Re: Is the transfer of embryo's between different species possible?

Hi Michael!

What a great question!  Before I plunge into answering your question, I 
want to briefly review the very early steps of mammalian development.

The fertilized zygote is formed when the nuclei of the oocyte and sperm 
fuse with each other to form a single, diploid nucleus.  A series of 
regulated mitotic cell divisions ensues (a process known as cleavage) 
wherein the number of daughter cells, called blastomeres, increases while 
the size of the embryo, as a whole, does not change.  The first division 
results in two equal-sized blastomeres, the second produces four, and so 
on.  By the 32-cell stage, the embryo resembles a small mulberry, and is 
called a morula (from the Latin word morum, meaning mulberry).  The cells 
of the morula will give rise not only to the embryo proper, but also to 
"extraembryonic" structures, such as the placenta.  These structures are 
vital for embryonic survival though they aren't a part of the mature 
organism.  The blastomeres that will give rise to mainly embryonic 
structures begin to adhere to each other and form a compact mass, called 
the inner cell mass, on one side of the morula.  The remaining cells along 
the periphery are called the outer cell mass, or trophoblast, and will 
contribute exclusively to the placenta and other extraembryonic tissues.  
At this stage the embryo is called a blastocyst.

Okay, so back to your question….  The experiment you propose in your 
question (the transfer of a cat embryo into the uterus of a cow) is an 
example of interspecific embryo transfer.  Historically, interspecific 
pregnancies have been studied in order to gain an understanding of 
maternal-fetal interactions.  For example, the developing fetus can be 
considered a specialized "tissue graft" and these studies provide insight 
into how it escapes immunological rejection by the mother under normal 
circumstances.  From a practical standpoint, interspecific embryo transfer 
may offer a means for the preservation of endangered species.  

In general, interspecific pregnancies do not yield viable offspring, for 
several reasons. Failure has been attributed primarily to immune 
incompatibilities between the mother and fetus. Physiological problems are 
also believed to be responsible, including insufficient production of 
pregnancy-specific hormones and the failure of the placenta to form 
correctly.  A few models of interspecific pregnancy have produced healthy 
progeny, but these are usually between very closely related species.  These 
involved Bos taurus and Bos indicus, cattle; Bos gaurus and Bos taurus; and 
Ovis musimon and Ovis aries, sheep.  Several interspecific combinations 
have been studied in detail.  These include Mus musculus (the domesticated 
lab mouse) and Mus caroli (a wild mouse from Southeast Asia); Equus 
caballus (the domestic horse) and Equus asinus (the donkey); and Ovis aries 
(the domestic sheep) and Capra hircus (the domestic goat). 

A very creative way in which the interspecies incompatibilities between 
mother and fetus have been circumvented in some model systems is through 
making chimeric embryos.  A chimera results when cells of differing genetic 
makeups are used to create a single organism.  For example, (if you 
remember what I told you in the first paragraph!) if the inner cell mass of 
one species (let's say the lab mouse) was injected into the blastocyst of 
another (like the wild mouse), a chimera would result.  This chimera would 
give rise to an embryo with both wild mouse and lab mouse cells, but its 
extraembryonic tissues (such as the placenta) would only be composed of 
wild mouse cells.  When these chimeras are transferred to wild mouse host 
mothers, the frequency of viable offspring is much greater.  For the mouse 
model system, this demonstrated the importance of the trophoblast in the 
survival of interspecific embryo transfers.  Incidentally, the converse 
experiment (a chimera whose inner cell mass is mixed but whose trophoblast 
is made of lab mouse cells only and is transferred to a lab mouse host) 
does not result in increased viability!  This suggests that the rules for 
interspecies pregnancy are very complicated and should be interpreted with 
caution…

Similar chimeric experiments have been used successfully in the sheep-goat 
system.  Here, both types of chimeras have produced viable offspring, as 
long as the trophoblast and recipient mother were from the same species.  

Equine species seem to have different rules when it comes to interspecific 
pregnancy.  Donkey-in-horse (donkey embryo transferred to a domestic horse) 
and horse-in-donkey (horse embryo transferred to a donkey) pregnancies have 
produced viable offspring, though it seems that donkey-in-horse pregnancies 
are not usually as successful as horse-in-donkey pregnancies.  Grants' 
zebra (Equidae burchelli) is the common zebra that is used as a model for 
studying endangered zebra species.  Interestingly, Grants' zebra embryos 
have yielded healthy progeny when carried in horse hosts.  

Interspecific pregnancy is a very complicated subject.  Although the exact 
experiment you proposed, a cat-in-cow transfer, would probably be 
unsuccessful due to the phylogenetic distance between these two species, 
other transfers have been successful.  The trophoblast seems to play an 
important role in determining the outcome of the embryo transfer 
experiments, as does the immune system.

I hope my explanation helps!  Please feel free to email me with further 
questions.  A few references are listed below.

Nikki
(nmdavis@fas.harvard.edu)   

This is a review that I found in my literature search but this particular 
volume was missing from my library.  It may be worth reading:
Allen, W. R. et al. Interspecific and extraspecific pregnancies in equids: 
anything goes. 1997, Journal of Heredity 88(5):384-92.

These references are rather old, but I wasn't able to find anything else 
that was recent.  You might consider consulting someone well versed in 
immunology to find out more detailed information.
Anderson, G. B., Interspecific pregnancy: barriers and prospects. 1988, 
Biology of Reproduction 38(1):1-15.  (this is a very dense and technical 
review, but parts of it are worth reading)

Kraemer, D. C., Intra- and interspecific embryo transfer. 1983, Journal of 
Experimental Zoology 228(2):363-371.

Rossant, J., Croy, B. A., Clark, D. A., Chapman, V. M. Interspecific 
hybrids and chimeras in mice. 1983, Journal of Experimental Zoology 
228(2):223-233.