Glossary
osteoblast
An osteoblast is the cell that builds bone: it lays down the new matrix and starts the mineral deposit that hardens it.
An osteoblast builds bone. It sits on the surface of existing bone, produces the protein matrix that gives bone its shape — chiefly collagen — and starts the deposit of calcium and phosphate that hardens it. Once the matrix around an osteoblast has calcified, the cell stops building and becomes an osteocyte, walled into a small cavity of its own making. Osteoblasts work where bone is growing: at the growth plate of a long bone, in the widening of the shaft, and at the site of a fracture. Their work is not independent of the cell that removes bone, the osteoclast. The two sit on opposite sides of the same balance, and bone mass at any moment is the difference between what has been laid down and what has been taken away. Hormones and load shift that balance in either direction.
Taught in
Related terms
- osteoclast — An osteoclast is the large, multinucleated cell that breaks bone down, releasing calcium into the blood and reshaping the skeleton.
- osteocyte — An osteocyte is a mature bone cell, once an osteoblast, sealed inside the matrix it made — it maintains that matrix and senses load.
- periosteum — The periosteum is the tough membrane wrapped around a bone's outer surface, carrying its blood vessels and the cells that widen and mend it.
- diaphysis — The diaphysis is the long shaft of a long bone — a tube of compact bone wrapped around a marrow cavity.
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)