In a paper recently published in Human Reproduction, Prof Bruce Campbell and his team at the School of Medicine, Nottingham set out some exciting results on the restoration of fertility - the cryopreservation and transplantation of whole adult sheep ovaries. This is the first time that a whole organ from an adult large mammal has been successfully frozen, successfully transplanted and successfully produced immediate results.
Premature ovarian failure affects 1-2% of all women and is due to a number of causes such as genetic predisposition and cancer treatment - and this loss has serious clinical and psychological effects. Using a new method, a Planer controlled rate freezer and different post operative regimes, Prof Campbell restarted 100% of the ovarian function and produced high rates of natural fertility in his sheep: pregnancy rate 64%; live birth rate 29%.
Freezing and subsequent transplantation require successful cryo-preservation of both the ovary and its vascular supply. Work to date has not been so successful; five years ago a delivery was made, but 545 days after transplantation. Additionally the follicular survival rate in the grafted ovaries was just 1.7%-7.6%. The Nottingham work involved adult sheep, rather than lambs, where ovaries are more typical of human ovaries but much harder to freeze and was immediately successful.
A mature Greyface ewe with the 'results' of the transplant
What the Nottingham team discovered was that the duration of cryo-protectant perfusion was highly significant and that the degree of this penetration - and the maintenance of post-operative vascular viability - were the critical determinants of success.
Transplanting fragments of ovarian cortex for human fertility is a new procedure and has its problems - both practically and because these fragments contain only a fraction of a woman's ovarian reserve, and so can only provide the recipient with a relatively brief fertile window before the supply of oocytes contained within their graft is used up. The problem of longevity of the actual graft may be exacerbated by the freeze-thaw process and high rates of follicle loss occur. So, grafting pieces of ovarian cortex may have drawbacks as a fertility treatment with, for example, older patients in whom follicle density is already low. The cryogenic preservation of the whole ovary, complete with vascular pedicle, for later transplantation provides a very promising alternative strategy.
All methods of tissue cryopreservation require the replacements of intra-cellular water with penetrating cryo-protective agents. During organ preservation this has usually been tried by vascular perfusion. The new Nottingham technique, using Planer special slow rate freezing equipment, has the advantage of providing very short diffusion distances for the cryo-protectant to penetrate, but has the potential to cause microvascular damage. The cellular injury and loss caused by the formation of intra-cellular ice during cooling and warming can usually be avoided using controlled rate slow freezing methods, whereas the formation of ice in extra-cellular locations, and particularly in blood vessels, can produce serious injury and prevent organ function.
Before this technology can be used in human preservation the perfusion rates and the levels of anti-coagulant will need to be optimised for different sized ovaries and of course trials on the normality of offspring will be required.
This method holds great promise for the preservation of fertility in women. Additionally it could also perhaps be applied to the cryopreservation of other reproductive organs or even major ones such as kidneys where there are considerable difficulties in storing donated tissue.
Whilst small, the ovary is an organ, with multiple cell types and a separate blood supply. So far attempts to freeze and store whole organs such as the heart, liver and kidney have proved uniformly unsuccessful. The primary challenge with whole organ cryopreservation is that of preserving an intact vascular system and the Nottingham work, funded by the UK's Medical Research Council - is a major step in the right direction.