How it works
A simple bone cell diagram: a piece of bone cut through, with its cells on the surface and inside it. It shows bone formation on the left, where osteoblasts lay down new matrix, and bone being taken away on the right.
Read the full guide
The cells
Flat osteoprogenitor cells lie on the surface at the left. Next to them a row of osteoblasts stands on the osteoid they have laid down. Osteocytes lie inside the matrix. The large cell in the hollow on the right is an osteoclast.
What each one makes or sits in
Osteoid is the soft layer under the osteoblasts; below it the matrix is mineralized and hard. Each osteocyte sits in a lacuna, and canaliculi join one lacuna to the next. Under the osteoclast are its ruffled border and the resorption pit it has made.
- Blast builds and clast breaks: an osteoblast makes bone, an osteoclast takes it away.
Questions and answers
What are the types of bone cells?
Osteoblasts build bone, osteocytes are mature bone cells sealed inside it, and osteoclasts break it down. Osteoblasts and osteocytes come from osteoprogenitor cells, the fourth kind on this diagram.
How is new bone formed?
Osteoblasts lay down osteoid, a soft matrix that is not yet mineralized. The osteoid then mineralizes into hard bone. Osteoblasts caught inside it stay in lacunae and mature into osteocytes.
What is the difference between an osteoblast and an osteoclast?
An osteoblast builds bone: it has one nucleus and lays down osteoid. An osteoclast breaks bone down: it is a large cell with several nuclei that sits in a pit on the bone surface.
Where are osteocytes found?
Inside the bone matrix, each in a small space called a lacuna. Thin channels called canaliculi join the lacunae, and the osteocytes reach into them with long processes.