Osteoclast

An osteoclast (from the Greek words for "bone" (ὀστέον), and "broken" (κλαστός)) is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodelling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium.

Morphology

Osteoclasts were discovered by Kolliker 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. Giant osteoclasts can occur in some diseases, including Paget's disease of bone and bisphosphonate toxicity. When osteoclast-inducing cytokines are used to convert macrophages to osteoclasts, very large cells that may reach 100 µm in diameter occur. These may have dozens of nuclei, and typically express major osteoclast proteins but have significant differences from cells in living bone because of the not-natural substrate. The size of the multinucleated assembled osteoclast allows it to focus the ion transport, protein secretory and vesicular transport capabilities of many macrophages on a localized area of bone.