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Oocyte cryopreservation is playing an increasingly important role in the field of human infertility treatment. The ability to store viable oocytes for later use has given many women the option to delay childbearing in order to pursue other ventures in life, without the concern of losing the opportunity to have a family. Furthermore, oocyte cryopreservation is very valuable for diseased patients who have to undergo treatments that may compromise fertility. Also, infertility patients who produce large numbers of oocytes during a retrieval cycle now have the option of storing some eggs prior to fertilization, thereby reducing the number of embryos that have to be managed. Lastly, oocyte cryopreservation enables egg donation programs that are independent of fresh donations, which makes it possible for numerous recipients to benefit from a single donor.Traditionally, slow freezing was the only method available for oocyte cryopreservation. However, recent years have shown that ultrarapid cooling of oocytes results in higher survival and developmental rates. Thus, vitrification is today's preferred method of oocyte cryopreservation and therefore the only technique described.In this chapter, we present two reliable methods of oocyte vitrification that have been in use for several years and that have been experimentally validated. Since no single vitrification method is clearly superior to the rest, other systems are also briefly described to give the reader options when deciding which methods to utilize in their practice.

Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models has provided the opportunity for research that may not have been possible with human material. Today, results of these studies still continue to form the basis of oocyte cryobiology. This review discusses these studies, especially the physiological impacts of cryopreservation on oocyte biology. It will also focus on the role that animal models have played in improvement strategies, validation before translating new techniques into the human model and the advances made in the human in IVF because of these animal models. Finally, existing investigations and their potential impact in other areas of research will be discussed. Until recently, success in oocyte cryopreservation has been very limited mainly due to poor understanding of the complex physiological processes that lead to cell damage during cryopreservation. In the past three decades, however, a wealth of information has been collected using various different animal models, which has led to development of new technologies and optimization of existing ones. The use of these models provided the opportunity for research that may not have been possible with human material. Today, animal models still continuously provide imperative data that facilitate further advancements in oocyte cryobiology. This review will focus on the physiological impacts, current improvement strategies and future applications of oocyte cryopreservation using animal models as they benefit not only human oocyte cryopreservation procedures, but also the human species through their usefulness in agriculture, medicine and conservation., (Copyright © 2011 Reproductive Healthcare Ltd. Published by Elsevier Ltd. All rights reserved.)

Embryo viability assessment is one of the most important and challenging tasks in IVF. Evaluation of embryo quality is critical when selecting the best embryo(s) to transfer or cryopreserve. Until recently, the only instrument used for embryo evaluation was the inverted light microscope, which provided information based on morphological characteristics. Developmental and morphological information gained from microscopic assessment have been positively associated with IVF outcomes, including pregnancy and implantation rates. However, based on general statistics, it is clear that IVF currently still results in relatively low pregnancy rates, while simultaneously being associated with relatively high multiple implantation rates. Only with novel embryo assessment and selection procedures would it be possible to improve these outcomes. Accordingly, it has been proposed that it is possible to test the culture environment of a developing embryo to gain valuable information regarding its viability. Different approaches have been used. These include the measurement of oxygen consumption by the embryo and testing of the soluble HLA-G in the environment, as it was proposed that secretion of HLA-G is associated with higher implantation rates. Amino acid turnover, which appears to be correlated to blastocyst development, can be measured as an indication of embryo viability. Other approaches, such as time-lapse video observation or cumulus cell gene expression analysis, may be used in the future to gain a broader understanding of embryo viability. Proteomics and metabolomics are also useful tools for assessment of embryo developmental potential. Results from recent studies on predicting embryo viability by analyzing the metabolome of different stage embryos are promising, as increases in pregnancy and implantation rates were obtained using the metabolomic profile for embryo selection. Several novel approaches are currently being developed to aid in viability assessment. These need to be evaluated in prospective clinical trials, while considering their practicality in the clinical laboratory., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

The oocytes of many invertebrate and non-mammalian vertebrate species are not only asymmetrical but also polar in the distribution of organelles, localized RNAs and proteins, and the oocyte polarity dictates the patterning of the future embryo. Polarily located within the oocytes of many species is the Balbiani body (Bb), which in Xenopus is known to be associated with the germinal granules responsible for the determination of germ cell fate. In contrast, in mammals, it is widely believed that the patterning of the embryo does not occur before implantation, and that oocytes are non-polar and symmetrical. Although the oocytes of many mammals, including mice and humans, contain Bbs, it remains unknown how and if the presence of Bbs relates to mouse oocyte and egg polarity. Using three-dimensional reconstruction of mouse neonatal oocytes, we showed that mouse early oocytes are both asymmetrical and transiently polar. In addition, the specifics of polarity in mouse oocytes are highly reminiscent of those in Xenopus early oocytes. Based on these findings, we conclude that the polarity of early oocytes imposed by the position of the centrioles at the cytoplasmic bridges is a fundamental and ancestral feature across the animal kingdom.

The production of cloned offspring by nuclear transfer (NT) of semen-derived somatic cells holds considerable potential for the incorporation of novel genes into endangered species populations. Because oocytes from endangered species are scarce, domestic species oocytes are often used as cytoplasts for interspecies NT. In the present study, epithelial cells isolated from eland semen were used for intergeneric transfer (IgNT) into enucleated bovine oocytes and compared with bovine NT embryos. Cleavage rates of bovine NT and eland IgNT embryos were similar (80 vs. 83%, respectively; p > 0.05); however, development to the morula and blastocyst stage was higher for bovine NT embryos (38 and 21%, respectively; p < 0.0001), than for eland IgNT embryos (0.5 and 0%, respectively). DNA synthesis was not observed in either bovine NT or eland IgNT cybrids before activation, but in 75 and 70% of bovine NT and eland igNT embryos, respectively, cell-cycle resumption was observed at 16 h postactivation (hpa). For eland IgNT embryos, 13% had > or = 8 cells at 84 hpa, while 32% of the bovine NT embryos had > or = 8 cells at the same interval. However, 100 and 66% of bovine NT and eland IgNT embryos, respectively, that had > or = 8 cells synthesized DNA. From these results we concluded that (1) semen-derived epithelial cell nuclei can interact and be transcriptionally controlled by bovine cytoplast, (2) the first cell-cycle occurred in IgNT embryos, (3) a high frequency of developmental arrest occurs before the eight-cell stage in IgNT embryos, and (4) IgNT embryos that progress through the early cleavage stage arrest can (a) synthesize DNA, (b) progress through subsequent cell cycles, and (c) may have the potential to develop further.