Bone metabolism is a dynamic equilibrium process between bone resorption of osteoclasts and bone formation of osteoblasts. Osteoblasts are responsible for the synthesis, secretion, and mineralization of bone matrix, and they are the controllers of the activation or termination of bone reconstruction, which plays an important role in the repair of trauma to the bone tissues, growth and development of bone tissues, and bone balance, and it is the main focus of the research on osteoporosis, fractures, and other bone diseases. Osteoblast differentiation is the premise and foundation of its dominant bone formation, and its most basic biological characteristics in the process of osteogenic differentiation are bone matrix synthesis, secretion, mineralization and maturation, and the mineralized nodule is the sign of osteoblast differentiation and maturation, and it is also the main morphological manifestation of osteoblasts exercising their osteogenic function, and observation of osteoblasts mineralized nodule is one of the commonly used means of researching the osteoblasts' differentiation.

The commonly used methods for staining mineralized nodules of osteoblasts are tetracycline method and alizarin red S method. By staining osteoblasts with alizarin red solution, alizarin red can chelate with calcium ions to produce a deep red or purplish-red complex, which can be used for identifying whether the stem cells have been successfully differentiated to osteoblasts.