Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Abstract

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

Keywords: Tendon; extracellular vesicles; mesenchymal stem cells; tissue engineering.

Figure 1.

Trunk tendon differentiation model. In the axial tendon, signals from the myotome are essential for the initiation of tendon progenitor cells, whereas signals from the sclerotome, which are activated by the ventral midline sonic hedgehog signal, in turn, play an opposite role. Scx and Mkx promote axial tendon differentiation by activating extracellular matrix molecules.

Figure 2.

Development of limb tendon in embryogenesis. In the limb, Scx and Sox9 are the first signals for tendon progenitor cell initiation, whereas Mkx and early growth response 1 and 2 are the second signals for tendon differentiation and maturation. Sox9 is involved in the initial stage and changes to play an opposite role subsequently. Both are synergically involved in limb tendon development by interacting with related growth factors.

Figure 3.

Expression of tendon markers in tenocytes during tendon development. (a) Mesenchymal cells differentiate into Scx-expressing tendon progenitor cells, which also partially express Sox9. Scx+Sox9+ progenitor cells differentiate into the tenocytes which are located near the bone in the enthesis. (b) In mouse limbs, Scx expression begins to increase at E9.5 and continues to increase until tenocyte maturation. Slight Mkx expression is detectable in the tendon at E12.5, after the emergence of Scx and robust Mkx mRNA expression at E13.5 and E14.5, stages at which the tendon progenitors undergo condensation and differentiation. Egr1 transcripts are first expressed at E12.5 in Scx domains forming tendons, and then they are expressed in long tendons at E16.5. Egr2 is first detectable in E14.5 limb tendons and is generally expressed in all limb tendons by E16.5. Tnmd is highly expressed in E14.5 and is considered a late tendon marker. Adapted from He et al.

Figure 4.

Location of different cells in tendon tissue. TSCs type I (orange) reside in the outer layer of the tendon (paratenon), TSCs type II (purple) reside in a niche in the inner part of tendons, and TNCs (brown) are aligned between fibres. Some of these subpopulations might overlap with each other, and perivascular TSCs may be present in the endotenon and peritenon (image partially created with https://www.biorender.com/).

Figure 5.

Differentiation of a TSPC to a tenocyte. The expression profile of a TSPC changes during differentiation to a tenocyte. Tenogenic differentiation is mostly driven by TGF-β2/3 and BMP12/13. Adapted from Schneider et al. (image partially created with https://www.biorender.com/).

Figure 6.

Stress–strain relationship in tendons. At low levels of stress, tendons stretch relatively easily. This is called the ‘toe’ portion of the stress–strain curve, a consequence of straightening of crimped collagen fibrils and orientation of fibres along the direction of the applied load. With higher levels of stress, the highly oriented collagen fibres respond with a linear level of strain. The slope of the linear region represents the elastic modulus of the tendon. Continuing increases in the level of stress applied to tendons and ligaments lead to irreversible changes at the interface between collagen fibres in the structure.

Figure 7.

Key molecular, cellular and matrix changes occurring during the three main phases of tendon repair. Each healing stage is characterized by the involvement of different growth factors, activation of specialized cell types, and production of essential matrix proteins. Collectively, they contribute to the replacement of the initial fibrous tissue with more tendinous regenerated. Adapted from Docheva et al. and James et al.

Figure 8.

EVs mechanism of healing for tendinopathies. Schematic diagram showing the three major categories of the increasing proliferation of tenocytes, attenuation of the inflammatory response, and improvement in enthesis to explain the mechanism of healing. Adapted from Fang et al.