High-magnification sections of chondrocyte–collagen sponges 1 week after culture. Cartilage layers with a laminar structure on the collagen sponges (green arrows) and a grey structure, which represents the walls of collagen sponges (black arrows), are visible at high magnification (100×). (a) to (d) Specimens at week 1 of culture. (a) and (b) Alcian blue staining, and (c) and (d) Safranin O staining: many chondrocytes with blue-stained and red-stained peripheral matrices could be observed, respectively. The chondrocytes exhibited a layer structure, and their infiltration into the sponge can also be observed. (e) to (h)Specimens at week 2 of culture. (e) and (f) Alcian blue staining, and (g) and (h) Safranin O staining: many chondrocytes with blue-stained and red-stained peripheral matrices can be observed, respectively. The layer of chondrocytes that formed on the surface of the sponge was found to be thicker in comparison with the week 1 cultures, and the volume of the extracellular matrix had also increased. The cartilage tissue that formed on the surface of the sponge consisted of more than 10 layers of chondrocytes. The staining of the extracellular matrix in the LIPUS group was also found to be stronger than in the control group.