Development of an injectable, in situ crosslinkable, degradable polymeric carrier for osteogenic cell populations. Part 1. Encapsulation of marrow stromal osteoblasts in surface crosslinked gelatin microparticles

This study investigated the temporary encapsulation of rat marrow stromal osteoblasts in surface crosslinked gelatin microparticles. Cells were encapsulated in uncrosslinked gelatin microparticles of average diameter of 630 μm containing 53 cells. Gelatin microparticles were crosslinked to shell thicknesses of 75 μm via exposure to 1 m dithiobis(succinimidylpropionate) (DSP) solution for 15 min or 5 m DSP solution for 5 min for the production of microparticles dispersing 60 min after placement into a physiologic fluid at 37°C. Formed microparticles were placed into culture wells at a cell seeding density of 5.3×104 cells/cm2 and, following the degradation and/or dissolution of gelatin, the cells were cultured in the presence of osteogenic supplements for 28 days. Samples were taken at specified time points and analyzed by a DNA assay for cell number and a 3H-thymidine incorporation assay for proliferative potential. Samples were also obtained and analyzed at several time points by alkaline phosphatase, osteocalcin, and mineralization assays for early and late phenotypic expression markers of osteoblastic differentiation. The measurements from the different assays for encapsulated cells (EC) in uncrosslinked and crosslinked gelatin microparticles were normalized with the cell numbers from the DNA assay and compared with those for nonencapsulated control cells. The results demonstrated that the marrow stromal cells survived the encapsulation procedure in uncrosslinked gelatin microparticles and also retained their proliferative potential and osteoblastic phenotype over a 28 day period, although at a slightly lower level than the nonencapsulated cells. The results further showed that the marrow stromal cells survived the encapsulation in crosslinked gelatin microparticles prepared via exposure to 5 m DSP for 5 min and also retained their proliferative potential and osteoblastic phenotype over a 28 day period, but at a slightly lower level than the EC in uncrosslinked gelatin microparticles. In contrast, exposure to 1 m DSP for 15 min led to severely limited cell viability and phenotypic expression probably due to the increased crosslinking time. These results suggest that temporary encapsulation of cells in gelatin microparticles may protect cells from short-term environmental effects such as those associated with the crosslinking of an injectable polymeric carrier for bone tissue engineering.