Tissue engineering of articular cartilage with biomimetic zones

Abstract

Articular cartilage damage is a persistent and increasing problem with the aging population, and treatments to achieve biological repair or restoration remain a challenge. Cartilage tissue engineering approaches have been investigated for over 20 years, but have yet to achieve the consistency and effectiveness for widespread clinical use. One of the potential reasons for this is that the engineered tissues do not have or establish the normal zonal organization of cells and extracellular matrix that appears critical for normal tissue function. A number of approaches are being taken currently to engineer tissue that more closely mimics the organization of native articular cartilage. This review focuses on the zonal organization of native articular cartilage, strategies being used to develop such organization, the reorganization that occurs after culture or implantation, and future prospects for the tissue engineering of articular cartilage with biomimetic zones.

FIG. 1.

Zonal organization in normal articular cartilage. (A) Three-dimensional histology and schematic showing cell and collagen fibril organization in the superficial, middle, and deep zones. Also depicted are changes in levels of oxygen, collagen cross-links (lysylpyridinoline (LP), hydroxylysylpyridinoline (HP)), and compressive modulus (H_A0) through the thickness of the tissue. (B) EPIC micro-CT map of GAG distribution in human articular cartilage. (C) PRG4 (reprinted from with permission from Elsevier) appears to suitable as a marker for the superficial zone, as does (D) developmental endothelial locus-1 (reprinted from with permission form Elsevier). (E) Cartilage intermediate layer protein is found in the interterritorial matrix of middle and deep zones, whereas (F) Jagged 1 is highly expressed in cells of the middle and deep zones. Color images available online at www.liebertonline.com/ten.

FIG. 2.

Schematic of methods that can be applied to zonal cartilage tissue engineering. Cells from different zones (s, m, d) of articular cartilage are isolated and, for example, (A) cultured in alginate, released from the alginate and layered in a scaffold- and matrix-free approach. Alternately, (B) cells are suspended in a hydrogel matrix and sequentially polymerized in layers to localize zonal cells and/or provide zonal variations in matrix. (C) To provide zonal structural differences, scaffolds can be printed with a gradient of pore sizes and seeded with full-thickness (f) cells. Finally, (D) a hybrid approach uses different cell populations within distinct hydrogel matrices that are printed to give zonal cellular, structural, and matrix properties.

FIG. 3.

Bioreactors for in vitro tissue conditioning. (A) A commercial dynamic compression bioreactor (Tissue Growth Technologies, Minnetonka, MN, www.tissuegrowth.com) allows for controlled simultaneous dynamic compressive loading and perfusion of up to 12 constructs. (B) A custom tensile bioreactor allows for tensile loading of 12 constructs, and has been used to show different responses from chondrocytes of different zones. (C) A hip simulator-based bioreactor applies surface motion as well as dynamic compression, and has been shown to enhance the surface properties of the engineered cartilage.

FIG. 4.

Cell re-organization and matrix remodeling in vivo. Zonal porcine cartilage constructs with fluorescently labeled superficial cells showed (A) well-defined layers after 4 weeks of in vitro culture, but (B) this structure was lost after 1 week in vivo. While no clinical trials have been performed with zonal engineered cartilage, several clinical approaches for regeneration result in tissues that generally lack the zonal organization of native articular cartilage. (C) ACI and (D) microfracture 2 years after implantation lack the zonal architecture, similar to (E) matrix-induced ACI 1 year postoperative. In rare cases, polarized light microscopy shows (F) good collagen alignment in ACI repaired regions, as opposed to (G) mixed hyaline-fibrocartilage and (H) more typical fibrocartilage formation. ACI, autologous chondrocyte implantation. Color images available online at www.liebertonline.com/ten.