An osteoclast is a large cell that breaks down bone. Osteoclasts work together with osteoblasts, the cells that build new bone. By removing old bone while osteoblasts lay down new bone, they keep the skeleton strong and allow it to repair itself. Each osteoclast contains many nuclei, which makes it one of the largest cells in the body.
Osteoclasts are a normal part of bone, not a diagnosis. Pathology reports may mention them when there is more bone breakdown than usual or when they are part of a tumor. This article explains what osteoclasts do, what they look like under the microscope, and why they may be mentioned in your report.
Bone is constantly being renewed throughout life. Osteoclasts attach to the surface of the bone and release acid and enzymes that dissolve it. Osteoblasts then fill in the space with new bone. This cycle is called bone remodeling.
Osteoclasts develop from the same family of blood cells that forms macrophages. Several cells start separately and then fuse, which is why a mature osteoclast has many nuclei. Hormones such as parathyroid hormone, along with signals from nearby bone cells, control osteoclast activity.
Under the microscope, osteoclasts are very large cells with pink cytoplasm and many nuclei, sometimes more than 20. They usually sit directly on the surface of the bone. As they break down the bone, they create small scooped-out pits underneath them.
An increase or change in osteoclasts can be a clue to several conditions. Osteoclasts may be mentioned in the following situations:
Several medicines used for bone conditions work by slowing down osteoclasts. These medicines treat conditions such as osteoporosis, and some are also used when cancer affects the bones.