Osteoclasts: Function & Overview of Bone Cells

Anne Kamiya, Catherine Konopka
  • Author
    Anne Kamiya

    Anne has experience in science research and writing. She has a graduate degree in nutrition (gut microbiome & nutritional microbiology) and undergraduate degrees in microbiology (immunology & medical microbiology) and English (myth & folklore). She has also worked as an ocean & Earth science educator.

  • 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.

Explore osteoclasts. Learn the definition and see the different functions of osteoclasts. Discover where osteoclasts are found and understand how they are formed. Updated: 05/30/2022

What are Osteoclasts?

Skeletal bones, like skin and other organs, are living tissues that are constantly being rebuilt and repaired. Bones are a mesh-like matrix constructed from both organic and inorganic components. The organic components in bone are primarily proteins, like collagen. The inorganic components of bone include hydroxyapatite, which is made from the minerals calcium and phosphorus. Bone health is maintained by cells that break down bone and build it back up again in a complex process called bone remodeling. Bone remodeling is normally kept in careful balance. However, if there is excessive bone breakdown, diseases of low bone density like osteoporosis can develop. The cells responsible for bone remodeling processes are called osteoclasts and osteoblasts, which are derived from progenitor stem cells in the inner bone.


Bones are constructed from a mesh-like matrix of proteins and minerals. Excessive breakdown of bone makes it too porous.

A black and white photo comparison of normal bone (left) and osteoporotic bone (right).


Osteoblast and Osteoclast Definition

  • What are osteoblasts? Osteoblast cells are a type of cell involved in remodeling bones and maintaining bone mass by building bones, which is called bone synthesis. Osteoblasts create new bone cells, which are called osteocytes.
  • What are osteoclasts? Osteoclast cells are a type of cell involved in remodeling bones and maintaining bone mass by dismantling bones, which is called bone resorption. In resorption, the components in bone are returned to the body for reuse.

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.

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Where are Osteoclasts Found?

When it is time for bone remodeling, osteoclasts bind to the bone. But where are osteoclasts found within the bone? Before they can bind to bone, osteoclasts hover in proximity to the bone surface. Here, they release enzymes that erode the collagen in bone, creating a groovelike depression. The osteoclast then binds to the surface of the boney matrix at these depressions, which are called resorptive pits or Howship's lacunae. Contact with the resorptive pit occurs at a specific location called the ruffled border of the osteoclast. The area around the resorptive pit and the osteoclast is then sealed by actin proteins, forming a ''sealing zone.''


In this illustration, the osteoclast is nestled inside of a resorptive pit in bone. The numerous cells on the periphery of the bone surface are osteoclasts, ready to rebuild.

An illustration of an osteoclast inside of a groove in a bone.


Formation of Osteoclast Cells

Osteoclast cells are relatively large with several nuclei, formed by the fusion of many progenitor stem cells that became monocytes. Osteoclast formation is dependent on the signaling molecules RANKL and MCSF, which are both inflammatory cytokines. The RANKL and MSCF molecules are produced by osteoblast or osteocyte cells and bind to monocytes at their RANK receptors, stimulating monocytes to aggregate and transform into osteoclasts.

Some situations that prompt increased formation and activity of osteoclasts, leading to increased bone resorption, include low blood calcium levels, microfractures of bone, large fractures of bone, the release of RANKL by cancer cells, increased parathyroid hormone (PTH), decreased calcitonin hormone from the thyroid, and increased IL-6, which is another inflammatory cytokine.

Osteoclast Formation Steps when Blood Calcium is Low

  • Parathyroid hormone releases PTH when blood calcium is low.
  • The PTH tells osteoblasts and osteocytes it is time for bone resorption.
  • Osteoblasts or osteocytes release signaling molecules, like RANKL, that bind to monocytes.
  • Many monocytes fuse together to create an osteoclast cell.

Osteoclast Function

Osteoclastic activity serves multiple functions in the body when osteoclasts facilitate bone resorption. Boney deconstruction is not just a localized process because it puts the components of bone back into blood circulation. For instance, when blood calcium dips it is dangerous to vital nerve and heart functions, so the release of calcium from bones by osteoclasts is important to maintain a consistent blood calcium level, regardless of dietary intake. Osteoclasts also remove damaged parts of bone so it can be rebuilt by osteoblasts in cases of bone fractures. Three important osteoclast functions during bone resorption are listed below.

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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Video Transcript

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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Frequently Asked Questions

What are osteoclasts and osteoblasts?

Bones are constantly being restructured in processes called bone synthesis and bone resorption. Two types of bone cells are responsible for this process, called osteoblasts and osteoclasts. Osteoblasts are the bone builders, whereas osteoclasts are the bone destroyers

What are three functions of osteoclasts?

Osteoclasts have numerous functions during bone resorption. Firstly, they produce acids and digestive enzymes to break down the minerals and proteins in bone. Secondly, they release calcium into the bloodstream when hydroxyapatite is digested. Lastly, they release amino acids into the bloodstream when collagen proteins are digested.

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