Observation of chondrocyte aggregate formation and internal structure on micropatterned fibroin-coated surface

Condensation/aggregation process of rabbit-derived chondrocytes on a fibroin-coated patterned substrate was observed to estimate initial aggregation process in fibroin sponge. Chondrocytes were seeded on array of 160 microm diameter pits in three densities: 5 cells/pit (2.5 x 10(4) cells/cm(2), LOW), 15 cells/pit (7.5 x 10(4) cells/cm(2), MID) and 25 cells/pit (12.5 x 10(4) cells/cm(2), HIGH). In the MID and HIGH groups, cells tended to form aggregates after 24 h after cell seeding. In the LOW group, cell aggregate were not seen in a majority of the pits. Observation of aggregates using confocal laser scanning microscope showed that the chondrocytes at the interface of the fibroin surface tended to extend to the surface, developing an extensive network of stress fibers throughout the cytoplasm. On the other hand, chondrocytes in the other part of the aggregates maintained spherical shape, and most of the actin was localized in the cell cortex as opposed to in stress fibers. These results suggest two functional structures in the aggregates, which may explain the good balance between the maintenance of their differentiated phenotype and proliferation rate in the fibroin sponge.