Glossary
osteoclast
An osteoclast is the large, multinucleated cell that breaks bone down, releasing calcium into the blood and reshaping the skeleton.
An osteoclast breaks bone down. It is a large cell with several nuclei, and it dissolves both the mineral and the protein of bone where it sits. Two needs call for that. Remodeling replaces old or damaged bone with new, and calcium balance needs a store it can draw on: bone holds the body's largest reserve of calcium, so when the blood level falls, osteoclast activity raises it. Growth needs it as well — a bone gets wider because new bone is laid on the outside while old bone is removed on the inside, which is how the shaft grows without becoming solid. Osteoclasts are the target of the drugs that slow bone loss, which is the practical reason the difference between the two bone cells is taught at all.
Taught in
Related terms
- osteoblast — An osteoblast is the cell that builds bone: it lays down the new matrix and starts the mineral deposit that hardens it.
- osteocyte — An osteocyte is a mature bone cell, once an osteoblast, sealed inside the matrix it made — it maintains that matrix and senses load.
Source: The anatomy and physiology guide in this course, whose organ and tissue descriptions follow the US National Cancer Institute's SEER Training Modules and MedlinePlus consumer health pages. Both are US federal government material, not subject to copyright in the United States. The definition above is written for this course in our own words. (SEER Training Modules accessed 2026-10-26; definition written 2026-10-27)