The pH in the Microenvironment of Human Mesenchymal Stem Cells Is a Critical Factor for Optimal Osteogenesis in Tissue-Engineered Constructs

The present study aimed at elucidating the effect of local pH in the extracellular microenvironment of tissue engineered (TE) constructs on bone cell functions pertinent to new tissue formation. To this aim, we evaluated the osteogenicity process associated with bone constructs prepared from human, bone marrow-derived stromal cells (hBMSC) combined with 45S5 bioactive glass (BG), a material that induces alkalinization of the external medium. The pH measured in cell-containing BG constructs was around 8.0, i.e., 0.5 unit more alkaline than that in two other cell-containing material (hydroxyapatite/tricalcium phosphate (HA/TCP) and coral) constructs tested. When implanted ectopically in mice, there was no de novo bone tissue in the BG cell-containing constructs, in contrast to results obtained with either HA/TCP or coral ceramics which consistently promoted formation of ectopic bone. In addition, the implanted 50:50 composites of both HA/TCP:BG and coral:BG constructs, that displayed a pH of around 7.8, promoted 20-30 fold less amount of bone tissue. Interestingly, hBMSC viability in BG constructs was not affected compared to the other two types of material constructs tested both in vitro and in vivo. Osteogenic differentiation (specifically, ALP activity and gene expression of RUNX2, ALP and BSP) was not affected when hBMSC were maintained in moderate alkaline pH (≤ 7.90) external milieu in vitro, but was dramatically inhibited at higher pH values. Formation of mineralized nodules in the extracellular matrix of hBMSC was fully inhibited at alkaline (> 7.54) pH values. Most importantly, there is a pH range (specifically, 7.9-8.27) at which hBMSC proliferation was not affected but the osteogenic differentiation of these cells was inhibited. Altogether, these findings provided evidence that excessive alkalinization in the microenvironment of TE constructs (resulting, for example, from material degradation) affects adversely the osteogenic differentiation of osteoprogenitor cells.