We have fabricated a hepatic tissue construct using a multilayer photopatterning platform for embedding cells in hydrogels of complex architecture. We first explored the potential of established hepatocyte culture models to stabilize isolated hepatocytes for photoencapsulation (e.g., double gel, Matrigel, cocultivation with nonparenchymal cells). Using photopolymerizable PEG hydrogels, we then tailored both the chemistry and architecture of the hydrogels to further support hepatocyte survival and liver-specific function. Specifically, we incorporated adhesive peptides to ligate key integrins on these adhesion-dependent cells. To identify the appropriate peptides for incorporation, the integrin expression of cultured hepatocytes was monitored by flow cytometry and their functional role in cell adhesion was assessed on full-length extracellular matrix (ECM) molecules and their adhesive peptide domains. In addition, we modified the hydrogel architecture to minimize barriers to nutrient transport for these highly metabolic cells. Viability of encapsulated cells was improved in photopatterned hydrogels with structural features of 500 microm in width over unpatterned, bulk hydrogels. Based on these findings, we fabricated a multilayer photopatterned PEG hydrogel structure containing the adhesive RGD peptide sequence to ligate the alpha5beta1 integrin of cocultured hepatocytes. Three-dimensional photopatterned constructs were visualized by digital volumetric imaging and cultured in a continuous flow bioreactor for 12 d where they performed favorably in comparison to unpatterned, unperfused constructs. These studies will have impact in the field of liver biology as well as provide enabling tools for tissue engineering of other organs.

S U M M A R Y Articular cartilage is a heterogeneous tissue, with cell density and organization varying with depth from the surface. The objectives of the present study were to establish a method for localizing individual cells in three-dimensional (3D) images of cartilage and quantifying depth-associated variation in cellularity and cell organization at different stages of growth. Accuracy of nucleus localization was high, with 99% sensitivity relative to manual localization. Cellularity (million cells per cm 3 ) decreased from 290, 310, and 150 near the articular surface in fetal, calf, and adult samples, respectively, to 120, 110, and 50 at a depth of 1.0 mm. The distance/angle to the nearest neighboring cell was 7.9 m/31 Њ , 7.1 m/31 Њ , and 9.1 m/31 Њ for cells at the articular surface of fetal, calf, and adult samples, respectively, and increased/decreased to 11.6 m/31 Њ , 12.0 m/30 Њ , and 19.2 m/ 25 Њ at a depth of 0.7 mm. The methodologies described here may be useful for analyzing the 3D cellular organization of cartilage during growth, maturation, aging, degeneration, and regeneration.

It is likely that effective application of cell-laden implants for cartilage defects depends on retention of implanted cells and interaction between implanted and host cells. The objectives of this study were to characterize stratified cartilaginous constructs seeded sequentially with superficial (S) and middle (M) chondrocyte subpopulations labeled with fluorescent cell tracking dye PKH26 (*) and determine the degree to which these stratified cartilaginous constructs maintain their architecture in vivo after implantation in mini-pigs for 1 week. Alginate-recovered cells were seeded sequentially to form stratified S*/M (only S cells labeled) and S*/M* (both S and M cells labeled) constructs. Full-thickness defects (4 mm diameter) were created in the patellofemoral groove of adult Yucatan mini-pigs and filled with portions of constructs or left empty. Constructs were characterized biochemically, histologically, and biomechanically, and stratification visualized and quantified, before and after implant. After 1 week, animals were sacrificed and implants retrieved. After 1 week in vivo, glycosaminoglycan and collagen content of constructs remained similar to that at implant, whereas DNA content increased. Histological analyses revealed features of an early repair response, with defects filled with tissues containing little matrix and abundant cells. Some implanted (PKH26-labeled) cells persisted in the defects, although constructs did not maintain a stratified organization. Of the labeled cells, 126 +/- 38% and 32 +/- 8% in S*/M and S*/M* constructs, respectively, were recovered. Distribution of labeled cells indicated interactions between implanted and host cells. Longer-term in vivo studies will be useful in determining whether implanted cells are sufficient to have a positive effect in repair.

Abstract3-D imaging and analysis techniques can be used to assess the organization of cells in biological tissues, providing key insights into the role of cell arrangement in growth, homeostasis, and degeneration. The objective of the present study was to use such methods to assess the growth-related changes in cell organization of articular cartilage from different sites in the bovine knee. Threedimensional images of fetal, calf, and adult cartilage were obtained and processed to identify cell nuclei. The density of cells was lower with growth and with increasing depth from the articular surface. The cell organization, assessed by the angle to the nearest neighboring cell, also varied with growth, and reflected the classical organization of cells in adult tissue, with neighboring cells arranged horizontally in the superficial zone (average angle of 20°) and vertically in the deep zone (60°). In all other regions and growth stages of cartilage, the angle was ~32°, indicative of an isotropic organization. On the contrary, the nearest neighbor distance did not vary significantly with growth or depth. Together, these results indicate that cartilage growth is associated with distinctive 3-D arrangements of groups of chondrocytes.

To analyze the effects of prolonged storage time, at warm and cold temperatures, on the viability of human nasal septal chondrocytes and to understand the implications for tissue engineering of septal cartilage.

A significant decline in chondrocyte viability occurs after intra-articular fractures of the calcaneus. This may contribute to the development of post-traumatic arthritis.

Various biomaterial scaffolds have been investigated for cartilage tissue engineering, although little attention has been paid to the effect of scaffold microstructure on tissue growth. Non-woven, fibrous, bioabsorbable scaffolds constructed from a copolymer of glycolide and trimethylene carbonate with varying levels of porosity and pore size were seeded with mesenchymal stroma cells with a chondrogenic lineage. Scaffolds and media were evaluated for both cell and extracellular matrix organization and content after up to 28 days of culture in a spinner flask. Analysis of DNA and glycosaminoglycan contents showed that the most porous of the three scaffold types, with a porosity of 81% and a porometry determined mean flow pore diameter of 54 microm, supported the most rapid proliferation of cells and accumulation of extracellular matrix. Analysis of the high porosity scaffold system, using Western Blot and immunohistochemistry confirmed the presence of collagen type II and absence of collagen type I, and demonstrated cells with a chondrocyte morphology with aggrecan and collagen II accumulation attached to the scaffolds. It was concluded that the 3D-microstructural characteristics of the scaffold (interconnecting porosity and pore size) play an important role in proliferation and phenotype of chondrogenic cells and accumulation of extracellular matrix molecules.

Synovial fluid (SF) contains lubricant macromolecules, hyaluronan (HA), and proteoglycan 4 (PRG4). The synovium not only contributes lubricants to SF through secretion by synoviocyte lining cells, but also concentrates lubricants in SF due to its semi-permeable nature. A membrane that recapitulates these synovium functions may be useful in a bioreactor system for generating a bioengineered fluid (BF) similar to native SF. The objectives were to analyze expanded polytetrafluoroethylene membranes with pore sizes of 50 nm, 90 nm, 170 nm, and 3 μm in terms of (1) HA and PRG4 secretion rates by adherent synoviocytes, and (2) the extent of HA and PRG4 retention with or without synoviocytes adherent on the membrane. Experiment 1: Synoviocytes were cultured on tissue culture (TC) plastic or membranes ± IL-1β + TGF-β1 + TNF-α, a cytokine combination that stimulates lubricant synthesis. HA and PRG4 secretion rates were assessed by analysis of medium. Experiment 2: Bioreactors were fabricated to provide a BF compartment enclosed by membranes ± adherent synoviocytes, and an external compartment of nutrient fluid (NF). A solution with HA (1 mg/mL, MW ranging from 30 to 4,000 kDa) or PRG4 (50 μg/mL) was added to the BF compartment, and HA and PRG4 loss into the NF compartment after 2, 8, and 24 h was determined. Lubricant loss kinetics were analyzed to estimate membrane permeability. Experiment 1: Cytokine-regulated HA and PRG4 secretion rates on membranes were comparable to those on TC plastic. Experiment 2: Transport of HA and PRG4 across membranes was lowest with 50 nm membranes and highest with 3 μm membranes, and transport of high MW HA was decreased by adherent synoviocytes (for 50 and 90 nm membranes). The permeability to HA mixtures for 50 nm membranes was ~20 × 10 −8 cm/s (− cells) and ~5 × 10 −8 cm/s (+ cells), for 90 nm membranes was ~35 × 10 −8 cm/s (− cells) and ~ 19 × 10 −8 cm/s (+ cells), for 170 nm membranes was ~74 × 10 −8 cm/s (± cells), and for 3 μm membranes was ~139 × 10 −8 cm/s (± cells). The permeability of 450 kDa HA was ~40× lower than that of 30 kDa HA for 50 nm Correspondence to: Robert L. Sah, rsah@ucsd.edu. NIH Public AccessAuthor Manuscript Biotechnol Bioeng. Author manuscript; available in PMC 2011 May 1. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript membranes, but only ~2.5× lower for 3 μm membranes. The permeability of 4,000 kDa HA was 250× lower than that of 30 kDa HA for 50 nm membranes, but only ~4× lower for 3 μm membranes. The permeability for PRG4 was ~4 × 10 −8 cm/s for 50 nm membranes, ~48 × 10 −8 cm/s for 90 nm membranes, ~144 × 10 −8 cm/s for 170 nm membranes, and ~336 × 10 −8 cm/s for 3 μm membranes. The associated loss across membranes after 24 h ranged from 3% to 92% for HA, and from 3% to 93% for PRG4. These results suggest that semi-permeable membranes may be used in a bioreactor system to modulate lubricant retention in a bioengineered SF, and that synoviocytes adherent on the membranes may serve as both a lubricant source and a barrier for ...