Characterization of cell bricks and injectable cell brick–platelet-rich plasma gel. (A), (B) Schematic description of the strategy for chondrocyte bricks and in vivo cell implantation. A chondrocyte sheet was cultured, harvested and embedded to be fragmented in a net cutting system (by multiple blades) (B.a, b, c). The thickness of the cell brick is about 50 to 80 μm; bar = 50 μm, magnification × 200 (B.c2, c3). Moreover, we used a stereomicroscope to measure the cell brick, which was 1,032 μm × 1,016 μm; bar = 200 μm, magnification × 30 (B.c1). (C) The obtained cell bricks or chondrocytes and cultured bone marrow-derived mesenchymal stem cells (BMSCs) mixed together or BMSCs alone were suspended in platelet-rich plasma (PRP), so that the injectable constructs were formed and injected subcutaneously into nude mice. B-C-P, BMSCs–chondrocytes–PRP; B-CB-P, BMSCs–cell bricks–PRP; B-P, BMSCs–PRP. (D) The graft maintained structural integrity at the fourth week postoperatively; bar = 200 μm, magnification × 40. (a) Scanning electron microscopy images showed BMSCs distributed evenly in the spaces formulated by chondrocyte bricks. (b) Even at the 12th week postoperatively, 5-bromo-2-deoxy uridine-labeled BMSCs (arrowheads) still occupied these spaces and further differentiated into tissues (c: low magnification, bar = 200 μm; d: high magnification, bar = 50 μm). Labeled BMSCs differentiated into newborn chondrocytes characterized by safranin-O staining and immunohistochemistry staining of collagen II (D.e, f).