In spite of justified optimism, several major challenges remain to be met. Foremost is the problem of mass transport during scale-up of engineered tissue constructs. Any TE modality that aspires toward clinical translation must consider vascularization. This hurdle is currently viewed as the limiting factor to the size of tissue constructs that can realistically be achieved. Supply of nutrients and oxygen to cells located deep in bulk tissue or complex organs must be resolved in order for them to be maintained in the body for any meaningful duration. Thrombogenic occlusion of microconduits or micropores introduced into biomaterial constructs is a common problem faced in tackling this limitation. The incorporation of antithrombogenic molecules into biomaterials is one of the strategies employed to overcome the problem. Alternatively, angiogenic factors can be incorporated into biomaterials to induce de novo
vasculogenesis and/or angiogenesis from tissues surrounding the implants. Spontaneous vasculogenesis observed under certain conditions, such as in human ESC EBs growing in suspension cultures,66,68
lends hope to surmounting this challenge.
Another challenge is the requirement for innervation. In fact, this requirement has been the major obstacle in the development of an implantable hybrid liver assist device. The liver is richly innervated via both the sympathetic and parasympathetic pathways from the hypothalamus and adrenal glands, which regulate functions such as blood flow through the hepatic sinusoids, solute exchange, and parenchymal function. Innervation is also required by other organs such as muscles, the pulmonary system, the kidney, and endocrine glands. Therefore, selection of biomaterials and the design of a tissue construct for repairing these organ systems would have to take into account the provision for innervation.
Organ systems are not composed of a homogenous cell type, but rather an assembly of different cell types either intermingled together or partitioned into discrete sublocations. Each of these cell types may have unique substratum requirements. Engineering of complex organs would, therefore, need to cater to each component cell type. A challenge remains to find the correct balance between the biological and physical properties of the scaffold material to suit each cell type. In this respect, TE using stem cells has clear advantages, because the plasticity of the cells can allow for de novo formation of tissues depending on scaffold composition. In situ remodeling at the interface between different cell types, akin to events that occur between germ layers during embryogenesis, can give rise to new tissues. This may theoretically relax the stringency for precise substratum requirements.
The creation of relevant disease models to evaluate the efficacy of the engineered tissue constructs is as important as overcoming the engineering hurdles. Often, small rodent models with mechanically or pharmacologically induced lesions do not accurately recapitulate human disease conditions, causing disparate outcomes between preclinical and clinical trials. Non-human primate models may in theory, provide the most relevant animal models, but these are not readily available for practical and ethical reasons. The creation of non-human primate models for various human diseases by gene targeting and nuclear transfer has been proposed.192,193
However, cloning of monkeys remains unsuccessful to date. Success in this arena may positively impact stem cell TE.