MadSci Network: Development

Re: In vitro fertilization of a oocyte with the nuclei from another oocyte...

Date: Wed Mar 21 12:42:21 2001
Posted By: Paul Szauter, Staff, Mouse Genome Informatics, The Jackson Laboratory
Area of science: Development
ID: 982192084.Dv

As far as I can tell, no one has done the experiment that you describe. 
There are at least two reasons for this.

First, there is a technical issue, because of the unusual biology of mammals 
with respect to egg development. The mature mammalian egg is arrested at 
metaphase of meiosis II until fertilization. Following fertilization, the 
second meiotic division is completed, and one of the resulting haploid 
nuclei is cast off as the second polar body. So until fertilization, there 
is no haploid egg nucleus that could be used as a source in the experiment 
that you describe.

Please see 'Molecular Biology of the Cell' by Alberts et al., or any other 
comprehensive cell biology textbook, for a good description of the process 
with figures. You can see Alberts online, although it is usually slow:

Second, even if we were to rescue such a haploid egg nucleus to substitute 
for the haploid sperm nucleus, the two nuclei must undergo karyogamy (or 
nuclear fusion) to make a diploid zygotic nucleus. It is not clear that two 
egg nuclei can do this. There might be some way to work this out 
technically, but the scientific purpose behind these experiments has been 
addressed by other means.

Recall that in cloning experiments, the egg nucleus is removed, and a 
diploid nucleus from another cell is transferred. Cloning experiments 
address the question: are nuclei other than newly formed zygotic nuclei able 
to undergo development into normal adults? The answer is clearly yes.

The much more interesting question that you have asked is whether there is 
anything lacking in the genetic material of an egg that is necessary for 
development. This question has been addressed by other means.

At first glance, the answer to this question would appear to be no, based on 
the fundamental principles of genetics. Half of the genes of an individual 
come from each parent. Confining our discussion to mammals, the only thing 
that an egg lacks is the Y chromosome; if the sperm carries one, the zygote 
is male, while if the sperm carries an X, as the egg does, the zygote is 
female. So for a female zygote, the two chromosome complements from each 
parent appear to be identical in terms of their genetic content. The 
mitochondrial genome comes from the egg, of course, because the sperm 
carries virtually no cytoplasm.

However, it has become clear that there are epigenetic modifications to the 
genome that make the maternal and paternal genomes nonequivalent. The 
epigenetic modifications are called 'imprinting'. Imprinting is epigenetic 
because the actual nucleotide sequence of genes is not changed, but there 
are modifications to the genetic material that are heritable; principally 

Early evidence for imprinting appeared from unusual cases in which a 
particular set of genes was not inherited from one parent. There is a human 
disease called Prader-Willi Syndrome, which is caused when a particular 
chromosomal deletion on chromosome 15 is inherited from the father. The 
identical deletion inherited from the mother causes a different syndrome. 
See the OMIM entry at:

Even more unusual are those cases of Prader-Willi Syndrome in which the 
affected individuals are euploid, but have inherited both copies of 
chromosome 15 from the mother and no chromosome 15 through the father. In 
this case, there are no missing genes, extra genes, or mutated genes, yet 
the individuals have a disease identical to that caused by inheriting a 
chromosome 15 deletion from the father.

A limited set of genes is known to be imprinted in mice. You can find out 
more at this site:


Here is a good paper on the acquisition of developmental competence by the 
oocyte nucleus that involves nuclear transplantation. The issue of 
imprinting is raised:

Bao S, Obata Y, Carroll J, Domeki I, Kono T (2000): Epigenetic modifications 
necessary for normal development are established during oocyte growth in 
mice. Biol Reprod 62:616-621.

You might also find the MGI Glossary useful in reading from these sources, 
or even reading this answer:


Good luck, and thank you for an interesting question.


Paul Szauter
Mouse Genome Informatics

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