing with preexisting macrophage-like or osteoclast-like cells. Thus, instead of belonging to the bone cell lineage and being derived from osteoblasts as was originally believed, osteoclasts represent an extension of the monocyte cell line of differentiation.
