Re: some questions about stem cells...

First, let's define what stem cells are and then address these issues as we go 
along...

Stems cells are "blank slates" and have the potential to develop into a variety of 
different types of cells that perform different functions in the body.  This process of 
developing into a specific cell type is called cellular differentiation; as cells go 
though this process they start to express specific sets of genes and making specific 
proteins that allow them to perform their specific function.  As stem cells start to 
divide, the daughter cells "choose" to either to remain a stem cell or to differentiate, 
or to develop into a specifc type of cell.  Once the choice is made to develop into a 
specific cell type, there is no turning back.  The cells become terminally 
differentiated and stop dividing.  These differentiated cells then perform their 
function until they die and are replaced by new cells that have also undergone the 
process of differentiation.  

In general, stem cells are of two classes, those that are totipotent and those that are 
pluripotent. These terms are used to describe stem cells in terms of their potential to 
develop into different types of cells.  

Embryonic stem cells are totipotent, that is they are stem cells capable of 
differentiating into every type cell in the body.  Each totipotent cell could divide and 
differentiate and give rise to any type of cell in a human being.  All cells within the 
early human embryo are totipotent until approximately the 16 cell stage 
(somehwere around 3-5 days after fertilization). This is the major reason embryonic 
stem cells are considered to have such great potential in medicine because they 
can be used for any tissue type in the body.

Pluripotent stem cells are those that can develop into a limited number of cell types 
of different tissues of the body.  These stem cells can undergo differentiation to form 
cells of specialized tissues like skin or blood cells, but are more limited than 
totipotent cells. These are often the stems cells that are referred to as adult stem 
cells.  Nonetheless, pluripotent stem cells are also of considerable potential use in 
medicine.  

Under appropriate growth conditions, most stem cells can continue to divide and 
reproduce more stem cells indefinitely without differentiation occurring. So, the 
lines of embryonic stem cells that exist today, could give rise to more stem cells for 
use later on, so getting embryonic stem cells is not such a big issue. However, one 
issue is the limited number of embryonic stem cell lines that exist. As you know, all 
humans are different genetically, so the embryonic stem cell lines are limited in 
terms of the genetic differences that might be important in some medical use; 
scientists don't know how important this may be. 

Remember that human stem cells are blank slates, but are still human cells. Using 
animal stem cells in humans, even if you replace the animal nucleus with a human 
nucleus does not alleviate the problem of animal cells being recognized as 
"foreign" to the human body.

It's possible that stem cells may one day used to treat diseases like schizophrenia 
or autism; however, not enough is known about the basic defects in these diseases 
for anyone to take a stem cell based approach to treatment.

Hope this helps answer some of your questions.