Osteoclasts arise from macrophage precursors, so they share many of the same antigens found on macrophages and giant cells. Therefore, identifying osteoclasts relies on the presence and lack of certain antigens. Antigens associated with osteoclasts are CD13, CD14, and CD68, which has a particularly strong association with osteoclasts. The lack of CD56, GrB, and Ki67 expression help distinguish osteoclasts from macrophages and giant cells.[5]

On light microscopy, osteoclasts are identifiable as containing red in their cytoplasm if stained with tartrate-resistant acid phosphatase (TRAP). The multinucleated appearance of osteoclasts can typically be visualized on light microscopy, and large vacuoles can also be seen. Light microscopy allows visualization of the osteoclasts being surrounded by osteocytes and other components of the bone matrix, allowing for viewers to gain an understanding of the complex interactions between different cells within the bone matrix.[11] Additionally, since osteoclasts contain a large number of mitochondria, observing the increased cytochrome C oxidase activity of osteoclasts in bone marrow can help differentiate them from other cells.[1]

On electron microscopy, which has much higher magnifying power than light microscopy, the extensive multinucleated appearance of osteoclasts can be visualized, along with with a closer view of the ruffled border adjacent to the bone undergoing resorption. The lacunae in which osteoclasts are contained, resorbing bone, are known as Howship Lacunae. Lying under the ruffled membrane are numerous visible vacuoles. Some of the vacuoles contain cells visibly undergoing apoptosis.[11]

The RANK-RANKL interaction is essential for osteoclastogenesis, and that interaction is under the regulation of osteoprotegerin (OPG). Osteoblasts and stromal cells secrete OPG, and it binds to RANKL on osteoblasts and stromal stem cells, and prevents the interaction between RANKL and RANK, inhibiting osteoclastogenesis. This serves as a control mechanism to prevent excess bone resorption from taking place.[12] OPG opposes the effects of 1, 25-dihydroxy-vitamin D, parathyroid hormone (PTH), parathyroid hormone-related peptide (PTH-rP), and prostaglandin E2 (PGE), which all increase bone resorption.[6] Therefore, the interplay between RANK-RANKL-OPG is crucial in the regulation of osteoclastogenesis and the maintenance of bone mineral density and plasma calcium levels. Dysregulation of the RANK-RANKL-OPG system plays a vital role in the pathogenesis of many conditions, such as osteoporosis, osteopetrosis, and bone tumors.

A complex interplay exists between the musculoskeletal and immune systems. Bone metabolism and immune activity are directly and indirectly connected, and immune system dysregulation can lead to osteoclast-mediated pathology. Two mechanisms can cause immune system-mediated osteoclast generation: T-cell mediated generation of osteoclasts and innate immune system generation of osteoclasts[2]:

T-Cell Mediated Generation of Osteoclasts

Similar to osteoblast expression of RANKL, arguably the most important osteoclast determining factor, activated T-cells also express RANKL. When there is a persistent of elevated active T-cells, it can lead to increased osteoclast differentiation, and ultimately, excess bone resorption.

Additionally, active T-cells produce pro-inflammatory cytokines, which can stimulate the expression of RANKL on osteoblasts. Inflammatory cytokines that can stimulation osteoclast-mediated bone resorption are IL-1, IL-6, IL-11, IL-15, IL-17, and TNF-alpha. Additionally, IL-1 and TNF-alpha stimulate osteoblasts, which can cause RANKL-induced osteoclasts differentiation by the osteoblasts. Inflammatory cytokines that inhibit osteoclast-mediated bone resorption are IL-4, IL-10, IL-12, IL-13, IL-18, IFN-alpha, and GM-CSF.

Innate Immune System Regulation of Osteoclasts

Immune cells in the body have toll-like receptors (TLRs), which recognize pathogen-associated molecular patterns (PAMPs) on invading organisms, allowing the body to mount an immune response to the foreign organisms. Upon activation of the TLRs and immune cells, pro-inflammatory cytokines are produced, which can indirectly affect bone metabolism through activation of osteoclast differentiation, as previously mentioned. This is an indirect mechanism through which the innate immune system can cause osteoclast-mediated resorption of bone. Additionally, osteoclasts themselves have TLRs, which can be directly activated by foreign organisms. When TLRs on late osteoclast progenitors and osteoclasts are active, that increases the generation of osteoclasts. However, when TLRs on early osteoclast progenitors are activated, it decreases the generation of osteoclasts. This has been theorized to be the body’s way of preventing excess osteoclast generation.

Aside from the interaction between the musculoskeletal and immune systems, the formation of many of the osteoclast structures previously described is important to prevent malfunctioning of osteoclasts. For example, failure of the ruffled border to form may result in the failure of bone resorption.[3]

Osteoarthritis

• Osteoarthritis (OA), a degenerative disorder of the hyaline cartilage and subchondral bone at the ends of bones, is the most common joint condition in the United States, and it affects many individuals throughout the world.[13] Osteoarthritis is associated with an altered ratio of osteoblasts to osteoclast, with an increased number of osteoclasts causing bone resorption of subchondral bone, leading to thinning of the subchondral bone plate. Not only is there an increased number of osteoclasts, but their activity is also increased, with a study finding that their surface area was increased in OA, leading to increased bone resorption.[8] An interesting finding of the study was that loading a joint that had osteoarthritis decreased the expression of RANKL, Cathepsin K, and TNF-alpha, which may provide a link between mechanical loading of a joint and reduced activity of osteoclasts.[8] Therefore, osteoclasts play a significant role in the pathogenesis of osteoarthritis.

Osteopetrosis

• Known as “marble bone disease,” osteopetrosis refers to several genetic diseases where patients have an increase in bone mass, but the bone is brittle and prone to pathological fractures.[9][10] There are many presentations of osteopetrosis, but they have a common link in that osteoclast function is decreased. The number of osteoclasts is typically normal or even increased. However, they are not functional and typically have problems with adhesion to the bone and activation, leading to impaired secretion of acids that cause bone resorption.[9]

Osteoporosis

• Osteoporosis is an age-related microarchitectural deterioration of bone, hypothesized to occur as a result of an imbalance between osteoblasts and osteoclasts. While osteoporosis is common in postmenopausal females, it can occur in males as well. Postmenopausal females have lower levels of estrogen, and estrogen is known to be a potent inhibitor of osteoclasts. Therefore, postmenopausal females lose that osteoclast inhibition, which then leads to increased osteoclast bone resorption. The hypothesis is that decreased levels of testosterone in men also increase osteoclast activity, but it is not clear through what mechanism.[4] A significant cause of morbidity and mortality in elderly individuals, osteoporotic-related fractures (especially of the vertebrae and spine) should be prevented through effective patient counseling and treatment.[14]

Paget’s Disease

• Paget’s disease is a systemic bone disorder that starts occurring in middle-aged individuals and older. It is the second most common bone disorder in that age group in the United States and the United Kingdom. While it can manifest with bony abnormalities and disfigurations of the skull, pelvis, and spine, it can also include deafness and nerve compression. In the majority of patients with the disease, it is asymptomatic. Paget disease has correlations with increased osteoclastic activity, but the exact cause is still unknown. Some studies have linked it to paramyxoviruses, but there has not been concrete evidence found regarding its pathogenesis. Paget disease typically occurs in four stages, with the first stage primarily showing increased osteoclastic activity, the second stage is a mix of osteoclastic and osteoblastic activity, the third stage being primarily osteoblastic activity, and the fourth stage is the degeneration of the bone. This progression ultimately leads to softening and enlargement of bone, which makes patients prone to pathological fractures. One rare complication of Paget’s disease is osteosarcoma, which can be deadly in patients. Histologically, there is typically a hypercellularity of osteoblasts and osteoclasts, with the osteoclasts normally having a very large number of nuclei (can reach up to 100) and intracytoplasmic inclusions.[9][4][15]

Juvenile Paget’s Disease (JPD)

• JPD is an autosomal recessive condition where there is a loss of function mutation of OPG, which causes a loss of regulation of the RANK/RANKL interaction, leading to increased osteoclast differentiation and activity, and increased osteoblast activity. Patients have a high turnover in their bones, which can lead to skeletal abnormalities such as skull thickening, axial spine vertebral compression fractures, kyphosis, widening of the diaphysis of long bones, and pelvic deformity. The key feature of JPD is its early onset, with newborn infants potentially showing symptoms of the disorder. JPD has a highly variable clinical presentation, and the histology of patients with JPD is also variable, with some patients not having any woven bone, and some patients having a highly disorganized bone matrix.[4][9]

FEO (Familiar Expanile Osteolysis), Expansile Skeletal Hyperphosphatasia (ESH), and Early Onset Paget’s Disease of Bone (Early Onset PDB)

• FEO, ESH, and Early Onset PDB are all characterized by focal skeletal abnormalities located in many regions throughout the body. Their onset of symptoms are during the second decade of life, and they range from progressive hearing loss found in all of the conditions (due to degeneration of resorption of the bones in the inner ear), to mainly the involvement of the appendicular skeleton in FEO, with the axial and appendicular skeleton involvement in early-onset PDB. ESH and early onset PDB are differentiated from FEO by episodic hypercalcemia. Skeletal abnormalities can lead to pathological fractures in patients. In all of these conditions, osteoclasts are hypothesized to have activating mutations of the RANK (not requiring the RANKL signal), which can cause excess resorption and lead to the widespread bony involvement. Osteoclasts become enlarged in this condition, but the number of osteoclasts does not typically elevate in these conditions.[9] With histological examination, there is a high amount of woven bone present in patients with these conditions, which ultimately can progress to lytic lesions of the bone and replacement with adipocytes and fibrovascular tissue.[9]

Pycnodysostosis

• Pycnodysostosis is an extremely rare genetic disease that presents as short stature with dysmorphic features of the skull, clavicles, and digits. It results from a genetic mutation of Cathepsin K, a proteinase secreted by osteoclasts that destroys Type I collagen. As a result, osteoclasts can resorb the mineralized portion of the bone matrix, but they are unable to resorb the organic portion of the bone matrix. This impairment of type I collagen degradation leads to osteoclasts phagocytosing the organic components of the bone matrix, which can be visualized in osteoclasts as intracellular vacuoles containing fibrillary collagen.[4][9] Patients with Pycnodysostosis typically have an increased number of osteoclasts.

Adult T-Cell Leukemia (ATL)

• As previously mentioned, T-cells can have RANKL ligands that bind to RANK on osteoclast precursors, causing osteoclastogenesis. In patients with ATL, a common side-effect is hypercalcemia, caused by increased osteoclast bone resorption activity. This directly results from increased RANK/RANKL interaction, and indirectly by the production of inflammatory cytokines such as PTHrP, IL-1, and TGF-beta. Other forms of cancer have correlations with increased expression of RANKL, such as breast cancer and multiple myeloma, which can present as hypercalcemia via osteoclastogenesis.[16][17][16]

Periodontitis

• The pathophysiology of periodontitis is that as bacterial cells increase in number, they ultimately attract immune cells such as macrophages. Those immune cells release inflammatory cytokines such as IL-6, which increases the migration of osteoclasts and results in increased bone resorption of the alveolar bone of teeth, destroying the insertion sites of teeth.[11] Additionally, T-lymphocytes can be called on to help destroy the bacterial infection, and they have RANKL, which can directly stimulate osteoclastogenesis and cause bone resorption of alveolar bone.[18]