Mature Cells

In the human body, there are approximately 300 different types of differentiated cells, each with its own specific function. During differentiation, these cells lose the ability to divide and, by adapting to a specific task, can no longer proliferate. Since cell therapy and tissue engineering rely on the implantation of considerable quantities of well-characterized autologous cells that display defined properties, differentiated cells must be manipulated so that they multiply in vitro. Taking primary cells from the healthy tissues of choice is mandatory for therapeutic applications. It is unimaginable that immortalized cells could be reintroduced into humans, as they could develop into a cancer.

There are different methods for reinitiating the cell division of differentiated cells. One way is to modify the cells genetically and conditionally immortalize them to proliferate for a limited period of time without inducing epigenetic changes or changes in the cell properties. Ideal candidate transgenes to control conditional expansion of therapeutic cell populations could be Notch48 or antisense p27Kip1.

However, since these strategies enable proliferation induction in a limited number of cell lines viral genes, such as the simian virus large T antigen, the Herpes virus-16 E6/7 or the ubiquitous telomerase reverse transcriptase, are sometimes used generically to trigger proliferation. In any case, proliferation-inducing transgenes must be tightly and timely controlled using either state-of-the-art transcription or translation control modalities or site-specific recombination technology to excise transgene expression loci prior to implantation.
Another way to trigger the proliferation of differentiated cells is to reprogram them under controlled conditions. The phenotypic conversion of a cell or a tissue type into another cell or tissue is referred to as ‘metaplasia’, with many examples prevalent in humans. Metaplasia frequently begins with the de-differentiation of the cell, whereby endogenous proliferation genes are turned on and cell mass can be generated for tissue engineering. This can be achieved either by adding extracts of pluripotent cells to the media or by implanting the nucleus of the cell in an ovum, from which the nucleus has been removed. However, a drawback of this approach is that the de-differentiated cells must differentiate correctly after the proliferation step.
An alternative to obtaining the desired cell type is trans-differentiation, which is a specific form of metaplasia in which the phenotype of a mature cell is converted into another fully differentiated cell type.
Mammalian cells have been trans-differentiated by co-culture, by modification of cell culture conditions or by genetic modification of the cells. Hu et al. reported the transdifferentiation of myoblasts into mature adipocytes by ectopic expression of the adipogenic transcription factors PPAR gamma and C/EBPα under conditions conductive to adipogenesis.
Schiller et al. showed that inhibition of gap-junctional communication between osteoblasts results in adipocyte formation as well. Furthermore, it has been shown that pancreatic cells can be converted into hepatocytes by the induction of hepatic transcription factors using dexamethasone and that myoblast transdifferentiation can be controlled precisely by introducing differentiation determinants such as MyoD and msx1.4 The microenvironment of the cells, including the extracellular matrix, the surrounding cells, the local milieu and growth or differentiation factors, plays a crucial role in redirecting cell fate.
Injection of endothelial cells into damaged heart tissue has led to their trans-differentiation into beating cardiomyocytes. By the same means, pancreatic epithelial cells have also trans-differentiated successfully into hepatocytes. Trans-differentiation plays an essential role in tissue engineering since large amounts of certain cell types, such as adipocytes, can be grafted from patients with reintroduction of the tissue into the body not resulting in immunogenic rejection.