Informing Stem Cell-Based Tendon Tissue Engineering Approaches with Embryonic Tendon Development

From the perspective of embryonic tendon development: various cells applied to tendon tissue engineering

There is a high risk of injury from damage to the force-bearing tissue of the tendon. Due to its poor self-healing ability, clinical interventions for tendon injuries are limited and yield unsatisfying results. Tissue engineering might supply an alternative to this obstacle. As one of the key elements of tissue engineering, various cell sources have been used for tendon engineering, but there is no consensue concerning a single optimal source. In this review, we summarized the development of tendon tissue from the embryonic stage and categorized the used cell sources in tendon engineering. By comparing various cell sources as the candidates for tendon regeneration, each cell type was found to have its advantages and limitations; therefore, it is difficult to define the best cell source for tendon engineering. The microenvironment cells located is also crucial for cell growth and differentiation; so, the optimal cells are unlikely to be the same for each patient. In the future, the clinical application of tendon engineering might be more precise and customized in contrast to the current use of a standardized/generic one-size-fits-all procedure. The best cell source for tendon engineering will require a case-based assessment.

Regenerative Engineering of the Rotator Cuff of the Shoulder

Rotator cuff tears often heal poorly, leading to re-tears after repair. This is in part attributed to the low proliferative ability of the resident cells (tendon fibroblasts and tendon-stem cells) upon injury to the rotator cuff tissue and the low vascularity of the tendon insertion. In addition, surgical outcomes of current techniques used in clinical settings are often suboptimal, leading to the formation of neo-tissue with poor biomechanics and structural characteristics, which results in re-tears. This has prompted interest in a new approach, which we term as "Regenerative Engineering", for regenerating rotator cuff tendons. In the Regenerative Engineering paradigm, roles played by stem cells, scaffolds, growth factors/small molecules, the use of local physical forces, and morphogenesis interplayed with clinical surgery techniques may synchronously act, leading to synergistic effects and resulting in successful tissue regeneration. In this regard, various cell sources such as tendon fibroblasts and adult tissue-derived stem cells have been isolated, characterized, and investigated for regenerating rotator cuff tendons. Likewise, numerous scaffolds with varying architecture, geometry, and mechanical characteristics of biologic and synthetic origin have been developed. Furthermore, these scaffolds have been also fabricated with biochemical cues (growth factors and small molecules), facilitating tissue regeneration. In this Review, various strategies to regenerate rotator cuff tendons using stem cells, advanced materials, and factors in the setting of physical forces under the Regenerative Engineering paradigm are described.