• plurpotent cells can cause teratomas (in fact this is one mandatory criterium of these cells). So transplantation of these cels can give rise to tumors and therefore usage is risky.
• The differentiation of cells is a proces which, in the body, is reglated by gradients of cytokines, chemokines and ligands for certain athways. further more the stability of recombinant ligands is not optimal therefore you can not mmimic the situation in the body optimal. Other reasons are that in the body you have multicellular systems whcih tend to interact

regards

Sven

Dear Charlotte Worrall,

When using the term "pluripotent stem cells", don't always assume someone is referring to induced pluripotent stem cells or embryonic stem cells. There are endogenous pluripotent stem cells present in adult animals, including humans. They are located within connective tissue niches throughout the body. 

My group has extensively studied endogenous pluripotent stem cells from both non-human mammals and humans. We have isolated these cells, expanded them in culture, cryopreserved them, cloned them from single cells, and performed extensive characterization studies using karyotyping; molecular analyses and differentiation assays, both in vitro and in vivo, with chemicals, recombinant growth factors, and conditioned medium (2013 Ency).

These stem cells do not form teratomas when placed in animal models in vivo (attached files, 2004, 2014). Indeed, unless the tissue has been previously damaged in some way the endogenous pluripotent stem cells remain in a quiescent state. If the tissue had been damaged the stem cells repair the damage, but stop when the tissue has been restored to its pre-damaged state. We have used genomically-labeled endogenous pluripotent stem cells to treat induced myocardial damage in animal models (2004 AR); induced Parkinson disease in an animal model (2005 J); and utilized autologous pluripotent stem cells in Parkinson disease in an IRB-approved human clinical trial (attached files, 2013, 2013).

It is relatively easy to induce the cells into selected cell types. The easiest method is to co-culture the pluripotent stem cells with the selected cell type you want them to become. We have performed this strategy with neurons, pancreatic islets and cardiac muscle, just to name a few. I would suggest using differentiated cells from a different species so you can identify which cell type is which in double labeling studies with species-specific antibodies.  One can also use media conditioned from the differentiated cell type if you do not want to worry about mixed culture systems. The differentiated cells secrete paracrine factors that induce stem cells to commit to progenitor cells of their particular tissue lineage and also secrete paracrine factors that accelerate phenotypic expression in tissue-committed progenitor cells to form differentiated cell types. 

Alternatively, one can use published cocktails of growth factors and cytokines to induce differentiation. We have done this using endogenous pluripotent stem cells with different published cocktails for our bone marrow reconstitution experiments (2004) and pancreatic islet experiments (2005).

1 Generally, pluripotent stem cells have capacity to generate all cell types of the body whereas adult stem cels are restricted to generating cells of a particular tissue. Embryonic stem cells out of context, e.g. injected under kidney capsules, will form teratomas containing cell types from all three germ layers. Since adult stem cells are  more restricted in their differentiation capacities, they would be less likely to be out of context if injected into a tissue of the body.

2 To produce many different cell types from embryonic stem cells is relatively easy. To produce simply one cell type alone is much more tricky. While the correct combination of molecules and cytokines might be added during a differentiation procedure, if the cells you are trying to manufacture are not among other cell types which they normally would be within the body, then they are mssing their natural niche from which they are normally effeciently generated.