Mesenchymal Stem Cells: Adult Stem Cells with Multiple Destinies
Although totipotent stem cells exist only during early embryogenesis, many adult tissues retain adult stem cells, which are multipotent and can differentiate into multiple differentiated cell types. One of these adult stem cells is the mesenchymal stem cell (MSC), which is mainly found in the bone marrow, adipose tissue, and umbilical cord. MSCs can differentiate into mesenchymal cell types, including adipocytes (fat), chondrocytes (cartilage), and osteocytes (bone).
In vitro models systems have developed elegant ways to study MSCs and their differentiation using culture media containing factors that stimulate differentiation into each respective cell type. These systems can then be used to study these processes in detail. Additionally, culture media that maintains MSCs in an undifferentiated state can be used to perpetuate MSC cultures to study this cell state.
Lifeline® Mesenchymal Stem Cells and Recent Studies on Osteogenesis
Lifeline® human MSCs are derived from three different locations, including the bone marrow, adipose tissue, and Wharton’s Jelly of the umbilical cord. Each MSC type is optimized for growth as undifferentiated MSCs in StemLife medium. However, Lifeline® MSCs can be differentiated into adipocytes, chondrocytes, and osteocytes using AdipoLife™, ChondroLife™, and OsteoLife™ media, respectively,
Vibrations are thought to positively affect bone formation by promoting osteogenesis and preventing resorption. In a 2013 study, Uzer et al. used Lifeline® adult adipose-derived mesenchymal stem cells (MSCs) to determine the effects of vibration-induced fluid shear on osteogenic differentiation. They evaluated osteogenesis by culturing MSCs in OsteoLife™ Medium, and measured mineralization following exposure to one of six fluid shear conditions, generated using one of two vibration frequencies (100 Hz or 30 Hz), combined with one of three vibration magnitudes (0.15g, 1g, or 2g). Their first assessment of mineralization demonstrated that all three 100 Hz conditions and the largest 30 Hz condition increased osteogenesis. Additionally, all vibration conditions increased cell proliferation over three days, although only 100 Hz-0.15g and 30 Hz-1g promoted a proliferative advantage in collagen, which increases cell attachment.
The authors also measured osteogenesis though the transcriptional markers RUNX-2, OPG, and RANK-L. They found that only RANK-L was upregulated by all 30 Hz conditions. Interestingly, RUNX-2 and OPG were upregulated when each fluid shear condition was combined with addition of LPA, which stimulates actin stress fiber formation. These results suggested to the authors that cytoskeletal factors might modulate the osteogenic response to vibrations.
To test this, they performed a PCR array on cells experiencing different fluid shear conditions during osteogenesis. They found that the majority of genes with a significant response were involved in actin remodeling, including WAS, whose activity is important for cytoskeletal actin dynamics, including formation of stress fibers. Together, the results of this study suggest that the MSC osteogenic response to vibrations is not due to fluid shear, but likely regulated in part by cytoskeletal remodeling.
Bone grafts are extremely common and may utilize different types of bioengineered scaffolds to stimulate bone regeneration. An ideal osteoinductive scaffold stimulates osteogenesis of existing MSCs in the bone and provides a surface to support this new growth. Additionally, these scaffolds should eventually dissolve without any significant adverse effects.
Recently, amorphous calcium phosphate (ACP) has been investigated as a scaffolding material, particularly in its polymer-induced liquid precursor phase, which can enter bone and be incorporated into mineralized collagen, which stimulates osteogenesis. However, the delivery and subsequent bioactivity of ACP has not yet been optimized. Therefore, in a 2017 study, Yang and colleagues set out to develop a system by which poly(allyamine)-stabilized ACP (PAH-ACP) could be delivered using mesoporous silica nanoparticles (MSNs) to induce osteogenesis.
The group synthesized expanded-pore MSNs (pMSNs), onto which PAH-ACP was successfully loaded to form PAH-ACP@pMSNs. Importantly, the researchers demonstrated that the majority of calcium and phosphate were released from PAH-ACP@pMSNs within 10 days, and silicon was released over the course of 30 days, suggesting that PAH-CAP was successfully released from pMSNs, which effectively dissolved over time. Additionally, using a single layer collagen model, the authors showed that released PAH-ACP was able to infiltrate and mineralize collagen fibrils.
To definitively demonstrated that PAH-ACP mineralized larger collagen scaffolds and that pMSNs were completely degraded following release of PAH-ACP, the group observed that three months after addition of PAH-ACP@pMSNs, completely demineralized collagen became mineralized, and pMSN dissolution could be visualized as voids in the matrix.
Finally, the authors investigated the effects of PAH-ACP@pMSN on Lifeline® human bone marrow-derived Mesenchymal Stem Cells. They found that PAH-ACP@pMSN had low cytotoxicity and promoted osteogenesis, illustrated by upregulation of markers of osteogenic differentiation, including increased alkaline phosphatase expression and activity. Although not tested in vivo, this study demonstrates that PAH-ACP@pMSN may be a viable scaffold delivery system for use in bone graft technology.
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