Cell biology
References 1 Cell and Tissue Culture Supplies: Marketing Report. Global Industry Analysts Inc. (2006). 2 Ben-Ze’ve, A (2005). Animal cell shape changes and gene expression. BioEssays, 13, 207-212. 3 Schmeichel, KL, Bissell, MJ (2003). Modeling tissue-specific signalling and organ function in three dimensions. J Cell Sci, 116, 2377-2388. 4 Bhadriraju, K, Chen, CS (2002). Engineering cellular microenvironments to improve cell-based drug testing. Drug Dis Today, 7, 612-20. 5 Sun, T, Jackson, S, Haycock, JW, MacNeil, S (2006). Culture of skin cells in 3D rather than 2D improves their ability to survive exposure to cytotoxic agents. J Biotechnol, 122, 372-381. 6 Zimmerman, H, Shirley, SG, Zimmerman, U (2007). Alginate-based encapsulation of cells: past, present, and future. Curr Diab Rep, 7, 314- 320. 7 Dvir-Ginzberg, M, Elkayam, T, Cohen, S (2008). Induced differentiation and maturation of newborn liver cells into functional hepatic tissue in macroporous alginate scaffolds. FASEB J, 22, 1440-1449. 8 Blackshaw, SE, Arkison, S, Cameron, C, Davies, JA (1997). Promotion of regeneration and axon growth following injury in an invertebrate nervous system by the use of three- dimensional collagen gels. Proc Biol Sci, 264, 657-61. 9 Sakiyama, SE, Schense, JC, Hubbell, JA (1999). Incorporation of heparin- binding peptides into fibrin gels enhances neurite extension: an example of designer matrices in tissue engineering. FASEB J, 13, 2214-24. 10Yu, X, Dillon, GP, Bellamkonda, RB (1999). A laminin and nerve growth factor-laden three-dimensional scaffold for enhanced neurite extension. Tissue Eng, 5, 291-304.
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removed from the environment which cells experi- ence in living tissues. One of the key differences between these two growth conditions is the impact of the environment on the physical shape and geometry of the cell. In vivo, cells have evolved to acquire their natural 3D structure, that is optimal for their normal growth and function. Furthermore, cells are supported by a complex 3D extracellular matrix (ECM) which facilitates cell-cell communi- cation via direct contact and through the secretion of cytokines and trophic factors. Many of these fac- tors are changed when cells are extracted from liv- ing tissues and explanted into 2D cell culture where they are generally confined in 2D monolayers with- out many of the physical and chemical cues which underlie their identity and function in vivo. The scope for cells to adopt natural morphologies or to communicate efficiently with their neighbours is significantly reduced in 2D culture. It has been known for many years that cell shape and contacts influence the cytoskeleton which in turn can regu- late gene and protein expression and hence cell function2. As a consequence, it is widely recognised that this 2D confinement is far removed from the aforementioned 3D complexities in living tissues and that this impacts on the validity of the data generated from 2D cell culture models. The scientific literature describes many instances in a broad range of applications where 3D cell growth is different and is advantageous over con- ventional 2D culture. For example, it has been shown that the growth and function of cells as multi-cellular 3D structures is significantly differ- ent to their growth as conventional 2D monolayer cultures3. Furthermore, engineering the cell culture micro-environment to create growth conditions that more accurately mimic the in vivo behaviour of cells is an essential step for improving predictive accuracy during drug discovery4. The design of 3D culture systems for use in pharmaceutical develop- ment is an important part of this process. Data show that refinement of the in vitro environment significantly influences the way in which cells respond to small molecules5.
A now overwhelming amount of evidence sug- gests that enabling cells to grow in a 3D spatial environment will overcome some of the restric- tions associated with 2D cell culture. As a conse- quence, a significant amount of effort is now focused on engineering materials to create such a 3D space for cell growth, which will begin to bridge the gap between conventional 2D cell cul- ture and living tissue environments. This article introduces some currently available technologies that enable 3D cell culture.
Technologies that enable 3D cell culture
A search of the scientific literature will reveal that there are many different approaches that enable the growth of cells in 3D. Traditionally, 3D cell growth has been an aspiration of tissue engineers, particularly for the generation of tissues to be used in transplantation. Comparatively less attention has been devoted to the development of technolo- gy for 3D culture for exclusive application in the laboratory. Furthermore, very few examples have been developed commercially into products that are readily available 3D cell culture technologies designed to improve the accuracy of in vitro analy- ses in a routine and cost-effective manner.
In
response to demand and interest in fabricating materials for 3D cell culture growth, there are now a number of basic approaches that can be cate- gorised. These include naturally occurring materi- als as well as products fabricated from naturally- derived and synthetic polymers.
Natural scaffold substrates
Alginate is a seaweed-derived material that has been used as a natural substrate to support the growth of cells in a number of ways including cell encapsulation6. Alginate materials have also been developed into macroporous scaffolds which have been employed to support the development of 3D aggregates or ‘spheroids’ of hepatocytes7. Related technology has been developed commercially in the form of Algimatrix™ (Invitrogen). This is promot- ed as an animal-free 3D substrate for the develop- ment of high-fidelity cell culture models that enhance the predictive of drug responses in certain disease states. Algimatrix™ enables 3D cell growth, however, the growth of cells as individual spherical masses may not be suitable for all appli- cations. Furthermore, the distribution of cells throughout the material is not entirely uniform and there are issues about the thickness of the scaffold in relation to mass transfer of oxygen, nutrients and waste products, particularly in the absence of media perfusion. While cell growth on alginate- based materials may have certain advantages, it is not clear how cultured mammalian cells that have been studied in contact with polystyrene for many years will respond to an alginate substrate.
Hydrogels
The use of hydrogels has been established for many years and are a common form of material used to support 3D cell growth in vitro for a broad range of tissues including bone, cartilage and nervous tissues8-12. As would be expected
Drug Discovery World Spring 2011
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