Unique biomaterial compositions direct bone marrow stem cells into specific chondrocytic phenotypes corresponding to the various zones of articular cartilage Lonnissa H. Nguyen, Abhijit K. Kudva, Nicole L. Guckert, Klaus D. Linse, Krishnendu Roy * Department of Biomedical Engineering, The University of Texas at Austin, Austin, TX 78712, USA article info Article history: Received 24 July 2010 Accepted 6 October 2010 Available online 10 November 2010 Keywords: Bone marrow stromal cells Degradable hydrogel Chondroitin sulfate Hyaluronic acid MMP (matrix metalloproteinase) Articular cartilage tissue engineering abstract Numerous studies have reported generation of cartilage-like tissue from chondrocytes and stem cells, using pellet cultures, bioreactors and various biomaterials, especially hydrogels. However, one of the primary unsolved challenges in the field has been the inability to produce tissue that mimics the highly organized zonal architecture of articular cartilage; specifically its spatially varying mechanical properties and extra-cellular matrix (ECM) composition. Here we show that different combinations of synthetic and natural biopolymers create unique niches that can “direct” a single marrow stem cell (MSC) population to differentiate into the superficial, transitional, or deep zones of articular cartilage. Specifically, incorpo- rating chondroitin sulfate (CS) and matrix metalloproteinase-sensitive peptides (MMP-pep) into PEG hydrogels (PEG:CS:MMP-pep) induced high levels of collagen II and low levels of proteoglycan expres- sion resulting in a low compressive modulus, similar to the superficial zone. PEG:CS hydrogels produced intermediate-levels of both collagen II and proteoglycans, like the transitional zone, while PEG:hyalur- onic acid (HA) hydrogels induced high proteoglycan and low collagen II levels leading to high compressive modulus, similar to the deep zone. Additionally, the compressive moduli of these zone- specific matrices following cartilage generation showed similar trend as the corresponding zones of articular cartilage, with PEG:CS:MMP-pep having the lowest compressive modulus, followed by PEG:CS while PEG:HA had the highest modulus. These results underscore the potential for composite scaffold structures incorporating these biomaterial compositions such that a single stem-progenitor cell pop- ulation can give rise to zonally-organized, functional articular cartilage-like tissue. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Regeneration of articular cartilage, also known as hyaline cartilage, is one of the most critical challenges in arthritis, joint disorders and trauma. Although, tissue-engineered cartilage offers a promising solution, most efforts to date have focused on gener- ating homogenous tissues whose bulk properties are similar to native articular cartilage [1e3]. However, anatomically and func- tionally, articular cartilage consists of four, spatially distinct zones; the superficial, transitional (middle), deep, and calcified zones. Each zone is characterized by unique extra-cellular matrix (ECM) compositions, mechanical properties and cellular organization. The cartilage-ECM is primarily composed of type II collagen (90e95% of total collagen content) and glycosaminoglycans (GAGs) (10e20% of the cartilage wet weight) whose relative concentrations vary spatially from the superficial to the deep zone leading to varying mechanical properties [4]. The superficial zone contains high levels of collagen II and low levels of GAG [5]. The transitional zone has lower collagen II content while the GAG concentration increases [6]. The deep zone contains the highest concentration of GAGs and the lowest level of collagen II fibers [7]. Finally, the calcified carti- lage zone contains high levels of collagen X and integrates the cartilage to the subchondral bone [5e7]. The mechanical properties of articular cartilage are sensitive to changes in the ECM compo- sition because any compressive forces that are experienced by the tissue, stimulates GAG synthesis and ECM remodeling [8,9]. Specifically, the compressive modulus increases significantly from the articular surface to the deep zone and is dictated by the varying ECM composition of the various zones and the structural organi- zation of the ECM molecules [4]. One of the major unsolved challenges in cartilage tissue engi- neering has been our inability to regenerate tissue that mimics the highly organized zonal architecture of articular cartilage. Classical cartilage tissue engineering approaches did not mimic the struc- tural organization or the zonal properties of articular cartilage, * Corresponding author. Department of Biomedical Engineering, The University of Texas at Austin, Biomedical Engineering Bldg. 2.312, Austin, TX 78712, USA. Tel.: þ1 (512) 232 3477; fax: þ1 (512) 471 0616. E-mail address: kroy@mail.utexas.edu (K. Roy). Contents lists available at ScienceDirect Biomaterials journal homepage: www.elsevier.com/locate/biomaterials 0142-9612/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2010.10.009 Biomaterials 32 (2011) 1327e1338