The cultures of rabbit chondrocytes embedded in collagen gels were conducted to investigate the cell behaviors and consequent architectures of cell aggregation in an early culture phase. The chondrocyte cells seeded at 1.0 X 10(5) cells/cm(3) underwent a transition to spindle-shaped morphology, and formed the loose aggregates with a starburst shape by means of possible migration and gathering. These aggregates accompanied the poor production Of collagen type II, while the cells seeded at 1.6 x 10(6) cells/cm 3 exhibited active proliferation to form the dense aggregates rich in collagen type II. Stereoscopic observation was performed at 5 days to define the migrating cells in terms of a morphology-relating parameter of sphericity determined for individual cells in the gels. The frequency of migrating cells decreased with increasing seeding density, while the frequency of dividing cells showed the counter trend. The culture seeded at 1.0 x 10(5) Cells/cm(3) gave the migrating cell frequency of 0.25, the value of which was 25 times higher than that at 1.6 x 10(6) cells/cm(3). In addition, the analysis of mRNA expression revealed that the chondrocyte cells seeded at 1.0 x 10(5) cells/cm(3) showed appreciable down-regulation in collagen type II relating to differentiation and up-regulation in matrix metalloproteinases relating to migration, as compared to the cells seeded at 1.6 x 106 cells/cm(3). These data supports the morphological analyses concerning the cell migration and aggregate formation in the cultures with varied seeding densities. it is concluded that the seeding density is an important factor to affect the cell behaviors and architecture of aggregates and thereby to modulate the quality of cultured cartilage. Biotechnol. Bioeng. 2009;102: 294-302. (C) 2008 Wiley Periodicals, Inc.