Tissue Engineering Coursework - Thanos & Yiangos

Cell Survival in Tissue Engineering: challenges and strategies

Yiangos Psaras

Thanos Sofroniou

 

Introduction

Tissue engineering as an interdisciplinary field has had significant progress over the last decade with various improvements being made in many areas affecting tissue engineering such as biomaterials, surface chemistry, physiology, laboratory techniques and others[T1] . Despite this progress, the main goal of tissue engineering, which is to create in vitro tissues and organs for in vivo replacement to repair, maintain or enhance biological function or for in vitro usage of tissues and organs as biological modeling for basic research and drug development, is far from realized[T2] . Simple tissues such as skin and cartilage have been successfully developed and even used in the clinic but complex organs face several obstacles that prevent them from being translated for patients.

The main challenge of complex tissues and organs from being engineered in vitro is the ability of cells to survive throughout tissue construction. Cell survival in tissue engineering refers to several different features that cells exhibit when cultured in vitro. Cell viability, cell proliferation and cell differentiation and proper cell function as well as the absence of ischemia, hypoxia or necrosis indicate the desirable characteristics of cell survival in tissue engineered grafts.

There are several problems that impede cell survival in complex tissues but these can be categorized in two ways: vascularization of graft and niche mimickery.

Vascularization of tissues provides oxygen, nutrients and various chemical factors that are essential for cell survival and behavior. The accumulation of cellular waste can also hinder cell viability significantly, which marks a vascular system essential for tissue formation. The vascular system in vivo is characterized by blood vessels that branch into different sizes and dependent on their structure and size they exhibit different functions, with capillaries being of great importance as they are responsible for diffusion of substances into the interstitial fluid and into cells. The specific three-dimensional structure of the ECM of a particular tissue is also important in cell survival within scaffolds as it delivers essential biological and physico-chemical signals to the cells. This is often referred to as the niche microenvironment of the cells, which is analogous to the ecological niche of a species. Creating scaffolds in vitro which obtain vascular networks and mimic the niche microenvironment are considered to be the main challenges in tissue engineering which will be discussed in further detail below.

Challenges

Cell survival is the initial step to be considered when producing a graft in TE, however in translating TE products to the clinic, it is essential to consider a more survival at a larger scale. Hence, this review will not only consider survival at the cellular level, but also to the level of graft survival. The major challenges in graft survival are concerned with ischemic damage and the niche.

In the past, production and even clinical application of engineered grafts has been successful. Indeed the biggest success is possibly that of engineered trachea being implanted []. However, grafts of low complexity seem to belittle the challenges faced in producing successful grafts. It must be pointed out that successful production has so far concerned largely avascular grafts with low oxygen demands. Langer et al., report that skeletally mature articular cartilage, the major component of the trachea, is an avascular tissue [19]. While some allowance is made on the basis that tracheal tissue does contain a cellular population, albeit one of limited diversity and number as well as limited innervation, it remains a fact that low metabolic demands must be met in such a graft [21]. Thus, successful production of equivalents for metabolically active tissues such as the heart, kidney or liver has so far been hindered.

It has been reported that ischemic damage causes cell death due to lack of oxygen. This is a universal problem, despite the different oxygen requirements of different cell types; cardiomyocytes require 27.6mmol of oxygen per mg of protein per minute, whereas hepatocytes require 18mmol of oxygen per mg of protein per minute [12 ^{147, 148}]. These are unable to be supplied by simple diffusion of oxygen across thick tissue. Indeed, oxygen diffusion in tissue is restricted to a distance of 150-200nm from the source [5 ¹³]. Simple oxygen diffusion limits are dependent on the type of tissue, particularly its cell density and that of the extracellular matrix and its components. In vitro viability of cardiomyocytes can in fact be maintained on sheets no thicker than 100micrometers [668] (although this is presumed to be dependent on cell density, and involve the diffusion of nutrients as well -> must check!).  Perfusion for in vivo work presented in ref. 6.Mechanism of Ischemic damage.

Niche imitation

Strategies

            Several strategies have been proposed and studied for the enhancement of cell survival. Although the literature is relatively poor in the term cell survival, it is apparent that positive evidence to cell survival is represented in cell proliferation, differentiation, specific morphologic cellular response, migration and growth in cell size.

Vascularization

The major cue in driving cell survival is addressing systemic delivery of oxygen, nutrient and removal of waste material such as lactate and urea []. As such, there is an abundance of literature exploring the development of vascularity in grafts. This concerns angiogenesis, neovascularization and inosculation and anastomosis (define), although lacking a qualitative assessment as to the effect of increasing vascularity in grafts to cell survival [].

Niche reproduction

Other strategies to enhance cell viability

Future outlook

Conclusion

References