**9. Conclusions**

Attempts to enhance the repair and regeneration of injured connective tissues of the musculoskeletal system using cell therapies has been the subject of intense research over the past 30+ years. With the discovery of cells with the ability to differentiate into several relevant lineages (e.g., chondrocytes, bone cells, and others) reviewed in [122], this effort intensified. Using cells labeled mesenchymal stem cells (MSC), expectations ran high, but achieving success was more challenging.

Early after the discovery of adult "stem or progenitor" cells, there was considerable anticipation that they would rapidly be used to repair a variety of tissues damaged by

injury or disease, particularly during the aging process. This hope rapidly became hype, and a number of what have been called rogue clinics and companies began selling stem cell-based cell therapy approaches directly to patients or consumers for a number of conditions [199–201]. Such rogue entities preyed on desperate patients, and many such clinics in North America have been recently curtailed by the FDA and Health Canada in the USA and Canada, respectively. Such rogue applications of stem cell therapies emphasize the need to continue to develop methods and interventions to use these cells more effectively and with a solid base of scientific and clinical justification. This will require building on past successes and failures to evaluate new directions and approaches.

Learning from these past scientifically and ethically approved research efforts, it is emerging that many of the relevant connective tissues that could benefit from stem cell interventions have complex structures, are designed to work in complex mechanical environments, and when injured this creates an inflammatory environment. Furthermore, when injured or subjected to a disease process, the situation arises as an adult or an elderly individual when the anabolic environment of youth (growth and maturation) is no longer evident. Thus, attention to the environment that cells, such as MSC, are placed in, either as individual cells or incorporated into constructs, needs to be addressed if the MSC are to achieve more of their potential to impact the return of functionality in these connective tissues. That is, control of an environment where a catabolic inflammatory process is needed, supplementation of the environment with appropriate anabolic mediators is also needed (either as molecules, PRP or extracellular vesicles), and for some circumstances using cellular therapy early in a disease process while the remaining endogenous tissue can serve a template function may additionally be critical. Finally, controlling the impact of co-morbidities (i.e., diabetes) may also be required. Thus, improving the environment into which the cells are placed may be critical for further success. Similarly, picking the right cells for the job may also be critical as MSC from different sources can exhibit different properties even though they can have a similar phenotype, as discussed in [123,124]. Thus, the right cells in the right environment at the right time are needed are discussed in [202–204], and there is likely a need for a more "precision medicine" approach as "one size does not fit all" [202].

While some progress is being made in the applications of cellular therapy, including MSC use in tissue engineered constructs as reviewed in [165,198], and many lessons have been learned as outlined above, several questions related to the issue of tissue regeneration still remain. The first relates to human heterogeneity and how such heterogeneity translates to variation in connective tissue structure and function. A second relates to the question of whether absolute regeneration is required to obtain optimal functioning in a specific mechanical environment? That is, would 80 or 90% regeneration at a structural level be sufficient for people in the 60–70 years old age range, but perhaps not acceptable for someone 30–40 years of age and wanting to maintain a very active lifestyle? Some of these philosophical questions may also need to be factored into the expectations of how success is defined going forward.

**Author Contributions:** D.A.H. drafted the initial version and then made subsequent changes. N.N. made additional revisions and edits to the drafts. All authors have read and agreed to the published version of the manuscript.

**Funding:** The preparation of this review was supported by funds from the Strategic Clinical Networks Program of Alberta Health Services.

**Institutional Review Board Statement:** N/A This review did not involve patients or patient data.

**Informed Consent Statement:** N/A This review did not involve patients or patient data.

**Data Availability Statement:** N/A No original data was presented in this article. **Acknowledgments:** The authors thank the several colleagues for fruitful discussions on the topics presented over the past decade including Cyril B. Frank (deceased), Wataru Ando, Kazunori Shimomura, Ronald Zernicke, Nigel Shrive, Arin Sen and Roman Krawetz, as well as the contributions of many trainees over the past decades. We also apologize to all authors whose publications were not cited due to space considerations. This article is dedicated to the memory of Cyril B. Frank who was central to the initiation of many of the relevant studies discussed.

**Conflicts of Interest:** The authors declare no conflict of interest.
