The osteoclasts are constituent cells of the bone . Its function is to destroy and absorb dead or damaged cells from bone tissue. They make up the so-called remodeling unit together with osteoblasts, bone-forming cells.


What are osteoclasts?

Osteoclasts are bone cells. They constitute the bone together with the osteoblasts and osteocytes. Its name comes from the Greek words osteon = bones and klastos = fragmented. They are part of the bone resorption line. They go from one state to another, they are transitional, keeping the bone tissue in constant activity, a continuous process of remodeling. Contrary to what is thought, due to its calcified nature, bone is a living tissue. It is repaired and has the property of maintaining its internal condition thanks to the regular exchange of matter and energy with the environment. It undergoes constant modeling and remodeling processes, which intervene in its structure and function. It is there where the osteoclasts intervene.

  • Osteoclast characteristics
  • Training
  • Location
  • Osteoclast function
  • How are they different from osteoblasts?
  • Importance of osteoclasts

Osteoclast characteristics

Osteoclasts are independent , polarized, large and polynuclear cells , which come to have dozens of nuclei. They have a basal area with quantities of organelles, abundant vesicles and mitochondria. They have irregular or rough edges, brush-shaped, structures specialized in wear and resorption.


They are mobile cells, they move through the area of ​​the bone to “remodel”. They have an eminent metabolic flow. They are very active, they can produce acid phosphatases, although their main characteristic is the force of destruction of the bone. By destroying it, it initiates and actively participates in the calcium homeostasis process.

Osteoclasts act under normal conditions , they destroy bone tissue during growth stages, as part of a physiological process. They also act driven by various pathologies, generally chronic inflammatory processes, which cause an imbalance between bone formation and resorption. And they are stimulated -with the use of hormones- in case of alterations and fractures, for the destruction and resorption of altered and / or dead cells.

The cells are closely associated with local growth factors and with systemic hormones: PTH and calcitonin. Also with vitamin D. Sometimes its activity increases out of proportion, causing excessive wear of the bone, related to various pathologies.


Osteoclasts are formed by the fusion of mononuclear cells from a multipotent hematopoietic stem cell. All blood cells are also derived from these stem cells. Its cellular precursor is common and undifferentiated. They acquire differentiation in the bone microenvironment according to the stimuli of regulatory molecules.

There are at least three models that explain the emergence of osteoclasts. The first one says they result from the Granulocyte-Macrophage Colony Forming Units (CFU-GM). The second model exposes the theory of an Osteoclast Colony Forming Unit derived from the stem cell. And the third, presents them as the fusion of the monocytes of the UFC-GM.

Osteoclasts share some properties of macrophages, which is why they are included in the macrophage monocyte system. Although they present peculiarities such as the formation of resorption lagoons, the presence of rough edges, receptors for vitronectin and calcitonin. As well as the expression of tartrate resistant acid phosphatase (TRAP) and reactivity when activating the 121F protein, one of its markers.


Osteoclasts are found in the inner layer of the periosteum , attached to the bone. Also in the endosteum of the anatomical and functional unit of bone tissue, known as the Haversian system.

Due to their so-called clear zone, a ring of the cell without vesicles, they stick to the bone surface through integrins. Integrins are glycoproteins that facilitate the interaction of substances with the mechanisms of cellular metabolism. They are also known as receptors, in this case specialized bone receptors, they translate signals into cells.

Osteoclast function

Osteoclastic cells, together with osteoblasts , participate in the synthesis of glycoproteins and other molecules necessary for the bone remodeling process.

In the event, parathyroid hormone, calcitriol, prostaglandins and calcitonin, among other systemic hormones, mediate. Stimulating factors are also involved: interleukins, O2 radicals, tumor necrosis factor (TNF) and transforming growth factor alpha (TGF-α). And inhibitory factors: transforming growth factor beta (TGF-β), interferon-y, interleukin 4, nitric oxide, sex steroids and bisphosphonates.

The bone microenvironment is another factor to consider in the formation, differentiation and activity of osteoclasts. The remodeling cells secrete substances that stimulate you; while bone marrow stromal cells and osteoblasts improve their activity.

Osteoclasts fulfill the destructive function within the remodeling unit. They are the first cells to act. Their comb or brush-shaped extensions allow them to pull out old cells (without firmness and flexibility), altered or dead, from the bone. They do it in a sweep or back-and-forth motion. As they pass they excrete lysosomal enzymes and protons that degrade the matrix and create the environment for the mineral salts of the bone to dissolve.

Its activity destroys and creates osteoclastic cavities , also called Howship lagoons. Osteoclasts release minerals while removing bone tissue, they develop the process of bone resorption. The osteoblasts act behind them, absorbing and synthesizing the matrix that will fill the cavities of the bone to form its new tissue. The work of osteoclasts and osteoblasts is done in a synchronized way, in precise times and spaces.

The continuous remodeling process allows the normal function of the bone and the entire skeletal structure. Repairs fractures that compromise their strength. It also facilitates the adaptation of the bones to predominant physical activities. In addition to fulfilling a second function, the maintenance of homeostasis or regulation of the metabolism of various minerals, including calcium. Osteoclasts release calcium that goes into the bloodstream.

How are they different from osteoblasts?

The osteoclast is a large , multinucleated cell , without organelles, that wears, absorbs and remodels bone; It is born from hematopoietic cells. The osteoblast contains only one nucleus and is a cell specialized in the growth, development and remodeling of the tissue that supports the body. Its origin is mesenchymal.

Osteoblasts are also bone remodeling cells. But its main function is to process –extract- the collagen, proteoglycans, glycoproteins and organic salts that form the bone matrix. They synthesize cytokines and other protein substances responsible for intercellular communication. They induce changes in neighboring cells, altering their behavior and differentiation. Their predecessors produce substances that stimulate osteoclasts. They regulate the process of bone resorption.

When osteoclasts complete resorption in a remodeling unit they undergo apoptosis, programmed cell death. When osteoblasts complete matrix synthesis, a minimal portion of the cells die, another becomes impregnated with the synthesized matrix and becomes osteocytes. Osteoblasts that remain on the bone surface transform into lining cells.

Importance of osteoclasts

In the process of bone remodeling, it is vital that there is a coupling between osteoclast and osteoblast cells. The balance between the forces of destruction and bone formation allows the repair of weak areas and microfractures. It also optimizes vascular distribution and maintains mineral homeostasis.

Bone remodeling is essential for the normal functioning of the body during the growth stages. Also for the repair of injuries. It is a physiological process that fulfills two functions: the repair of damaged bone tissue and the regulation of the metabolism of various minerals. Through the process of bone resorption, calcium levels essential for muscle contraction and other important functions are sent to the blood.

Lack of stimuli for osteoclasts to maintain bone tissue can lead to decompensation of bone resorption. By increasing or decreasing the work of bone destruction, the body becomes prone to diseases of endocrine and nutritional origin. Decompensation contributes to the progression of chronic inflammatory bone diseases. As well as the loss of bone density due to the existence of high levels of certain minerals.

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