Microenvironmental Stiffness Directs Chondrogenic Lineages of Stem Cells from the Human Apical Papilla via Cooperation between ROCK and Smad3 Signaling

Microenvironments appear important in stem cell lineage specification but can be difficult to adequately characterize or control with soft tissues. Naive mesenchymal stem cells (MSCs) are shown here to specify lineage and commit to phenotypes with extreme sensitivity to tissue-level elasticity. Soft matrices that mimic brain are neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. During the initial week in culture, reprogramming of these lineages is possible with addition of soluble induction factors, but after several weeks in culture, the cells commit to the lineage specified by matrix elasticity, consistent with the elasticity-insensitive commitment of differentiated cell types. Inhibition of nonmuscle myosin II blocks all elasticity-directed lineage specification-without strongly perturbing many other aspects of cell function and shape. The results have significant implications for understanding physical effects of the in vivo microenvironment and also for therapeutic uses of stem cells.

Integrin-mediated cell adhesions provide dynamic, bidirectional links between the extracellular matrix and the cytoskeleton. Besides having central roles in cell migration and morphogenesis, focal adhesions and related structures convey information across the cell membrane, to regulate extracellular-matrix assembly, cell proliferation, differentiation, and death. This review describes integrin functions, mechanosensors, molecular switches and signal-transduction pathways activated and integrated by adhesion, with a unifying theme being the importance of local physical forces.

Mesenchymal stem cells (MSCs) have been isolated from the pulp tissue of permanent teeth (dental pulp stem cells or DPSCs) and deciduous teeth (stem cells from human exfoliated deciduous teeth). We recently discovered another type of MSCs in the apical papilla of human immature permanent teeth termed stem cells from the apical papilla (SCAP). Here, we further characterized the apical papilla tissue and stem cell properties of SCAP using histologic, immunohistochemical, and immunocytofluorescent analyses. We found that the apical papilla is distinctive to the pulp in terms of containing less cellular and vascular components than those in the pulp. Cells in the apical papilla proliferated 2- to 3-fold greater than those in the pulp in organ cultures. Both SCAP and DPSCs were as potent in osteo/dentinogenic differentiation as MSCs from bone marrows, whereas they were weaker in adipogenic potential. The immunophenotype of SCAP is similar to that of DPSCs on the osteo/dentinogenic and growth factor receptor gene profiles. Double-staining experiments showed that STRO-1 coexpressed with dentinogenic markers such as bone sialophosphoprotein, osteocalcin, and growth factors FGFR1 and TGFbetaRI in cultured SCAP. Additionally, SCAP express a wide variety of neurogenic markers such as nestin and neurofilament M upon stimulation with a neurogenic medium. We conclude that SCAP are similar to DPSCs but a distinct source of potent dental stem/progenitor cells. Their implications in root development and apexogenesis are discussed.