Re: How is Cross-Species Gestation acheived following In Vitro Fertilization

Dear Michael,

I must admit I'm a little confused by your question, I hope we're talking 
theoretically here! But let me give you a brief description of how assisted 
reproduction is achieved.

In vitro fertilisation (IVF) is the process of bringing a sperm and egg 
together outside of the body. Sperm is taken from the male and eggs are 
'harvested' from the female usually after they have ovulated. The eggs are 
taken from the woman's reproductive tract. As women only ovulate one egg at 
a time, they are often given medication which causes them to ovulate 
several eggs to increase the chances of one of them being fertilised.

Fertilisation of the egg takes place in a petri dish in a solution designed 
specifically for survival of both the eggs and sperm, this solution 
contains salts, proteins and amino acids, and is specific for the 
particular species because even at this early stage, different species of 
egg and sperm have different requirements. The petri dish is incubated for 
a set period (12 - 48 hours, again depending on the species) in an 
incubator set to body temerature and with a particular mixture of gases 
(oxygen, carbon dioxide and nitrogen). After this time the egg is washed 
and transferred to a new solution which will provide for its new 
requirements. Successful fertilisation is assessed by viewing the egg over 
the next few days, looking for cell divisions.

The fertilised egg, or embryo, is then put back into the reproductive tract 
of the female and basically you just hope that it attaches itself to the 
uterus wall. Attachment of the embryo to the uterus is called implantation. 
An interface between the mother and embryo is set up which allows transfer 
of all the nutrients which the embryo needs. As you can imagine, this is a 
very intimate attachment, embryonic cells grow right into the mother's 
tissue to make the placenta and blood supplies are connected. If the embryo 
doesn't produce the right signals, or if the mother doesn't produce the 
right signals, the embryo will not be able to implant and in these cases 
the mother may see the embryo as 'foreign', and reject it, like an organ 
transplant is often rejected.

Artificial insemination is the process where sperm is taken from the male 
and put into the female. So fertilisation occurs in her reproductive tract 
instead of in a petri dish.

Now, concerning your question about cross species gestation, I'm afraid I 
don't know of any examples where this has been done and as far as I know, 
it's not possible. I am proud to call myself a mad scientist, but not that 
mad!!! What you may have heard about is a few cases where people have put 
an embryo into the reproductive tract of another species for a few days and 
it has been able to survive, but implantation has never properly occurred. 
If someone is reading this who has heard of cases where cross species 
gestation has occurred I would be interested to hear about it.

Michael, I hope this information has been helpful and has answered your 
question.