The meniscus of the knee is a fibrocartilaginous structure essential to the biomechanical integrity and function of the knee joint. Millions of people suffer meniscus injuries each year, and meniscus tears are common at all ages and stages of life. Meniscus injury has long-term consequences:​ loss of meniscus function has been definitively linked to early-onset osteoarthritis. Treatment options for meniscus injury remain limited;​ although there has been a recent movement to surgically repair the meniscus whenever possible, partial meniscectomy remains one of the most commonly performed orthopedic surgeries. Due to this urgent need, there is currently great interest in methods to stimulate meniscus healing, augment repair, and improve long-term outcomes following meniscus injury using pharmaceutical, biological, or tissue engineering methods. Research on meniscus regenerative medicine, however, is greatly limited by a lack of understanding of meniscus cellular biology.

In this dissertation, this gap in knowledge is addressed with a thorough characterization of the meniscus cell phenotype. We have investigated the phenotypic identity of meniscus cells from inner and outer zones of the meniscus by RNA-sequencing, and provide comparisons to articular cartilage and isolated, monolayer cultured meniscus cells. We found that in situ meniscus cells from both the inner and outer zones are strikingly distinct from either articular chondrocytes or monolayer expanded meniscus cells at the transcriptomic level, and that inner and outer zone meniscus cells may be more similar to each other than to chondrocytes or monolayer cultured cells. Differences were also observed between inner and outer zone cells, and this dataset provides a wealth of novel targets for characterizing inner and outer zone cells to better understand regional cell biology of the meniscus.

We investigated the role of the physical microenvironment, including native extracellular matrix, monolayer culture, and biomaterial hydrogels, to modulate the meniscus cell transcriptional phenotype and support meniscus cell culture and expansion in vitro. Our findings provide new details on the meniscus cell dedifferentiation process, and demonstrate the utility of bioengineered hydrogels to reverse meniscus cell dedifferentiation for long-term in vitro culture.

We also investigated the effect of an injury-relevant inflammatory stimulus (IL1), and the potential for dynamic mechanical loading to modulate the inflammatory response of meniscus cells in two models of dynamic physiologic loading, cell stretch of isolated meniscus cells and compression of tissue explants. Results of RNA-sequencing, gene set enrichment analysis, and RT-qPCR from both models showed significant modulation of inflammation-related genes and pathways with mechanical stimulation, supporting the potential of mechanotransduction pathways as novel therapeutic targets to improve outcomes following meniscus injury.

Overall, this work provides a wealth of data characterizing the meniscus cell phenotype and lays the groundwork for future studies of meniscus regenerative medicine and tissue engineering. Furthermore, this work entailed considerable development and validation of methods for in vitro studies of meniscus cell biology and mechanotransduction, which will be valuable to the field of meniscus research. The work presented in this dissertation represents an enormous step forward in understanding the effects of physical, mechanical, and inflammatory environments on the meniscus cell phenotype, which is essential to the development of effective novel therapies to stimulate meniscus repair and prevent PTOA.