Re: What distinguishes a fertilised egg cell from say a skin cell

You are mostly correct in describing cloning by nuclear transfer: a somatic nucleus is transferred 
to an enucleated egg and development is initiated, leading to a complete organism. The success 
rate of this procedure is quite low in practice. To read more, search the Mad Sci archive under 
"Szauter" and "cloning."

You ask why a skin cell alone won't go on to make a whole organism by itself. Biologists describe 
skin cells as differentiated, because they show a specialization of function that is grossly 
apparent. A histologist could look at a skin cell and know that it was a skin cell. Prior to 
differentiation, the skin cell went through a process of determination, in which its developmental 
potential was restricted. The restriction of developmental potential is generally irreversible, 
although successful cloning shows that it can be done.

Determination and differentiation of cells generally does not involve any changes to the genome 
(except in the case of some cells in the mammalian immune system). Instead, the changes are in 
gene regulation. Skin cells express a particular set of genes, including some regulatory genes 
that "tell" the cell that it is a skin cell.

An oocyte, on the other hand, is a very complex cell that contains many specific protein and RNA 
molecules that prepare it for development. In some species (like Drosophila, the fruit fly) there is 
no gene expression at all early in development; the egg is filled with all the specific mRNAs and 
proteins necessary to carry out the early steps of development. Some of these molecules are 
localized to specific parts of the egg.

Can the skin cell be induced to behave the way an egg cell does? The easiest method is nuclear 
transfer to an enucleated oocyte. This subjects the nucleus of the skin cell to the regulatory 
proteins that are present in the oocyte cytoplasm. It is hard to imagine a set of procedures that 
could be used to change a differentiated cell into a different cell type that was not part of its 
normal developmental program, now or any time in the near future.

There are some excellent books covering developmental biology available online at:
 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books

Search using "differentiation" or "determination" to learn more.

Thank you for an interesting question.

Yours,

Paul Szauter
Mouse Genome Informatics