The next decade or two of medical science will be dominated by progress in cell engineering. Today that field is still largely a matter of building low cost and reliable research tools, with first generation therapies as a secondary benefit rather than a primary goal. Much of that work involves stem cells: understanding them, and then figuring out how to create and control them as needed. Stem cells are comparatively rare cells in the body, of many different types, that maintain and build tissue. All descend from the original embryonic stem cells that build the body in the first place:
In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues.
Somewhere along this road stand a complete toolkit for repair of injuries that do not normally heal, technology to build replacement organs as needed, and other feats of medical science. This is one of the technology platforms needed to reverse the course of aging - to be able to build replacement cell populations for those that have become worn and dysfunctional with age.
Embryonic stem cells are important in stem cell research because they are pluripotent: able to create all cell types. A few years ago, researchers figured out how to turn ordinary cells into pluripotent cells that seemed to be as powerful as embryonic cells - a much cheaper and more readily available source than embryonic cells, and with the added possible benefit of using a patient's own cells to grow any form of tissue desired. These engineered stem cells are called induced pluripotent stem cells, and one present focus of research efforts is to understand whether they are, in fact, the same as embryonic stem cells.
For example, see this open access paper:
After the hope and controversy brought by embryonic stem cells two decades ago for regenerative medicine, a new turn has been taken in pluripotent cells research when, in 2006, Yamanaka's group reported the reprogramming of fibroblasts to pluripotent cells with the transfection of only four transcription factors. Since then many researchers have managed to reprogram somatic cells from diverse origins into pluripotent cells, though the cellular and genetic consequences of reprogramming remain largely unknown. Furthermore, it is still unclear whether induced pluripotent stem cells (iPSCs) are truly functionally equivalent to embryonic stem cells (ESCs) and if they demonstrate the same differentiation potential as ESCs.
When compared to ESCs, iPSCs, as expected, share a common pluripotency/self-renewal network. Perhaps more importantly, they also show differences in the expression of some genes. We concentrated our efforts on the study of [a range of genes] (in ESCs) which are not expressed in ESCs, as they are supposedly important for differentiation and should possess a poised status in pluripotent cells, i.e. be ready to but not yet be expressed. We studied each iPSC line separately to estimate the quality of the reprogramming and saw a correlation of the lowest number of such genes expressed in each respective iPSC line with the stringency of the pluripotency test achieved by the line.
Cells are complex and wayward little beasts, and many similar but different methodologies are presently being using to produce iPSCs. The end results vary in some way from laboratory to laboratory because that is true of almost every form of cell engineering. Cells change their gene expression profiles in minor ways at the drop of a hat. But these researchers are at least demonstrating that there are ways in which iPSCs can be methodically assessed and rated against ESCs.
The purpose of this study was to determine the degree of molecular similarity [between] differentiated cell types [derived] from human iPS cells and conventional ES cells. Our data indicates that [iPS cells] are transcriptionally highly similar to [ES cells].
This is how the sausage is made in the life sciences: a lot of very painstaking measurement and assessment of complex, shifting cellular machinery.