Re: Why can't they insert a general cell nucleus into a stem cell

The first point is to clarify that stem cells have variable growth 
potential. Embryonic stem cells can divide in culture indefinitely, adult 
stem cells such as mesenchymal stem cells or MSC can be cultured for short 
periods of time and others like hematopoeitic stem cells have limited 
potential to be expanded in culture.  Normal adult cells (not stem cells 
but differentiated types) have limited potential to grow in culture but 
can grow indefinitely once transformed or immortalized.  Based on their 
potential to divide and form other types of cells, these different kinds 
of cells can be heirarchically arranged with embryonic stem cells at the 
top and differentiated cells at the bottom.

The question then is can we make a stem cell with limited growth 
potential similar to an immortalized/transformed cell type that has the 
potential to divide in culture for long periods of time?  
The answer is yes and this can be done relatively easily using certain 
viruses, by altering cell cycle check points or by overexpressing the 
protein hTERT to maintain telomere length.  These processes however leads 
to abnormal cells like cancer cells.  This is not favorable for a cell 
type such as stem cells which holds promise as a cell therapy agent for 
curing various degenerative diseases.  It is very important therefore to 
maintain these cells normal.  The perfect method would be to reverse the 
heirarchy towards a cell type like embryonic stem cell.  

The idea of transferring the nucleus from one cell type to another is an 
excellent point.  This process called Somatic Cell Nuclear Transfer (SCNT) 
or therapeutic cloning is indeed being carried out where a nucleus from an 
adult tissue when injected in to an enucleated Oocyte can reprogram the 
nucleus of the adult tissue transforming it to be totipotent (capable of 
forming an entire organism) allowing the cells to develop into an embryo 
that can grow upto blastocyst.  The inner cell mass of the blastocyst can 
then be used to derive embryonic stem cells.  It is therefore possible to 
make an embryonic stem cell from a  adult cell, though it is  a highly 
skilled,  low efficiency process that is not very time or cost 
effective.   

Experiments are also being carried out to reprogram adult cell types to an 
embryonic stem cell type.  A Japanese group has recently identified four 
factors when used in combination can change an adult mouse fibroblast to 
cells similar to mouse embryonic stem cells.