Bones are not static "reinforced concrete" but dynamic tissues that undergo continuous "renewal" throughout life. Osteoclasts are the cells responsible for the "demolition" task. Studying the differentiation and function of osteoclasts is crucial for elucidating the pathogenesis of osteoporosis, tumor bone metastasis, and delayed fracture healing. However, primary osteoclasts are difficult to expand and have an extremely short lifespan, so in vitro induction has become the mainstream research method. This article systematically sorts out the in vitro differentiation strategies of osteoclasts based on the latest experimental protocols.
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RAW264.7 mouse monocyte/macrophage cell line: Expresses M-CSF and its receptor c-fms, and only requires additional RANKL to initiate differentiation.
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Bone marrow monocytes (BMMs): Require simultaneous supplementation of M-CSF and RANKL.
M-CSF → c-fms (promotes proliferation of precursor cells and induces RANK expression)
RANKL → RANK → NFATc1 (master transcription factor that activates osteoclast-specific genes: Trap, Ctsk, Dc-stamp, etc.)
OPG acts as a "decoy receptor" that blocks RANKL and negatively regulates differentiation.
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Cell line selection: Use the RAW 264.7 cell line.
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Medium preparation: Use high-glucose DMEM medium supplemented with 10% fetal bovine serum (FBS) and 1% antibiotics (e.g., penicillin-streptomycin).
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Cell growth environment: Incubate cells in a 5% CO₂, 37°C incubator.
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Cell acquisition and preparation: Obtain cells from RAW 264.7 cultures, gently detach them using trypsinization or a cell scraper, and prepare a cell suspension.
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Cell seeding: Seed the cell suspension into 6-well plates at approximately 5 × 10⁵ cells per well, ensuring uniform distribution.
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Adherent culture: Allow cells to adhere and grow overnight to ensure good growth status.
- Medium replacement and factor addition: The next day, discard the original medium and add fresh complete medium containing M-CSF (25 ng/mL) and RANKL (100 ng/mL) to induce RAW cells to differentiate into osteoclasts.
- Continuous culture and observation: Continue culturing cells in medium containing RANKL for 5–7 days. During this period, replace the medium every 2–3 days while maintaining a constant RANKL concentration. Observe changes in cell morphology: over time, cells will gradually fuse and eventually form multinucleated cells.
① Take femurs and tibias from 6–8-week-old C57BL/6 mice;
② Rinse the bone marrow cavity 3 times with 1 mL PBS using a syringe, collect the liquid in a centrifuge tube, centrifuge at 300g for 5 min at 4°C, and discard the supernatant.
(Note: Avoid excessive force to prevent mechanical damage to cells);
③ Filter through a 70 μm sieve, centrifuge at 1500 rpm for 5 min; treat with red blood cell lysis buffer for 5 min, and wash twice with PBS.
(1) Cell resuspension and centrifugation: Resuspend cells in 5 mL of α-MEM medium containing 10% FBS, centrifuge, and discard the supernatant.
(2) Cell seeding and culture: Resuspend cells in 5 mL of α-MEM medium containing 25 ng/mL M-CSF and 10% FBS, transfer to 6-well plates (each well corresponding to cells from one mouse), and incubate overnight in a 37°C, 5% CO₂ incubator.
(3) Cell purification and expansion: Collect the culture supernatant and centrifuge, discard the supernatant containing red blood cells and granulocytes, resuspend cells in 5 mL of complete medium containing 50 ng/mL M-CSF, seed into 24-well plates at a density of 5×10⁴ cells/well (1 mL per well), and continue culturing for 2–3 days. The adherent cells are bone marrow-derived macrophages (BMMs).
(1) Discard the supernatant and wash twice with sterile PBS. Use complete medium containing 50 ng/mL M-CSF and 50–100 ng/mL RANKL for induction of differentiation;
(2) Change the medium every 3 days and supplement with 25 ng/mL M-CSF and 50–100 ng/mL RANKL. Perform induction identification after 4–6 days.
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Phase-contrast microscopy: Giant cells with ≥3 nuclei, with visible "ruffled borders" at the edges.
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Phalloidin staining: F-actin forms "sealing zones".
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TRAP staining: Positive cytoplasmic staining (wine-red).
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Bone resorption assay: Resorption lacunae appear on hydroxyapatite/bovine bone slices, which can be observed by SEM or confocal 3D reconstruction.
qPCR/WB: Significantly upregulated expression of Trap, NFATc1, CTSK, and DC-STAMP.
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Prepare induction medium fresh and prewarm to 37°C to reduce cell stress.
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RAW264.7 shows an "all-or-nothing" response to RANKL; it is recommended to set gradients of 0, 25, 50, and 100 ng/mL.
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For studying the effects of drugs on osteoclasts, the optimal administration time is Day 0 (early blocking) or Day 3 (late inhibition).
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For co-culture experiments (e.g., osteoblast-osteoclast co-culture), osteoblast supernatant can be added in a Transwell system or direct contact culture can be used.
Mastering the in vitro differentiation technology of osteoclasts is the "golden key" to opening the door to bone metabolism research. It is hoped that this guide will help novice researchers avoid detours, and experts are welcome to leave comments to share more exclusive tips. May your osteoclasts be large and round, with TRAP staining as red as purple!
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