This article discusses the role of osteoblasts in bone mineralization through the secretion of matrix vesicles. It details the process of visualizing mineralization using Alizarin Red staining.
Osteoblasts are bone-forming cells that secrete small vesicles into their extracellular matrix, ECM. These vesicles, called matrix vesicles, are equipped with membrane transporters and enzymes that enable calcium and phosphate ion entry, forming mineral crystals within. As crystals accumulate, they break through the vesicle membrane to form mineralized nodules in the ECM, making up the bone mineral density.
To visualize mineralization, begin with osteoblast cells on a biocompatible scaffold in a suitable growth medium. The osteoblasts attach to the scaffold and use growth factors from the medium to secrete matrix vesicles that subsequently form mineralized nodules in the ECM.
Aspirate the culture medium and rinse the cells to remove any media traces. Treat the cells with a formalin solution, which induces ECM crosslinking, fixing, and preserving the tissue structure. Wash the cells to remove any remaining formalin.
Next, add Alizarin Red staining solution and incubate the culture. The dye molecules bind to the mineralized nodule and associate with calcium, a key bone mineral, forming large stained complexes.
Aspirate the staining solution and wash thoroughly to remove any unbound dye. When observed under a microscope, the calcium-rich mineral deposits appear as bright-red nodules in the ECM, indicating mineralization by the osteoblasts.
Aspirate culture medium from primary osteoblasts, 14 days after the addition of osteogenic mineralization medium. Rinse twice with 0.5 milliliters of 1X PBS. Then, fix the cells by adding 0.5 milliliters of 10% buffered formalin solution, and incubating at room temperature for 10 minutes. After 10 minutes, aspirate the 10% buffered formalin with a single-use pipette, and wash twice with 0.5 milliliters of ultrapure water. Then, add 250 microliters of 40 millimolar Alizarin Red S staining solution to the fixed osteoblasts, and incubate the plate at room temperature for 10 minutes on a shaker at 100 shakes per minute.
Following the incubation with staining solution, aspirate the staining solution with a single-use pipette, and rinse with 1 milliliter of ultrapure water. Repeat this step 5 to 10 times until the rinsing solution comes off clear to remove non-specific staining. After aspirating the final wash, add 1 milliliter of cold PBS, and incubate the plate at room temperature for 10 minutes on a shaker as before. Next, transfer the stained beta-tricalcium phosphate disc to a new well, and scan the plates with a flatbed scanner to record mineralization.
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