Subcutaneous Regeneration of Engineered Cartilage: A Comparison of Cell Sheets and Chondrocyte-Scaffold Constructs in a Porcine Model

I read with great interest the article by Ding et al.1 on the comparison of chondrocyte sheets and chondrocyte-scaffold constructs for engineered cartilage. Compared with chondrocyte–polyglycolic acid/polylactic acid constructs, the chondrocyte sheet technique could effectively regenerate more mature and stable engineered cartilage without obvious inflammatory responses. Inspired by the authors, I would like to express several considerations. First, I am concerned that 12 weeks of in vitro chondrocyte sheet preculture may be unnecessary. To my knowledge, the duration of in vitro preculture is unknown, ranging from 1 week to 12 weeks.1–3 However, there are no comparative studies evaluating histologic outcomes of chondrocyte sheets at different durations. In other words, the quantity and quality of extracellular matrix and chondrocyte viability may not always be increased even though the in vitro preculture is prolonged. This study provided the gross and histologic outcomes of chondrocyte/polyglycolic acid composites after in vitro preculture for 6 and 12 weeks, but only shows the outcomes of chondrocyte sheets after in vitro preculture for 12 weeks. Thus, I would like to know whether the researchers conducted the gross and histologic evaluation of chondrocyte sheets after in vitro precultivation for 6 weeks. These comparative outcomes will be helpful for selecting appropriate duration. Actually, some methods are helpful in ensuring sufficient thickness to maintain the shape of chondrocyte sheets. The simplest technique is to increase the layers of chondrocyte sheets, such as the triple-layered chondrocyte sheets. The triple-layered sheets can be easily constructed by putting one chondrocyte sheet on top of others by making use of the supporting polyvinylidene difluoride membrane.3,4 The multilayered cell sheets can shorten the in vitro preculture to only 1 week, which could decrease the economic cost and the risks of infection. Compared with the monolayer, these multilayered cell sheets show a consistent cartilaginous phenotype and adhesive properties by significantly increasing the levels of SOX9, collagen type 2, integrin α10, and fibronectin.3 In addition, the study by Niyama et al.5 indicated that cell sheet thickness increased with increasing inoculum cell density, and the thickness of the cell culture insert was higher than that in the 96-well plate. Finally, autologous chondrocyte sheets are effective for repairing small and superficial cartilage defects in orthopedic surgery. However, I still have a long way to go before chondrocyte sheets can be applied to total ear reconstruction. The shape control, mechanical strength, biosafety, and long-term stability of the regenerated cartilage are main issues. Although the chondrocyte sheet technique shows superiority in regenerating mature and stable engineered cartilage, biological scaffolds still play an important role in engineered cartilage. Until now, many novel and biocompatible scaffolds, such as the GelMA and HAMA hydrogels, have been found and widely applied to regenerate engineered cartilage. However, it is unknown whether the chondrocyte sheets outperform these novel hydrogels. I would like to congratulate Ding et al.1 again for their innovative job. What is undeniable is that chondrocyte sheets can resolve the inflammation responses caused by polyglycolic acid/polylactic acid. Further efforts should be made to improve the cell sheets with good shape control, mechanical strength, and long-term stability. DISCLOSURE The author has no financial interest to declare in relation to the content of this communication. Jianguo Chen, M.D.Plastic Surgery HospitalChinese Academy of Medical Sciences and Peking Union Medical CollegeNo. 33 Badachu RoadShijingshan DistrictBeijing 100144, People’s Republic of China[email protected]