Although pellet culture and encapsulation of chondrocytes into gel-like biomaterials have lead to major advances in cartilage tissue engineering, a quantitative comparative characterization of cellular differentiation behavior during those cultivation procedures has not yet been performed. Our study therefore aimed at answering the following question: is the redifferentiation pathway of chondrocytes altered by slight changes in the type of alginate biomaterial (pure alginate, alginate-fibrin, alginate-chitosan) and how do the cells behave in comparison to biomaterial-free (pellet) three-dimensional culturing? Monolayer-expanded chondrocytes from healthy adult porcine knee joints were cultivated in alginate, alginate-chitosan, alginate-fibrin beads and as pellets up to 4 weeks. Quantitative PCR and Immunohistology were used to assess chondrogenic markers. Alginate-fibrin-encapsulated chondrocytes behaved almost like monolayer chondrocytes. Alginate- and alginate-chitosan encapsulation lead to a low chondrogenic marker gene expression. Although all 3D-cultured chondrocytes showed a considerable amount of Sox9 expression, only pellet cultivation lead to a sufficient Collagen II expression. This puts the usage of alginate-cultivated cartilage tissue engineering constructs under question. Fibrin addition is not beneficial for chondrogenic differentiation. Sox9 and Collagen II behave differently, depending upon the surrounding 3D-environment.

Recent research in tissue engineering for the treatment of cartilage defects have demonstrated that matrix-biomaterial, cell culture conditions, and cytokine-related factors influence the chondrogenic differentiation pattern, especially for the expression of matrix genes. However, little is known about the impact of cell seeding density in a three-dimensional environment on the key chondrogenic transcription factor Sox9. Here we investigated, whether the cell concentration of alginate encapsulated chondrocytes influences the Sox9 expression. Dedifferentiated passage-4 porcine chondrocytes were encapsulated in alginate beads at two different concentrations (4 x 10(6) versus 7 x 10(7) cells/mL) and cultivated for up to 4 weeks under TGF-ss stimulation. The expression of Sox9, Collagen I, II, and X was assessed via quantitative RT-PCR and compared to those observed in the initial monolayer culture. Cellular viability, cell morphology, and the sulphated glycosaminoglycan-production were monitored. Interestingly Sox9 expression was significantly upregulated in the low-cell-density group, whereas no difference between high-cell-density and monolayer culture group could be observed. Furthermore, the cellular survival and the sulphated glycosaminoglycan production were higher in the low-cell-density group. Collagen I expression was downregulated in the low-cell-density group whereas it was upregulated in the high-cell-density one. Surprisingly, only the high-cell-density group showed the expression of Collagen II, although it appeared not significant. Collagen X expression was upregulated in the low-cell-density group. Taken together our data indicate that a low concentration of cell seeding in a three-dimensional environment is beneficial for the overall chondrogenic development. However, this article reveals discrepancies between Sox9 and the chondrogenic pathway in redifferentiating chondrocytes that should be addressed in further work.