Engineering tissues for in vitro applications

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Engineered tissues can be employed for studies on the fundamental mechanisms of embryology and adult physiology and for investigating the evolution of disease processes. They also provide platforms to evaluate the behavior of new chemical entities in drug development. The recent development of three specific technologies has greatly facilitated the engineering of tissues for in vitro applications: the microfabrication tools that serve to both define the cellular microenvironment and enable parallelization of cell-based assays; synthetic, tunable hydrogels to create three-dimensional microenvironments; and bioreactors to control nutrient transport and fluid shear stress. Furthermore, convergence of these tools is providing investigators with the opportunity to construct and study tissues in vitro with unprecedented levels of sophistication.

Introduction

Engineering of tissues for therapeutic applications is one of the potential paths to replacing damaged tissues in ‘regenerative medicine’; however, it has been recently recognized that engineering of tissues for in vitro applications also holds tremendous potential value. In vivo, the responses of individual cells are regulated by spatiotemporal cues that reside in the local microenvironment such as the extracellular matrix (ECM), neighboring cells, soluble factors and physical forces, all presented in a three-dimensional context. Upon isolation from their in vivo milieu, a multitude of cell types display phenotypic instability [1, 2]; therefore, successful recapitulation of high-fidelity tissue models in vitro will require an understanding of just how cells respond to such microenvironmental stimuli towards defining structure/function relationships for tissues. A variety of novel tools have been developed recently that will aid in this effort including microfabrication-based tools to specify cell–substrate interactions [3, 4•], tunable synthetic hydrogels for the creation of three-dimensional tissues [5••], and controlled bioreactors for subjecting tissues to flow [6]. Here, we will provide selected examples of recent efforts to use such tools for engineering highly functional tissues. We will also discuss arenas in which these tissues are finding utility.

Section snippets

Microfabricated two-dimensional tissues: controlled cellular microenvironments and cellular microarrays

Conventional monolayer cultures are generated by randomly seeding cells onto homogenous substrates. Through the use of selective surface modification, microfabrication tools are now used to fabricate heterogeneous surfaces that offer control over cell–ECM and cell–cell interactions with micrometer precision [3]. A variety of such ‘micropatterning’ techniques have been developed and reviewed elsewhere [7, 8]. Briefly, photolithography is utilized to pattern photoresist (light-sensitive polymer)

Synthetic three-dimensional microenvironments

Considerable research has focused on mimicking the biochemical composition, fibrillar structure and viscoelastic gel characteristics of the natural tissue matrix [28, 29••]. Both naturally derived and synthetic biomaterials have been extensively explored as three-dimensional scaffolds [5••]. Of the many synthetic biomaterials being explored, hydrogels in particular have been widely adopted for three-dimensional cell culture because their high water content and mechanical properties resemble

Bioreactors for in vitro applications

Bioreactors are devices in which the biological and/or biochemical processes develop under closely monitored and tightly controlled environmental and operating conditions (i.e. pH, temperature, pressure, nutrient supply and waste removal, and shear stress) [6]. A plethora of different bioreactor designs have been described in the literature for the culture of two-dimensional and three-dimensional tissue constructs [53]. Here, our focus is primarily on small-scale bioreactors that have been

Conclusions

The ex vivo engineering of high-fidelity tissues is being facilitated by three specific technologies: microfabrication tools to precisely control cellular microenvironments and create miniaturized cell-based assays for screening applications; synthetic tunable hydrogels to create three-dimensional scaffolds that interact with cells in a bidirectional manner; and bioreactors to culture tissues under flow conditions towards controlling nutrient transport, enabling continuous culture and

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We thank Dirk Albrecht, Alice Chen, David Eddington, Elliot Hui and Gregory Underhill of the Laboratory for Multiscale Regenerative Technologies at MIT for insightful discussions. Funding was generously provided by NSF CAREER (SNB), NIH NIDDK, Deshpande Center at MIT, and the David and Lucile Packard Foundation.

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