Osteoclasts and Bone Resorption

The cell. The osteoclast is a multinucleated giant cell 100-150 μm in diameter carrying 10-100 nuclei, formed by the fusion of mononuclear precursors; multinucleation is required for function. It shares its origin with the macrophage, and its precursors circulate as monocytes.

Where it works. An osteoclast lives for approximately 2 weeks, and while active it sits in the resorption pit it has made, the Howship lacuna.

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The master switch. RANKL is the activator; OPG (osteoprotegerin), also produced by osteoblasts, is a decoy receptor that binds RANKL and so prevents RANK activation. The balance between the two determines osteoclast number and activity, and the RANKL:OPG ratio is the key determinant of the rate of bone resorption. It is the target of denosumab therapy.

Oestrogen deficiency. After the menopause, oestrogen deficiency increases RANKL and decreases OPG production, shifting the ratio towards resorption. This explains the accelerated bone loss of postmenopausal women and the efficacy of oestrogen replacement therapy.

Resorption is a dual process inside one sealed compartment: acidification dissolves the mineral, and enzymes degrade the organic matrix.

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The functional zones. An active osteoclast has distinct functional zones:

• Ruffled border - membrane invaginations facing the bone surface
• Sealing zone - an actin ring that creates a tight seal around the resorption site
• Clear zone - the organelle-free area where the sealing occurs
• Basolateral domain - the opposite surface, where degradation products transcytosed through the cell are released into the circulation

Attachment and sealing. The αvβ3 integrin on the osteoclast surface binds RGD sequences in bone matrix proteins (osteopontin and bone sialoprotein). F-actin filaments then polymerise into a ring within the clear zone, which is densely packed with actin. The resulting peripheral seal isolates the resorption lacuna from the extracellular space; this gasket is what allows acidification and maintains the low pH.

Dissolving the mineral. V-type H+ ATPase pumps in the ruffled border actively drive protons into the resorption lacuna, bringing it to pH 4.5, and ClC-7 chloride channels maintain electroneutrality by carrying Cl- alongside the H+. The protons are generated inside the cell by carbonic anhydrase II (see Sly syndrome below). In this acid the hydroxyapatite crystals dissolve, releasing calcium and phosphate.

Degrading the matrix. Cathepsin K is the dominant collagenase, accounting for most degradation of the organic matrix, and it functions optimally at acidic pH.

Too much resorption. Osteoclast activity is increased in:

• Osteoporosis (postmenopausal, steroid-induced)
• Paget disease (abnormal osteoclasts)
• Hyperparathyroidism
• Multiple myeloma
• Bone metastases

Too little. Osteoclast activity is decreased in:

• Osteopetrosis (RANK/RANKL/ClC-7 mutations)
• Pycnodysostosis (cathepsin K deficiency)
• Bisphosphonate therapy (excessive)
• Carbonic anhydrase II deficiency

Two kinds of osteopetrosis. In the common osteoclast-rich forms, caused by TCIRG1, CLCN7 or carbonic anhydrase II mutations, osteoclasts are present but unable to resorb. In the osteoclast-poor RANK and RANKL forms, osteoclasts fail to form.

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One enzyme, three tissues. Carbonic anhydrase II (CA-II) catalyses CO2 + H2O into H+ and bicarbonate, generating the very protons the osteoclast's H+-ATPase pumps into the resorption lacuna. The same enzyme is expressed in the renal tubule and the brain, so its autosomal-recessive loss produces a characteristic triad:

• Osteopetrosis - without CA-II-generated protons the osteoclast cannot acidify the lacuna or dissolve hydroxyapatite, so dense bone accumulates; the osteoclasts are present but non-functional
• Renal tubular acidosis - CA-II is required for renal tubular acid-base handling, so its loss impairs both proximal bicarbonate reclamation and distal urinary acidification, a mixed ("type 3") RTA producing a metabolic acidosis
• Cerebral calcification, typically of the basal ganglia, with developmental delay

How it differs. The renal and cerebral phenotype distinguishes CA-II deficiency from the TCIRG1 and CLCN7 acidification defects, which give isolated osteopetrosis. CA-II deficiency is also a milder, often later-presenting form. The detailed osteopetrosis subtypes and the role of HSCT are developed in the osteopetrosis topic.

When osteoclast number or function is abnormal, several conditions can present with overlapping radiographic or biochemical features. Distinguishing them rests on osteoclast biology.

The collagen fragments measure how actively osteoclasts are resorbing; TRAP-5b, which the osteoclasts secrete themselves, measures how many there are.

On treatment. Resorption markers fall rapidly when an antiresorptive is started, and a drop in CTX confirms response and compliance. They surge above baseline when denosumab is stopped, the biochemical signature of the rebound; its fracture consequence is set out under Denosumab below.

In osteopetrosis. TRAP-5b is high in the osteoclast-rich forms, because there are many osteoclasts, and low in the osteoclast-poor RANK/RANKL forms, because osteoclasts fail to form. It therefore separates the two subtypes and predicts whether haematopoietic stem cell transplantation can work. The bone-formation markers (P1NP, osteocalcin, bone-specific ALP) and the remodelling cycle itself are developed in the osteoblasts-bone-formation and bone-remodeling topics.

Bisphosphonates, denosumab and calcitonin all target osteoclast activity in osteoporosis.

Bisphosphonates. The nitrogen-containing bisphosphonates (alendronate, risedronate, zoledronic acid) inhibit farnesyl pyrophosphate synthase in the mevalonate pathway, preventing prenylation of the small GTPases essential for osteoclast function. They bind hydroxyapatite with high affinity and become incorporated into the bone matrix, and osteoclasts endocytose the bisphosphonate-containing bone as they resorb it. Inside the cell the drug inhibits the enzyme and disrupts GTPase signalling, and the loss of functional GTPases triggers osteoclast apoptosis, reducing bone resorption.

Adverse effects. Bisphosphonates can cause osteonecrosis of the jaw, which is rare, and atypical femoral fractures with prolonged use (greater than 5 years).

Denosumab. Denosumab is a fully human monoclonal antibody that binds RANKL with high affinity, preventing the RANKL-RANK interaction.

Stopping denosumab. Bone turnover surges and bone density falls back to baseline within about a year. The vertebral fracture rate returns to that of untreated patients, 7.1 per 100 participant-years after stopping denosumab against 8.5 after stopping placebo, and does not rise above the untreated level. What is specific to denosumab is that patients who do fracture are far more likely to sustain several vertebral fractures at once (60.7% versus 38.7%).

The bridge. Transition to a bisphosphonate is required. Which bisphosphonate, and how and when it is given, is not defined by prospective trials (see Controversies below).

Calcitonin. Calcitonin binds G-protein coupled receptors on the osteoclast surface and inhibits resorption directly, reducing it rapidly (within hours). It also has an analgesic effect, mechanism unclear, but its efficacy is modest compared with bisphosphonates, and tachyphylaxis (a reduced response with repeated dosing) limits long-term use. It is used for acute hypercalcaemia, Paget disease and as an analgesic for vertebral fractures.

Global Epidemiology

• Osteoporosis affects an estimated 500 million people worldwide; roughly 1 in 3 women and 1 in 5 men over 50 will sustain a fragility fracture.
• Over 8.9 million osteoporotic fractures occur globally each year (a fragility fracture roughly every 3 seconds), with hip fractures projected to rise sharply in Asia as populations age.
• Paget disease shows marked geographic variation — historically common in the UK and populations of British descent, and declining in prevalence in recent decades.
• Infantile (malignant) osteopetrosis has an incidence of approximately 1 in 250,000 births; the autosomal dominant adult form is far more common at around 1 in 20,000.

Major Guidelines, Side by Side

• Consensus across societies: nitrogen-containing bisphosphonates remain first-line; denosumab is a potent alternative but must never be stopped abruptly; teriparatide/romosozumab (anabolic) are favoured first for very-high-risk patients, followed by an anti-resorptive to consolidate gains.
• Anti-resorptive "drug holidays" apply to bisphosphonates (which persist in bone for years) but are explicitly contraindicated for denosumab.

Registry and Resource-Setting Notes

• Arthroplasty registries (NJR, AJRR, AOANJRR, SHAR, Norwegian, NZJR) track periprosthetic fractures, which are influenced by underlying bone quality and anti-resorptive status; bisphosphonate use around arthroplasty and its effect on aseptic loosening and revision is an area of ongoing analysis.
• In well-resourced settings, severe infantile osteopetrosis is managed with haematopoietic stem cell transplantation and bone-density-targeted pharmacotherapy is guided by DXA and bone turnover markers.
• In limited-resource settings, DXA access, the cost of denosumab and anabolic agents, and HSCT availability are major constraints; generic oral bisphosphonates and FRAX (which can be calculated without BMD) are the practical mainstays.

Related pages: Osteoblasts and Bone Formation for the cell that makes both RANKL and its decoy OPG, and therefore sets the resorption rate described here, and Bone Remodeling for the coupled cycle the two cells run together; Osteopetrosis for the natural experiments that prove this pathway in humans — TCIRG1, CLCN7 and carbonic anhydrase II mutations produce osteoclasts that cannot acidify, and the RANK/RANKL forms produce no osteoclasts at all, which is why TRAP-5b separates them; Paget's Disease of Bone for pathologically hyperactive osteoclasts; Osteoporosis and Glucocorticoid-Induced Osteoporosis for the diseases these drugs treat; Bisphosphonates for the mevalonate-pathway mechanism only sketched here; and Giant Cell Tumour of Bone for the RANKL-driven tumour in which denosumab is used oncologically rather than metabolically.

Cathepsin K inhibitors. Odanacatib reduced fractures in the LOFT trial but was withdrawn in 2016 after a signal of increased stroke risk. Whether selective cathepsin K inhibition can be achieved without off-target cardiovascular or cutaneous effects remains unresolved.

The denosumab exit strategy. The rebound phenomenon is established, but the ideal bridging regimen (which bisphosphonate, oral or IV, timing relative to the missed dose, and duration) is not defined by prospective trials and varies between guidelines.

Anti-resorptive drug holidays. Balancing the risks of atypical femoral fracture and osteonecrosis of the jaw, which rise with cumulative exposure, against rebound fracture risk on stopping is contentious. Holidays are reasonable for bisphosphonates but are explicitly unsafe for denosumab.

Osteoclasts as signalling cells. Beyond resorption, osteoclasts secrete "clastokines" that couple resorption to formation. Whether anabolic benefit can be preserved while resorption is suppressed, the basis of interest in coupling-sparing agents, is an active question.