Osteoclast: Definition


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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.

What do osteoclasts do?

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.

What do osteoclasts look like under the microscope?

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.

Why are osteoclasts mentioned in a pathology report?

An increase or change in osteoclasts can be a clue to several conditions. Osteoclasts may be mentioned in the following situations:

  • Overactive parathyroid glands. Too much parathyroid hormone, often caused by a parathyroid adenoma, makes osteoclasts more active. In severe cases, this can create a bone lesion called a brown tumor.
  • Diseases that change bone turnover. In Paget disease of bone, osteoclasts are larger and more numerous than normal. In osteoporosis, bone is broken down faster than it is rebuilt.
  • Cancer in bone. Some cancers that spread to bone cause osteoclasts to break down the surrounding bone. This can weaken the bone and lead to fractures.
  • Tumors that contain osteoclast-like cells. Some tumors, such as giant cell tumor of bone, contain many large cells that look like osteoclasts. These are called osteoclast-like giant cells.

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.

Questions to ask your doctor

  • Why does my report mention osteoclasts?
  • Is there more bone breakdown than normal in my sample?
  • Could my parathyroid hormone levels be affecting my bones?
  • Are the osteoclasts part of a tumor, or a reaction to something else?
  • What is the final diagnosis in my report?
  • Do I need any blood tests or imaging to look at my bones?
  • Do I need any further treatment or follow-up?

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