Bone marrow mesenchymal stem cells (BMMSCs) represent a subset of non-hematopoietic stem cells residing in bone marrow, characterized by their self-renewal capabilities, multi-directional differentiation potential, and supportive functions for hematopoiesis. These cells play pivotal roles in various physiological processes such as tissue regeneration, angiogenesis, and the regulation of cellular activities20. Consequently, BMMSCs are frequently utilized in tissue repair and regenerative engineering as an optimal seed cell source21.
Jaw bone marrow mesenchymal stem cells (JBMMSCs), initially isolated from human jaw bone marrow aspirates in 2005 by Matsubara et al.22, exhibit distinctive differentiation characteristics attributable to the unique origins and developmental pathways of the jaw compared to other skeletal sites4,23. Given the distinct developmental and pathological mechanisms governing craniofacial bone, prioritizing JBMMSCs in the repair of craniofacial bone defects appears imperative, owing to their homologous developmental traits23. Furthermore, JBMMSCs exhibit heightened histocompatibility with the oral microenvironment owing to their shared embryonic tissue origins24,25. Despite these advantages, a standardized and safe isolation protocol for JBMMSCs remains elusive, and research on JBMMSCs remains relatively limited.
The isolation and cultivation of BMMSCs currently lack standardization, with commonly employed methods including immunomagnetic bead sorting, flow cytometry sorting, density gradient centrifugation, and whole bone marrow adherent culture26. Among these, the immunomagnetic bead method and flow cytometry sorting method isolate cells by recognizing specific antigens on the cell surface. However, these methods involve cumbersome operations, require specialized instruments, and may affect cell activity despite yielding high-purity cells27. The density gradient centrifugation method separates cells through centrifugation and stratification, which can alter the cell microenvironment, leading to slower cell growth and increased cellular aging28. Conversely, the whole bone marrow adherent culture method separates and purifies BMMSCs by intermittently replacing the medium and passaging based on varying adherent abilities among different cell types in the bone marrow. This method inflicts minimal damage to cells and is straightforward to execute16.
Compared with the femur, the jaw is smaller in size and has a narrower bone marrow cavity, with interference from cells in the tooth or periodontal ligament. The bone marrow flushing and adhesion method requires a relatively larger amount of bone marrow and is suitable for large animals and humans29. JBMMSCs in rats extracted by applying the traditional bone marrow flushing method are rare in number, slow to proliferate, and of poor quality30,31. The content of JBMMSCs in bone marrow is very low, and they are mainly found in the compact bone and endosteum. Bu-Kyu Lee et al.32 used the mandibular aspirates to isolate JBMMSCs, while the total aspiration time for 10 mL of marrow blood for the mandible was five times longer than that for the iliac crest, and the initial yield of MSCs from the mandible was three times lower than that from the iliac crest. Flushing the bone marrow alone cannot effectively isolate the cells in the compact bone and endosteum.
In recent years, studies have found that compact bone is a new and reliable source of BMMSCs, and a relatively simple method of BMMSC isolation has been derived, the bone slice digesting culture method33. BMMSCs isolated by this method have high purity34. Guo et al.35have successfully isolated bone mesenchymal stem cells from mouse femurs using the bone slice digesting culture method. Yamazaet al.12 and Cheng et al.15 applied the bone slice digesting culture method to the isolation of mouse mandibular bone marrow mesenchymal stem cells and verified MSC properties by cell proliferation, immunophenotype, and multilineage differentiation. This experiment used a combination of flushed whole bone marrow adhesion and bone slice digesting to isolate and culture primary rat JBMMSCs. The bone marrow was thoroughly washed out from the cavity without filtration to maintain the original cellular composition and growth factors in the bone marrow. Then, the bone slices were cut into small pieces and digested using type II collagenase to facilitate cells to climb out of bone fragments. Finally, the bone slices were inoculated onto a culture dish and cultured to allow cells to crawl out of the bone slices. The combination method makes the culture system of the primary cell simultaneously contain components of hematopoietic stem cells, cancellous bone, and cortical bone, which can better simulate the microenvironment of bone marrow mesenchymal stem cells in the body and is more conducive to maintaining the original biological characteristics of cells. This method is therefore called "a niche-based approach on stemness"36. Luet al.37 also emphasized the importance of stem cell niches in the isolation of BMMSCs, but they only involved the microenvironment in the bone marrow without cortical bone. This method has been successfully applied in isolating bone marrow mesenchymal stem cells from mice jaw38.
We also successfully isolated and cultivated rat JBMMSCs with this niche-based approach. First, the isolated cells exhibited a fibroblast-like shape and adhered to plastic culture plates in vitro. Second, the cells positively expressed the CD90, CD29, and CD44 surface markers and negatively expressed CD45, CD34, and CD11b/c. Third, the cells could differentiate into osteocytes, adipocytes, and chondrocytes. The obtained cells had a good morphology, a fast proliferation rate, and differentiation ability, which met the minimum criteria for identifying human-derived mesenchymal stem cells proposed by the International Society for Cellular Therapy17.
The isolation of JBMMSCs has been developed and used to isolate cells in humans. Matsubara et al.22 were the earliest to isolate JBMMSCs from human alveolar bone marrow samples during oral surgery. Possible complications may occur during the mandible aspiration process, including damage to adjacent tissue, infection due to oral bacterial contamination, etc. Zong et al.39 flushed the bone marrow cavity of the cancellous bone specimen pieces to obtain human JBMMSCs. Park et al.40 isolated human JBMMSCs from alveolar bone chips obtained during implant drilling using a sequential digestion method. Mason et al.41 directly isolated human JBMMSCs from the jaw bone marrow cores and aspirate of patients undergoing routine dental implant placement. There was a high success rate of isolating cells from either aspirates or core and a higher success rate of the combination. However, although the above methods have successfully isolated JBMMSCs from humans, the differences in effectiveness between methods need to be compared. Currently, this presented method has only been used in mice and rats, and there is no research on applying this method to the isolation of human JBMMSCs. There are limitations to this method, especially when it is applied to humans. First, this method of combining bone marrow flushing and bone slice digestion is relatively complicated and requires more steps. Second, complex processes place higher demands on the sterility of operations, which is critical to the protocol. Third, it is impossible to flush the entire bone marrow cavity of the human jawbone, so exploring some equivalent methods, such as flushing cancellous bone fragments, is necessary.
In summary, this study successfully isolated and cultured rat JBMMSCs using a combination of bone marrow flushing and bone slice digestion based on the principle of "stem cell niche." The cell identification results confirmed that this method can isolate sufficient and high-purity JBMMSCs, providing ample cell sources for jawbone tissue engineering, particularly in bone defect repair, which is of great significance.