On Sun, 8 Nov 1998, Josh Glasstetter wrote:
> does anyone understand how a human embryo can be cryopreserved, revived,
> inserted into a uterus, and then develop naturally? it seems to me that if
> cryopreservation is so terribly destructive to human cells that
> cryopreservation of embryos would be impossible...
Not all mammal embryos can be cryopreserved 'as is' (pig embryos require a protocol which causes a temporary breakdown of the cytoskeleton), human are one of the hardy ones. IIRC, the embryos are frozen at a few cell stage, where the embryo is microscopically small (and so subthreshold to certain damage mechanisms) and the cells are still undifferentiated. At this stage a considerable fraction of the cells can die, the remaining ones still being able to compensate. Even so, a fair fraction of cryopreserved embryos is nonviable.
If you freeze a macroscopic adult organ (say, a human brain) the amount of damage caused by currently utilized protocols which are perfectly okay for embryos is unacceptable, probably even if we consider structural conservation as the only criterion. Certain organs (brain, liver) are many times more fragile than others (heart, kidney).
Much research is still needed to classify the kind and quantify the amount of damage occuring, so that better protocols can be devised. Basically the only accepted validation criterion of cryopreservation protocols is tissue viability, which makes it doubly difficult (you subject the tissue/organ to additional damage during thawing/devitrification, and viability can be difficult to measure).