Osteoclast: Definition, Function & Formation

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  • 0:00 Osteoclasts and Making Bone
  • 1:42 Osteoclast Morphology…
  • 4:41 Lesson Summary
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Lesson Transcript
Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Bone is continually made and remodeled as we grow and age. In this lesson, you will learn about a cell that degrades and reabsorbs bone in response to changes in nutrition and physical activity.

Osteoclasts and Making Bone

As we grow taller and stronger (and perhaps even wider), the shape and strength of our bones change. There are two types of cells that work together to alter your bones in response to many environmental factors: osteoblasts and osteoclasts. Bone is a hardened matrix composed mainly of the mineral calcium phosphate and the protein collagen. This matrix is produced and secreted by osteoblasts. Osteoblasts make bone in response to growth factors and mechanical stress on the bone.

Counteracting the osteoblast activity are osteoclasts - the bone reabsorbing cells. Osteoclasts make and secrete digestive enzymes that break up or dissolve the bone tissue. Osteoclasts then take up or 'absorb' the bone debris and further break it down inside the cell. The collagen is broken down into amino acids, which are recycled to build other proteins, while the calcium and phosphate are released to be used elsewhere in the body.

You can think of osteoclasts as a collection of miniature demolition machines. A wrecking ball and jack hammer break up the side of a building; a front loader scoops up the debris and loads it into a dump truck, which takes it to a recycling center where the material is further processed. All of this is done by osteoclasts!

Osteoclasts are found on top of or next to existing bone tissue, sometimes in close proximity to osteoblasts. There is on-going race between the two cell types; osteoblasts make bone tissue while osteoclasts reabsorb it. To recall the difference, remember osteo-B-lasts B-uild bone, while osteo-C-lasts C-ollapse bone.

Osteoclast Morphology and Lineage

Osteoclasts are quite different than osteoblasts, both in the way they look and where they come from. This makes sense because osteoblasts and osteoclasts do very different things. Osteoclasts are multinucleated, meaning that they are cells that have more than one nuclei and have a foamy-looking cytoplasm due to large numbers of lysosomes and enzyme-filled vesicles. In addition, the cell membrane closest to the bone tissue is ruffled, which increases the surface area for secretion of digestive enzymes and absorption of digested bone tissue.

Osteoclasts are derived from the same stem cells that make blood cells (red blood cells, various white blood cells, platelets, etc). Stem cells are like the queen of an ant colony; they continually reproduce while their 'daughters' have various functions in the body.

In your bone marrow, haemopoietic stem cells (HSCs) are constantly dividing.

Here is a step-by-step process of how this occurs:

1) When a haemopoietic stem cell divides into two cells, one remains an HSC.

2) The other cell can become either a myeloid or lymphoid stem cell.

3) The MSC can become several types of blood cells.

4) If a monocyte is formed it will become either a macrophage or osteoclast.

To become an osteoclast (step 5), several monocytes fuse together and become a multinucleated cell, which develops a ruffled border and many lysosomes in order to degrade and reabsorb the bone matrix.

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