In Vitro Innovation of Tendon Tissue Engineering Strategies

Abstract

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Keywords: in vitro; stem cells; tendon differentiation.

Figure 1

In vitro strategies for tendon tissue engineering. Tendon tissue engineering refers to a multidisciplinary field that aims at the inducement of tissue repair or regeneration. Therefore, it involves the combination of several key factors, such as cells, scaffolds, biochemical and mechanical inputs to produce a functional tendon-like construct. Abbreviations. PGA: polyglycolic acids; PLA: polylactic acids, PCL: polycaprolactones; PLGA: poly(lactic-co-glycolic) acids; PLCL: poly (lactil-co-captolactone) acids; ESCs: embryonic stem cells; iPSCs: induced pluripotent stem cells; AECs: amniotic epithelial stem cells; AMCs: amniotic mesenchymal stem cells; AFCs: amniotic fluid stem cells; UB-MSCs: umbilical cord mesenchymal stem cells; BMSCs: bone marrow mesenchymal stem cells; ADSCs: adipose derived mesenchymal stem cells; TPSCs: tendon progenitors stem cells; TGFβ: transforming growth factor beta; BMPs: bone morphogenetic proteins; CTGF: connective tissue growth factor; FGFs: fibroblastic growth factors; IGF-1: VEGF: vascular endothelial growth factor; PDGFs: platelet-derived growth factor.

Figure 2

Hierarchical arrangement of the structure of tendons: (a) Scanning electron microscopy (SEM) of a transverse section of collagen fiber bundles (scale bar = 10 μm); (b) SEM image of longitudinal collagen bundles in which their parallel arrangement along the longitudinal axis of the tendon is clearly shown; each collagen bundle is surrounded by the endotenon (scale bar = 20 μm); (c) SEM image that shows the multiple collagen fiber bundles that make up the tendon. The sample has been cross-sectioned, but it is clearly evident the parallel orientation of the collagen fibers (red arrows) (scale bar = 10 μm). Tendon images were obtained by field emission-scanning electron microscopy (FE-SEM, mod. LEO 1525; Carl Zeiss, Oberkochen, Germany). Samples were fixed in 4% paraformaldehyde (PFA), dehydrated with critical point dryer (mod. K850 Emitech, Assing, Rome, Italy), and cut before to be coated with a gold (250 Å thickness) using a sputter coater (mod.108 Å; Agar Scientific, Stansted, UK), courtesy of Giovanna Della Porta and Electron Microscopy Labs at Dept. of Industrial Engineering, University of Salerno.

Figure 3

Typical stress–strain curve for tendon tissue. The schematization illustrates the behavior of collagen fibers: under tensile strain, they stretch out absorbing shock, and when the stimulus disappears, they return to their initial configuration. If the stretching limit is exceeded, overcoming the physiological range, the tissue may suffer microscopic and macroscopic traumas. Adapted from [100].

Figure 4

Scientometric analysis aimed to compare the available publications on the Scopus database related to the main topics on tendon differentiation discussed in this review. The legend indicates the number of total publications for each topic, whereas the figure also represents the sub-class of papers exclusively referred to the in vitro conditions.

Figure 5

The comparative scientometric analysis of available publications on the Scopus database on tendon differentiation and in vitro tendon differentiation reveals four main common topics: stem cells, growth factors, biomaterials, and physical stimuli. The topic papers’ distributions amongst topics are independent of tendon differentiation sub-category (tendon or in vitro tendon differentiation). Stem cells are the most represented one (approximately 65%) followed by growth factors (approximately 20%), biomaterials (approximately 10%), and finally physical stimuli (for both 5%).

Figure 6

The light blue section shows the percentage/number of papers available on the Scopus database obtained by combining key words in vitro tendon differentiation with stem cells. In the histogram, the relative percentage of cited papers is expressed as well as the relative number in the legend.

Figure 7

The pink section shows the percentage/number of papers available on Scopus database obtained by combining key words in vitro stem cells tendon differentiation with hypoxia. In the histogram, the relative percentage of cited papers is expressed as well as the relative number in the legend.

Figure 8

The orange section shows the percentage/number of papers available on the Scopus database obtained by combining key words in vitro stem cells tendon differentiation with physical stimuli. In the histogram, the relative percentage of the cited papers is expressed as well as the relative number in the legend.

Figure 9

The green section shows the percentage/number of papers available on the Scopus database obtained by combining key words in vitro stem cells tendon differentiation with biomaterials. In the histogram, the relative percentage of cited papers is expressed as well as the relative number in the legend.

Figure 10

The yellow section shows the percentage/number of papers available on the Scopus database obtained by combining key words in vitro stem cells tendon differentiation with growth factors. In the histogram, the relative percentage of cited papers is expressed as well as the relative number in the legend.

Figure 11

The gray section shows the percentage/number of papers available on the Scopus database obtained by combining key words in vitro stem cells tendon differentiation with co-culture. In the histogram, the relative percentage of cited papers is expressed as well as the relative number in the legend.