Structure of Bones Quiz

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Which statement best describes osteoclasts?

A They form bone matrix and originate from mesenchymal stem cells.
B They secrete osteoid and regulate mineralisation of new bone.
C They are inactive bone lining cells derived from osteoblasts.
D They become osteocytes after being trapped in bone.
E They resorb bone and originate from haemopoietic stem cells. ✓

The answer is E: They resorb bone and originate from haemopoietic stem cells.

Explanation

Why E is correct

See Osteoclasts. Option E is correct. Osteoclasts are the cells that break down bone, a process called bone resorption. They sit on the bone surface, are large and multinucleated (they have many nuclei), and dissolve the mineral by releasing acid and break down the collagen with enzymes such as collagenase. They are not made from the same cells as bone-forming cells. Osteoclasts come from haemopoietic stem cells, the blood-forming cells of the bone marrow, and are in the monocyte and macrophage family of white blood cells, whereas osteoblasts come from mesenchymal stem cells. Bone is renewed all the time: osteoclasts remove old bone while osteoblasts make new bone, and the balance between them keeps bones strong. Osteoclasts also release calcium into the blood when needed, so they help to control blood calcium. When osteoclast activity outruns bone formation, bone density falls, which is the main process behind osteoporosis. A memory aid is that osteoclasts clear bone away, while osteoblasts build it.

Function: osteoclasts break down bone, a process called resorption.
Origin: they come from haemopoietic stem cells and are in the monocyte and macrophage family.
Contrast: osteoblasts build bone and come from mesenchymal stem cells.

Why the other options are incorrect

A. They form bone matrix and originate from mesenchymal stem cells.: This describes osteoblasts, which build bone and come from mesenchymal stem cells. Osteoclasts do the opposite job and have a different origin. It would be the answer to a question about the bone-forming cell.
B. They secrete osteoid and regulate mineralisation of new bone.: Secreting osteoid, the unmineralised bone matrix, and starting its mineralisation are the work of osteoblasts. Osteoclasts break bone down. It would be the answer to a question about the cell that makes new bone matrix.
C. They are inactive bone lining cells derived from osteoblasts.: Inactive bone lining cells are flattened cells on the bone surface that come from osteoblasts and do not resorb bone. Osteoclasts are active, multinucleated cells. It would be the answer to a question about resting cells on a bone surface.
D. They become osteocytes after being trapped in bone.: This describes osteocytes, which are osteoblasts that have become trapped in the bone matrix they made. Osteoclasts are large cells on the bone surface and are not trapped in bone. It would be the answer to a question about the cell that lives in a lacuna.

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More questions from this set. Start the quiz to answer them and see each explanation.

1
Which cell type is mainly responsible for laying down new bone matrix?
2
Which structure is formed mainly by intramembranous ossification during normal human development?

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