The structural properties of natural cartilage have proven challenging to recreate in tissue engineering, but researchers are making progress: "A lab discovery is a step toward implantable replacement cartilage, holding promise for knees, shoulders, ears and noses damaged by osteoarthritis, sports injuries and accidents. Self-assembling sheets of mesenchymal stem cells permeated with tiny beads filled with growth factor formed thicker, stiffer cartilage than previous tissue engineering methods. ... We think that the capacity to drive cartilage formation using the patient's own stem cells and the potential to use this approach without lengthy culture time prior to implantation makes this technology attractive ... The team put transforming growth factor beta-1 in biodegradable gelatin microspheres distributed throughout the sheet of stem cells rather than soak the sheet in growth factor. ... The microspheres provide structure, similar to scaffolds, creating space between cells that is maintained after the beads degrade. The spacing results in better water retention - a key to resiliency. The gelatin beads degrade at a controllable rate due to exposure to chemicals released by the cells. As the beads degrade, growth factor is released to cells at the interior and exterior of the sheet, providing more uniform cell differentiation into neocartilage. The rate of microsphere degradation and, therefore, cell differentiation, can be tailored by the degree to which the microsphere are cross-linked. ... After three weeks in a petri dish, all sheets containing microspheres were thicker and more resilient than the control sheet. The sheet with sparsely crosslinked microspheres grew into the thickest and most resilient neocartilage."