| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Fundamental and Applied Cryobiology Group,2 Department of Biochemistry and Cellular and Molecular Biology, University of Tennessee, Knoxville, Tennessee 37932
Department of Biological Sciences,3 University of New Orleans, New Orleans, Louisiana 70148
Audubon Center for Research of Endangered Species,4 New Orleans, Louisiana 70131
Animal Reproduction and Biotechnology Laboratory,5 Colorado State University, Fort Collins, Colorado 80523
ABSTRACT
Molecular genetics and developmental biology have created thousands of new strains of laboratory animals, including rodents, Drosophila, and zebrafish. This process will accelerate. A decreasing fraction can be maintained as breeding colonies; hence, the others will be lost irretrievably unless their germplasm can be cryopreserved. Because of the increasingly critical role of cryopreservation, and because of wide differences in the success with which various forms of germplasm can be cryopreserved in various species, the National Institutes of Health National Center for Research Resources held a workshop on April 10–11, 2007, titled "Achieving High-Throughput Repositories for Biomedical Germplasm Preservation." The species of concern were mouse, rat, domestic swine, rhesus monkey, and zebrafish. Our review/commentary has several purposes. The first is to summarize the status of the cryopreservation of germplasm from these species as assessed in the workshop. The second is to discuss the nature of the major underlying problems when survivals are poor or highly variable and possible ways of addressing them. Third is to emphasize the importance of a balance between fundamental and applied research in the process. Finally, we assess and comment on the factors to be considered in transferring from a base of scientific information to maximally cost-effective processes for the preservation of this germplasm in repositories. With respect to the first purpose, we discuss the three methods of preservation in use: slow equilibrium freezing, rapid nonequilibrium vitrification, and the use of intracytoplasmic sperm injection to achieve fertilization with sperm rendered nonviable by other preservation treatments. With respect to the last purpose, we comment on and concur with the workshop's recommendations that cryopreservation largely be conducted by large, centralized repositories, and that both sperm (low front-end but high rederivation costs) and embryos (high front-end but modest rederivation costs) be preserved.
assisted reproductive technology, embryo, in vitro fertilization, ovum, sperm
With the advent of molecular genetics and molecular developmental biology, thousands of new mutant strains of mice, Drosophila, Medaka, and zebrafish have been created by knockout- and knockin-transgenic and other techniques. With mice, the number of strains is so great that it has become impossible in terms of cost and space to maintain more than a fraction of them as breeding colonies. Consequently, an increasing number and proportion of strains are being maintained by cryopreservation of their germplasm. In the present context, "germplasm" refers collectively to cells that, singly or in combination (primarily preimplantation embryos and sperm), lead to the development of offspring. For example, the Jackson Laboratory in Bar Harbor, Maine, has cryopreserved some 5500 unique strains. Recently, at the Oak Ridge National Laboratory in Tennessee, several hundred mutant strains of mice were moved from a dysfunctional old building into new quarters 10 miles away, not by transferring the animals, but by rederiving the strains in the pathogen-free new quarters from embryos and sperm cryopreserved in the old building. Unfortunately, although the need is just as great, procedures to cryopreserve the germplasm of Drosophila, zebrafish, rhesus monkey, and other laboratory research species are not yet in a satisfactory state.
Large as it is, the impact of cryopreservation of mutant strains of research animals is only a fraction of the total impact of cryopreservation. In agriculture, over 250 000 000 doses of semen from genetically superior bulls were cryopreserved world wide in 1998, and over 100 000 000 cows received their first insemination from that frozen semen [1]. Undoubtedly, this ability to use frozen sperm has been a substantial factor in the large increase in milk productivity per cow over the last several decades. In human reproductive medicine, it is now standard practice to cryopreserve all donor semen until tests demonstrate that it is free of HIV and other pathogens. In human in vitro fertilization, excess embryos resulting from the fertilization of oocytes collected from superovulated patients are routinely cryopreserved, thus materially reducing the costs of repeat procedures if needed, and reducing potential ethical, religious, and legal problems. In clinical medicine, cryopreserved marrow stem cells are used in the treatment of leukemia and other disorders, and umbilical cord stem cells are being cryopreserved with increasing frequency. Cryopreserved erythrocytes, lymphocytes, and platelets are used in specialized transfusions, including the treatment of melanoma.
The problem of cryopreserving mutant strains of laboratory research animals will rapidly become more acute. More than 76 000 articles on transgenic mice have been published since 1988. Some estimate that, in the next decade, over 5000 new strains of mice will be created yearly, about equal in 1 year to the number of mouse strains that the Jackson Laboratory has cryopreserved over the past 30 years. On the basis of these concerns, William Rall and colleagues at the Comparative Medicine Division of the National Institutes of Health's (NIH) National Center for Research Resources (NCRR) organized a workshop titled "Achieving High-Throughput Repositories for Biomedical Germplasm Preservation," held April 10–11, 2007. Its purpose was to assess the current state of the cryopreservation of certain important laboratory research species and to discuss areas of needed research and approaches for achieving high throughput. The workshop focused on mouse, rat, domestic swine, nonhuman primates (primarily, rhesus monkey), and aquarium fish (primarily, zebrafish). There were 14 speakers and about 70 attendees, including the three of us. All the attendees use cryopreservation. In addition, some six were specialists in fundamental cryobiology.
Prior to the workshop, we were commissioned by the NIH/NCRR organizers to prepare for publication a commentary/review on the workshop and on the underlying cryobiological science. We have five goals in this paper. The first is to provide documentation on the accelerating impact that cryopreservation has had to date in biological and medical research. The second is to review and comment on the major findings that emerged from the NIH/NCRR workshop with respect to the status of the cryopreservation of the germplasm of important research animals. The third purpose is to explore, in some depth, what we see as the major underlying cryobiological factors adversely affecting survival of some germplasm cells after freezing and thawing. Fourth, we point out how a balance of fundamental and applied cryobiological research has contributed to past success, and the strong likelihood that such a balance will be essential to the solution of current and future problems. Finally, we comment on some of the strategies and managerial decisions that will be needed to achieve high throughput (i.e., the cryopreservation of germplasm and the rederivation of offspring with maximal efficiency and cost effectiveness). Except where we specify otherwise, the comments we make on the workshop and the views and interpretations expressed on the underlying science are our own.
CURRENT STATUS OF THE CRYOPRESERVATION OF VARIOUS FORMS OF GERMPLASM FROM LABORATORY RESEARCH ANIMALS
Moderated by Kent Lloyd (University of California, Davis), the scientific program began with an assessment of the relative success of germplasm cryopreservation by species and type of cell or tissue. These assessments, as subsequently modified during the meeting, are shown in Table 1. The species are mouse, rat, swine, rhesus and other monkeys, zebrafish, and medaka. The cell or tissue types are spermatozoa, embryos, oocytes, ovarian and testicular tissue, and embryonic stem cells. The table and the meeting agenda did not include human, bovine, equine, or other livestock, and did not include insects like Drosophila. Research on the cryopreservation of these latter species was peripheral to the workshop.
|
The assessments of the participants fall into three groups. Group I (++++) encompasses species and tissue types for which cryopreservation is quite satisfactory. It is represented by mouse sperm and embryos and rat embryos. Group III (0) includes species and tissue types that cannot yet be cryopreserved at all—a major example being the zebrafish embryo. Group II (+, ++, +++) includes species and tissue types for which the ability to cryopreserve ranges from poor to fair in terms of functional survivals and reproducibility. Examples are rat sperm, mouse oocytes, pig sperm and embryos, rhesus sperm and embryos, and zebrafish sperm. Properly, no attempt was made at the workshop to quasiquantify these scores; such an attempt would have been difficult and of doubtful value.
Cryopreservation of mouse sperm and embryos is clearly at a state in which the combination of successful cryopreservation techniques and high demand calls for emphasis on achieving high throughput. In the Group III cell types, it is not a question of achieving high throughput, but of achieving any throughput. Overcoming this deficiency will require either a far better understanding of the fundamental bases of the problems, or extraordinary luck in finding empirical solutions. For group II cell types, there are various approaches. One is to accept that functional viabilities after freezing are only poor to fair, and compensate for that in practical use. A good example is boar sperm. Their motility after freezing and thawing is reasonably high, but their functional survival is only about 10% (i.e., it takes an 
10-fold-greater number of cryopreserved sperm to produce an equal number of piglets by artificial insemination [AI] as it does with fresh sperm). Moreover, cryopreserved semen from 10%–20% of boars does not yield acceptable fertility, even at very high sperm doses. The alternative approach is to develop procedures to convert poor survivals into good survivals. The question is: what is the proper balance between fundamental and empirical approaches?
A third dimension to Table 1 would have been useful at the workshop; namely, the degree of success as a function of the procedure used to preserve a given cell or tissue type. Current procedures fall into three broad categories: slow equilibrium freezing; rapid nonequilibrium vitrification; and, in the case of sperm, procedures that kill the cells but compensate by using intracytoplasmic sperm injection (ICSI) to achieve fertilization and offspring. Slow equilibrium freezing, in essence, means the use of the maximum cooling rate that will produce sufficient osmotic dehydration to keep the chemical potential of the water in a cell at near equilibrium with the chemical potential of the water in the partly frozen external medium. If the cooling rate is too high, the residual supercooled water in the cell will undergo intracellular ice formation (IIF), and the cell will be killed. If the cooling rate is too low, the cell may succumb to injury or death from so-called solution effects. Low molecular weight cryoprotective agents (CPAs) protect against solution-effect damage, but not against IIF. The critical cooling rate for different cells can vary 1000-fold (Fig. 1). This is chiefly because cells vary that much in their permeability to water, its activation energy, and in their surface-to-volume ratio—three vital factors that determine the rate at which supercooled water can leave a cell, and thus minimize the chance that it undergoes IIF. This approach of regarding a cell as a physical-chemical compartment (i.e., one containing an aqueous solution separated from an external aqueous solution by a membrane possessing certain permeabilities to water and CPA) dominated fundamental cryobiology for several decades beginning in 1963, and it continues to have a major influence today. This concept has been reviewed recently by Mazur [3], and is the approach that led directly to the successful cryopreservation of mouse embryos [4] and to the formulation of a two-factor hypothesis in 1970 [5]. This hypothesis states that cells cooled too rapidly are killed by IIF, and cells cooled too slowly may be killed by long exposure to concentrated solutions resulting from the progressive conversion of water to ice, and that the overall balance of the two forces commonly results in plots of survival vs. cooling rate having the shape of an inverted "U" (Fig. 1) [5].
|
Although an understanding of cells as physical-chemical objects has been both necessary and sufficient for successful cryopreservation of a number of cell types by slow equilibrium freezing (e.g., tissue culture cells, mouse embryos, and various types of blood cells), it has not been successful in other cell types from other species (i.e., the physical-chemical understanding is necessary, but it may not be sufficient). It has not been sufficient to cryopreserve complex mammalian tissues and organs, nor to preserve cells or cell aggregates that are extensively damaged by being cooled to temperatures near 0°C, a phenomenon referred to as chilling injury.
The second category of cryopreservation procedures is vitrification. It involves suspending cells in sufficiently high concentrations of mixtures of CPAs that, in combination with sufficiently high cooling and warming rates, prevent the cells and the surrounding medium from undergoing ice formation during cooling or warming. Instead, supercooled water is converted into a highly viscous glass during cooling (vitrification) and is kept in that state during subsequent warming (i.e., devitrification is prevented). Commonly, the CPAs used to vitrify cells include those used in slow equilibrium freezing, but in much higher concentrations. Rall and Fahy [6] and Rall [7] developed vitrification solutions for mouse embryos that yielded as good survivals as those achieved by slow freezing methods. But the real benefit of vitrification arises in cells that do not survive classical slow freezing either because they are killed by chilling injury (e.g., pig embryos [8]) or because IIF occurs at too high a temperature (e.g., zebrafish embryos [9]). Where it works, vitrification is also simpler and quicker than slow equilibrium freezing; however, the conditions for achieving it can be very demanding. The high concentrations of CPA required are near the maximums tolerated by cells, and small differences in cooling and warming rates can spell the difference between success and failure.
Achieving vitrification depends on a reciprocal relationship between the cooling and warming rate and the CPA concentration; the lower the concentration, the higher (by an exponential factor) the required cooling and warming rates [10]. At the meeting, John Critser (University of Missouri) described a device under development that is projected to produce rates of some 107 °C/min. If achievable, this device ought to permit the vitrification of cells containing low CPA concentrations that are easily tolerated. It should be noted that human spermatozoa have been successfully cryopreserved by vitrification without cryoprotectants [11, 12], and mouse pups have been produced from oocytes cryopreserved by vitrification in CPA solutions of <1 M [13].
The third method of preservation, applicable only to sperm, is in a sense a misnomer. It involves the preservation of sperm that have been rendered nonviable by procedures such as freeze drying or air drying, the lethality of which is probably due to the removal of cellular bound water needed to structurally stabilize some macromolecules. The sperm are dead in that they are nonmotile, nonmetabolizing, and have disrupted membranes. But although they are dead in every sense of the word, they can effect fertilization when injected directly into oocytes, a method called ICSI. The technique of ICSI evolved from observations of Uehara and Yanagimachi [14], who injected freeze-dried human spermatozoa into hamster oocytes and found that the sperm developed into pronuclei. Fifteen years ago, Palermo et al. [15] used fresh spermatozoa to produce the first human pregnancies by ICSI, a procedure that has now resulted in the birth of over a million children. In 1998, normal mice were produced by ICSI with freeze-dried, nonviable spermatozoa [16]. Recently, live rabbits [17] and rats [18] have been produced by ICSI of oocytes with freeze-dried spermatozoa. Mehmet Toner (Harvard University) illustrated that point at the workshop by reporting high fertilization rates from nonmotile mouse sperm that had been air dried at ambient temperature and then stored at subzero temperatures for weeks to months [19].
It is not clear whether only the chromosomal DNA needs to be injected, or whether other components in the sperm head, such as phospholipase C zeta, centrosomal material, or RNA, are also needed. Although these and other aspects of ICSI remain undefined, it is a broadly applicable method of achieving fertilization. For example, in addition to injection with freeze-dried sperm, the injection of fresh sperm into oocytes by ICSI has yielded live offspring in mice [20], rats [21], rabbits [22], pigs [23], rhesus monkeys [24], hamsters [25], and zebrafish [26], as well as sheep, goats, cattle, horses, and cats.
The above has been a brief survey of the status of germplasm cryopreservation as presented and discussed at the workshop, and a discussion of the three principal methods used to achieve cryopreservation. We turn now to some of the major underlying problems and questions, and to possible approaches to overcoming or answering some of them.
At first blush, it may appear surprising that sperm and embryos from animals in the same mammalian family can behave so differently with respect to cryopreservation. Embryos from the mouse and the rat can be readily preserved with high functional survival [27]. On the other hand, whereas cryopreserved sperm from the mouse have rather high functional survival, those from the rat fertilize oocytes only when inseminated directly into the uterus [28]. Thousands to millions of human embryos and sperm have been successfully cryopreserved, whereas very few cryopreserved embryos and sperm from rhesus macaque have yielded live infants [29].
The large differences between closely related species is especially surprising and intriguing in view of the fact that several critical cryobiological factors apply to a wide range of cell types, from yeast to erythrocytes to marrow stem cells to mammalian tissue culture cells, and to the sperm and embryos of many mammals [4, 5]. These include lethal IIF above a critical cooling rate (Fig. 1), and the need for permeating CPAs to protect against slow freezing injury, a protection that rises with increasing CPA concentration.
A peculiar feature of sperm cryopreservation is that there are often significant differences in the postthaw survival from different males of outbred species, such as the rhesus monkey [30], even when the sperm are frozen by identical methods. These male-to-male differences occur in the sperm of all mammalian species studied [31], and even with sperm of fish [32] and birds [33]. In the cattle industry, such male-to-male differences may be of considerable economic importance, since a bull of valuable genetic merit may produce sperm that do not survive freezing. If such a "poor freezer" is simply culled, his genetics are lost. In the case of outbred animals from other laboratory species, like pigs or monkeys, the loss of important genotypic characteristics may be critical.
There are two aspects to this variability—one practical and one scientific. On the practical side, some of this variation could represent a modest male-to-male shift in the optimum value of a cryobiological variable, such as the optimum cooling rate, the optimum concentration of CPA, or the permeability of the sperm to water or CPA, which in turn affects their osmotic response. In such cases, one might convert a "poor freezer" into a "good freezer" by reoptimizing the several cryobiological values. In practice, that would seem an undesirable tactic for a repository; a reoptimization could take several months for just a single line, and might not even be successful. A more logical strategy would be to try to develop protocols in which near optimal results are not sensitive to moderate variations in the important individual cryobiological variables, such as cooling rate.
On the scientific side is the question of what parameters are responsible for individual-to-individual variability with respect to cryopreservation. Answers to that question are important for their probable fundamental and practical relevance to cryopreservation. A priori, the responsible factors must be physiological, epigenetic, or genetic. An example of the physiological aspect is that semen from bulls that have had long abstinence from ejaculation often freezes more poorly than that from bulls that have ejaculated recently, leading one speaker to quote the cattlemen's vernacular about the "need to clean the rust out of the pipes."
Epigenetic aspects were discussed by George Seidel (Colorado State University). "Epigenetic" refers to modifications of the phenotype, primarily brought about by methylation of the DNA, or by alterations in the associated histones. An example brought up in the meeting of a developmental factor that some might consider epigenetic is that ejaculated sperm in some species are more chill sensitive than epididymal sperm [34–37]. Another example of epigenesis is the variability in mating ability of males within litters of inbred mice. These have identical genetics and were gestated and reared in the same environment; yet some mate more reliably than others, possibly due to chance differences in the sex of adjacent fetuses during gestation [38].
The third component of variability is genetic, and it is undoubtedly of major importance. The sperm from inbred strains of mice or hybrids of inbred strains display less individual-to-individual variability (good or poor) in cryopreservation than those from outbred strains, and there can be large differences among different inbred strains [39–42]. An interesting converse situation exists within noninbred males, in that the genotype of individual sperm varies enormously; yet they are essentially identical phenotypically because of the intercellular bridges formed during spermatogenesis in addition to the limited postmeiotic gene expression and other mechanisms [43]. Phenotypic identity, of course, is essential for Mendel's law of independent assortment to operate.
Such male-to-male differences occur in sperm from boars [44], and genetic analysis is beginning to be used to identify specific genes affecting cryopreservation. This approach ought to apply the power of genetics in general—molecular genetics in particular—to investigate cryobiological fundamentals at a molecular level.
An intriguing question is, what is controlled by the genome that becomes phenotypically expressed as "good freezing" or "poor freezing" sperm? Do single genes have large effects, as was reported recently by Sutter et al. [45], to explain much of the 200-fold differences in the weights of breeds of dogs? Or, as would appear more likely, are the cryobiologically relevant factors multigenic? Where in the sequential procedure, from collection, to freezing and thawing, to penetration and fertilization of the oocyte, to subsequent development to living young, does the phenotypic defect appear?
MEAN SURVIVALS VS. FREQUENCIES OF ZERO SURVIVAL
Survivals after cryopreservation are usually reported as mean percentages. Probably even more important is the fraction of samples yielding 0% survival. A mean survival of 30% achieved by averaging four survivals of 60% and four survivals of 0% should be a major warning flag. It means either that some variable in cryopreservation is at the edge of a precipice, or that some purposeful or inadvertent change in procedure has pushed the results over the precipice. Furthermore, an excessive number of zeros would cast serious doubts on the reliability of cryopreserved samples in a repository. As one example, in their early work on cryopreservation of Drosophila embryos, Mazur et al. obtained occasional high survival, but mostly zeros [46]. They were using 13-h embryos. When they subsequently tried 15 (± 0.5)-h embryos, the zeros disappeared and mean survivals jumped to 68%. In other words, in Drosophila there is a very narrow developmental window that permits cryopreservation. At the meeting, several examples were displayed showing large variations (80% to 0%) in fertility rates from the cryopreserved sperm of individual males. One example mentioned was zebrafish sperm (Terrence Tiersch, Louisiana State University), although these differences may be confounded by differences in the cooling rates used in different cryopreservation runs.
THE SPERMATOZOON: A CRYOBIOLOGICAL ENIGMA
Sperm were among the first vertebrate cells to be successfully cryopreserved in frog, rooster, bovine, and human. The success in the latter three species stemmed primarily from the serendipitous discovery in 1949 of the cryoprotective effect of glycerol [47]. What is rather remarkable is that the procedures developed quasiempirically nearly 60 years ago are essentially still used today [31, 48, 49]. The lack of change does not mean that the procedures are optimal; rather, it means that they yield or can be made to yield functional survivals high enough to be practical. By functional survival, we mean fertilized oocytes and, particularly, living young. Remarkably, with ICSI, sperm can be used to fertilize oocytes even when 100% are dead.
Sperm exhibit high species variability with respect to cold sensitivity. Immediately after ejaculation, boar sperm are completely inactivated by rapid chilling (thermal shock); however, with time after ejaculation, they become resistant to cold shock [50]. Human sperm are scarcely affected by rapid chilling, whereas sperm from some bulls (but not all) are very sensitive. One can actually obtain high functional survival with zero motility via ICSI, or one can have high motility and low fertilization rates (boar sperm). The functional survivals after cryopreservation can differ between epididymal and ejaculated sperm. Moreover, as discussed above, the survival of sperm after cryopreservation exhibits large male-to-male variability.
Even in their fundamental cryobiology, spermatozoa exhibit enigmatic behavior. Physical-chemical modeling based on the measured water permeability of mouse sperm at 
0°C predicts that they should only undergo lethal IIF and death when cooled at rates exceeding 2000°C/min [51]. The experimental observation is that they are killed when cooling rates exceed 200°C/min [52]. Using differential scanning calorimetry, Devireddy et al. [53, 54] have estimated water permeabilities at temperatures below 0°C for mouse and boar sperm and reported that they are far lower than most estimates made by extrapolating from measurements above 0°C. If one accepts the estimates of Devireddy et al., it brings the predicted and observed lethal cooling rates into agreement. However, if one accepts their value, it leads to the conclusion that there is a very large decrease in water permeability associated with initial freezing of the suspension, and such a large difference has not been noted in, for example, mouse oocytes and zygotes [55–57]. Another recent enigmatic finding is that of Morris [58]. Using cryoscanning electron microscopy and freeze substitution, he found no evidence for IIF in human sperm cooled as rapidly as 3000°C/min. Sperm, like all cells, must "obey" the laws of physics and physical chemistry. Consequently, IIF occurred during cooling but Morris was unable to detect it, the water in the sperm vitrified, or the water in the sperm is so tightly bound that it cannot form ice. The latter two hypotheses have their own sets of problems. The several discrepancies noted in this paragraph simply reflect the fact that the investigation of water permeability and IIF in the presence of external ice is experimentally and analytically complex. The discrepancies will be resolved sooner or later.
One important part of cryopreservation is that the addition and removal of CPAs produces osmotic changes in cell volumes, which, if excessive, can be damaging. As reviewed by J. Critser during the workshop, the osmotic tolerance limits of sperm are rather narrow. Based on measured permeability coefficients for water and CPAs, Gao et al. [59] have designed protocols for the step-wise addition and removal of CPAs that keep volume excursions within the tolerated range.
There is an interesting anomaly with respect to the osmotic tolerance limits of sperm. Although at room temperature they are rendered irreversibly immotile by exposure to, say, 1-osmolal solutions, during freezing the sperm are exposed to more than 20-times such osmolalities and yet survive. The anomaly applies to other cell types as well.
A fundamental tenet in cryobiology is that, to be protective, a CPA must be able to permeate the cell. However, the best success in the cryopreservation of mouse sperm has been achieved suspending them in raffinose/skim milk solutions [60] or in raffinose solutions alone [42, 52, 61]. The trisaccharide raffinose cannot permeate cells, so the basis of its protective effect is not understood. Sperm are very sensitive to reactive oxygen species (usually in the form of free radicals), and survival after freezing can be substantially improved by rendering the medium anoxic with an Escherichia coli membrane preparation [61, 62], or by introducing antioxidizing agents into the medium (Robert Taft, the Jackson Laboratory).
Explanations exist for all these enigmatic findings—we just do not know them. The question from the viewpoint of germplasm preservation is, is it important to discover them, or will it suffice to accept the limitations of the current empirically derived knowledge?
CRYOPRESERVATION OF THE EMBRYO: AN INTERPLAY BETWEEN FUNDAMENTAL AND EMPIRICAL CRYOBIOLOGY
In the decade following the successful cryopreservation of mammalian sperm in 1949, a number of attempts were made to cryopreserve mammalian preimplantation zygotes and embryos. All failed. The highest reported survival based on cleavage was 1% for rabbit zygotes frozen slowly in 15% glycerol [63]. In 1971, Whittingham switched to polyvinylpyrrolidone (PVP) as the cryoprotectant, and reported that, using rapid cooling at 60°C/min, he was able to obtain survival of 8-cell mouse embryos cooled to –79°C and held 30 min [64]. However, a year later, Whittingham et al. [4], and subsequently others [65, 66], were unable to repeat these findings.
In the preceding decade, between 1963 and 1971, a degree of mechanistic understanding of cryobiological injury had emerged. In 1963, Mazur [67] published a mathematical model based on physical-chemical considerations showing that the probability of IIF in a cell depended on the degree to which it underwent osmotic dehydration during cooling, and that in turn depended on the water permeability of the cell, the temperature coefficient or activation energy of that water permeability, and the surface:volume ratio of the cell. Subsequent experiments with yeast and human red blood cells led to the so-called two-factor hypothesis [68]; namely, that cells cooled too rapidly are killed by IIF and cells cooled too slowly are killed by solution-effect injury. The consequence of the interaction of these two factors is that plots of survival vs. cooling rate have the shape of an inverted "U." An example of solution effects is the increase in salt concentration by ice formation that had been quantitatively described by Lovelock [69] 12 years previously. Lovelock [70] had also shown that protection against freezing damage by low molecular weight compounds, such as glycerol or dimethyl sulfoxide, was due to their ability to colligatively lower the salt concentration at a given subzero temperature. Then, in 1970–1971, experimental observations showed that plots of survival of mouse marrow stem cells [71] and hamster V79 cells [72, 73], as a function of cooling rate, also exhibited an inverted U, with highest survivals at an intermediate, optimum rate. These data provided the first direct experimental support for the applicability of the two-factor hypothesis to nucleated mammalian cells.
Whittingham's 1971 published procedure [64] for mouse embryos was not consistent with these concepts. Physical-chemical modeling indicated that, to avoid IIF, an assemblage of cells of the size and presumed permeability properties of mouse embryos would have to be cooled at <1°C/min, not 60°C/min [67]. The theory underlying protection from slow freezing solution-effect injury argued that PVP would be an ineffective CPA because of its high molecular weight and consequent impermeability. When Whittingham et al. [4] redesigned the protocol for mouse embryos with these fundamentals in mind, they immediately obtained high survivals.
For a number of years, the mouse embryo cryopreservation procedure published by Whittingham et al. [4] served as the standard for laboratories such as the Jackson Laboratory. In 1977, Willadsen [74] introduced a modification based on experiments with bovine and ovine morulae; namely, rather than cooling the embryos slowly to –70°C, he cooled them slowly to –36° (0.3°C/min to –30°C, then 0.1°C/min to –36°C), followed by a plunge in liquid nitrogen. That modification was entirely compatible with and explicable by the underlying fundamentals. Slow cooling to –36°C was sufficient to cause most of the intracellular freezable water to flow out of the cell and freeze externally; consequently, little intracellular ice formed during the subsequent plunge into liquid nitrogen. A small amount of ice or glass apparently does form during the plunge, for the Willadsen procedure generally requires that the embryos be warmed rapidly. Rapid warming minimizes devitrification and minimizes recrystallization of small, pre-existing ice crystals. As Leibo and Songsasen have reported [75], and as Leibo pointed out in the workshop, Willadsen's now standard procedure has been used to successfully cryopreserve embryos from some 22 mammalian species. In the case of cattle, sheep, mice, and humans, more than a million offspring have developed from embryos cryopreserved by this "simple" method.
From this success, one might conclude that the problem of embryo cryopreservation has been solved for all species; but such is not the case. The standard procedure, derived from cryobiological fundamentals, was and still is unsuccessful in a number of cases, and these cases are important in germplasm preservation. They include embryos of swine (with a few exceptions), fish, and insects. In each of these species, the underlying fundamentals are not wrong, but they are overlaid by other injurious factors or blockades. Perhaps the most important of these "other factors" is chilling injury. If an embryo or oocyte is highly chill sensitive, the low cooling rate required to prevent IIF may produce such long exposure times to low temperature as to produce major chilling injury. That conflict appears to apply to embryos of the pig, zebrafish, and Drosophila, and to oocytes of many species. The proven or suggested remedy in all these cases has been to "outrun" the chilling injury by using high cooling rates to pass through the damaging temperature zone so rapidly that there is no time for injury to occur. But since high cooling rates normally induce lethal IIF, IIF has to be prevented by introducing multimolar concentrations of CPA to induce vitrification of the cell water. That, in turn, can introduce serious osmotic and toxicity problems.
In the cell types of some species, there are other overlying, sometimes multiple, injurious factors. At ovulation, oocytes of most mammalian species are arrested in metaphase II of meiosis, and cooling them to temperatures near 0°C causes disassembly of microtubules and disruption of the meiotic spindle [76, 77]. Furthermore, the high concentrations of CPA required for vitrification appear to harden the zona pellucida, making it difficult or impossible for the sperm to penetrate the egg [78]. In zebrafish embryos, one major problem is that IIF occurs at just about the same temperature as external freezing [9]. Put another way, the water in the zebrafish embryo can tolerate very little supercooling before it freezes intracellularly. Without supercooling, the fundamental considerations do not apply, and the classical slow freezing approach will not succeed. Finally, the zebrafish embryo is compartmentalized, and the compartments possess significant permeability barriers to water and CPA [79].
This leaves us with a strategic question: will research on fundamentals lead to new sets of principles to explain and overcome some or all of these injurious overlying factors, principles that are probably unrelated to the physical chemical fundamentals applied previously? Or conversely, are empirical studies more likely to lead to solutions? If the answer to this pair of questions were known, then the problems would have already been solved. It is interesting in this regard that there was a 40-yr gap between the cryopreservation of bull and mouse sperm, where research proceeded empirically, but only a 1-yr gap between the cryopreservation of mouse and bovine embryos, where success in the former was based on fundamentals.
We cite one example that could conceivably be a new set of principles applied to chilling injury. There is a correlation between the amount of intracellular lipid or yolk that embryos contain and their susceptibility to chilling injury. For example, early cleavage stages of bovine and porcine embryos contain substantial amounts of internal lipid droplets and exhibit high sensitivity to chilling injury. Mouse and human embryos contain few apparent lipid droplets, and exhibit no chilling injury. Zebrafish embryos contain large amounts of yolk and exhibit high chilling sensitivity. When lipid droplets are removed from pig embryos by differential centrifugation and micropipetting [80], their chilling sensitivity is markedly reduced. When yolk is removed from zebrafish embryos by similar procedures, their chilling sensitivity is also substantially reduced [81]. What is the causal relationship between intracellular lipid and susceptibility to chilling injury? Does the same cause and effect operate with intraembryonic yolk in zebrafish, even though there is no apparent chemical relationship between yolk and lipid droplets? A favored hypothesis is that chilling injury arises because of lipid phase changes in the plasma membrane at low temperatures. If so, why should events in intracytoplasmic lipid droplets affect the properties of the plasma membrane?
As noted several times in this review, chilling injury can block successful cryopreservation of a number of cell types. The subject was discussed in some detail at the workshop by Stanley Leibo. By chilling injury, we mean damage or killing of cells by exposure to low temperatures in the absence of ice. Chilling injury, however, needs to be clearly separated into adverse affects of exposure to cold (cold injury) and adverse effects from a high rate of cooling (thermal or cold shock). The two differ in their phenomenology, and may well differ in their cause. Cold injury is injury brought about by exposure to temperatures below some threshold. The extent of injury is proportional to the exposure time, and the rate at which it develops usually increases with decreasing temperature. It, therefore, is usually ameliorated by rapid cooling. Thermal shock is injury brought about by high rates of cooling. It is ameliorated by cooling the cells very slowly. Chilling injury in Drosophila embryos is a clear example of the first category: cold injury [82]. Damage to boar spermatozoa is a good example of thermal shock. The boar sperm are rendered irreversibly immotile by being rapidly cooled from ambient temperatures to 0°C, but not by slow cooling over that range. Similarly, bovine sperm are irreversibly damaged by rapid cooling from ambient to 0° to –5°C. If they are cooled very slowly over this range, motility remains high (however, a holding period of 2 h at –5°C is actually required to obtain high motilities after subsequent cryopreservation).
Cold injury and thermal shock may well differ mechanistically. Cold injury is usually ascribed to phase changes in membrane lipids. That is possibly why adding cholesterol via cyclodextrins [83] reduces the chilling sensitivity of bull and stallion sperm, as reported by J. Graham (Colorado State University). In addition, cold injury in oocytes is partly explained by the fact that tubulin, from which the meiotic spindle of oocytes is constructed, undergoes disassembly at temperatures near 0°C. But even this cannot be the full explanation, since the kinetics of chilling injury in germinal vesicle-stage oocytes, which do not contain a spindle, is identical to that in metaphase II-stage oocytes, which do contain a spindle [84].
In contrast, McGrath [85] has proposed a quantitative theory of cold shock based on thermal mechanical considerations. In brief, he points out that, during cooling, the plasma membrane lipid bilayer contracts thermally along its plane, thus diminishing its surface area. However, this force to diminish the surface area is opposed by the fact that the membrane surrounds an essentially incompressible volume of aqueous solution. That, in turn, generates a hydrostatic pressure that leads to tension in the plane of the membrane. Plasma membranes, however, can withstand only very small tensions before yielding or rupturing. Consequently, if no other compensating mechanism were present, that would be the outcome. But there is a compensating mechanism; namely, the increased hydrostatic pressure increases the activity of the intracellular water, creating an essentially osmotic or filtration force that tends to drive water out of the cell. The amount of water that can be driven out of the cell per unit time is dependent on water permeability and the cooling rate. If the water permeability is high and the cooling rate low, sufficient cell water will be forced out so that the thermally contracting membrane can enclose a decreased cell volume. On the other hand, if the water permeability is low or the cooling rate too high, cell water is not able to leave rapidly enough to prevent the buildup of tension in the membrane, and consequent membrane damage. This hypothesis is based on the membrane's being a smooth structure akin to the skin of a balloon, and it is interesting that the two cell types most susceptible to thermal shock—sperm and red blood cells—have smooth membranes. The surface of most other cells (including oocytes and embryos) is folded into pleats and microvilli. These constitute an excess membrane surface area that has been estimated to be 200% of the geometric smooth surface membrane. This excess membrane provides ample reserve to compensate for any decrease in area due to thermal contraction, leading to the conclusion that cells with pleats, folds, or microvilli in their membranes ought not to be subject to injury from thermal shock. To our knowledge, they are not.
HIGH THROUGHPUT FOR MOUSE GERMPLASM
A central aim of the NIH/NCRR sponsored workshop was stated in its title: "Achieving High-Throughput..." When dealing with this aspect, one is essentially changing one's viewpoint from that of the acquisition of the underlying science to the equally important matter of its applications, important components of which might aptly be referred to as process engineering combined with cost-benefit analysis. It is akin to making the transition from semiconductor and transistor theory and practice to the production of millions of central processing unit chips.
Several clear conclusions emerged from the presentations and discussions in the workshop. One was that only in the mouse is the demand high enough and cryopreservation procedures successful and mature enough to seriously require high throughputs at this time. Second, it became clearly evident that the superior strategy is that both the cryopreservation and the rederivation of functioning mouse germplasm be carried out in large central repositories. In other species, especially nonhuman primates and zebrafish, the need for cryopreservation is great, but one is scarcely in the position of obtaining any throughput, much less high throughput. Third, both sperm and embryos should be cryopreserved. Fourth, there should be more than one institution offering services for cryopreservation and rederivation.
The arguments in favor of cryopreservation and rederivation being carried out in central repositories rather than individual laboratories are persuasive:
NCRR currently supports six large repositories of cryopreserved mouse germplasm. Most of these (e.g., University of Missouri, University of California, Davis, University of North Carolina, Chapel Hill, and the Jackson Laboratory) now have systems and services for accepting outside mice, the germplasm of which is to be preserved, and, if necessary, the animals rederived. Still other mouse repositories are supported by other agencies in the United States and elsewhere.
To point out the obvious, a centralized cryopreservation facility separated geographically from the users of the animals is fine for small animals like mice, rats, and zebrafish. It has, shall we say, limitations with respect to the shipment of monkeys, pigs, elephants, and the like.
The third area of consensus at the workshop was the need to preserve both embryos and sperm. Sperm can be cryopreserved at considerably lower cost than embryos. Standard procedures almost always yield some survivors. However, if frozen embryos fail to yield progeny after thawing and implantation, the genome can, with reasonably high probability, be rederived from frozen-thawed sperm through AI or in vitro fertilization, or, if these fail, through ICSI. With mice, there is even a fourth option; namely, that the mutant line can be rederived from cryopreserved embryonic stem cells. These stem cells can be readily preserved with high functional survival. Not discussed in the workshop, but a possibility for the future, is the preservation and rederivation of strains from the cryopreservation of spermatogonial stem cells [86], which can also be readily cryopreserved by simple existing techniques.
The fourth consensus point, that there should be more than one central repository, is mostly a matter of establishing motivation to develop the most successful and cost-effective processes possible. It would also be advisable to store subsamples of cryopreserved materials in at least two repositories, even when each has subrepositories. At the Jackson Laboratory, the subrepository is located a day's drive distant from Bar Harbor.
The workshop participants frequently referred to front-end and back-end aspects and costs of cryopreservation. The front-end costs include the maintenance of the donor animals, the acquisition of samples, and the suspension of embryos or sperm in appropriate media, their freezing, and their storage in the frozen state. The back-end costs refer to those involved with thawing, physical recovery of the cells, removal of the CPA, tests for viability and disease, and the rederivation of living young with the same genetic characteristics as the original animals. Spermatozoa are an example of a cell type with low front-end costs, but potentially very high back-end costs if many backcrosses have to be made to return to the initial homozygous state. Embryos have modest front-end costs and medium high back-end costs.
The achievement of high throughput involves two separate but somewhat interacting elements. One involves increasing the productivity of the processes of cryopreservation and rederivations. The second involves "managerial" decisions as to what goes into the processes. With respect to the first element, a number of items must be considered. An important one is a computerized inventory of cryopreserved samples and procedures. Bar codes affixed to straws are an existing approach. Devices for accelerating processes such as filling straws and carrying out the freezing were discussed by Brad Belstra of IMV Technologies and John Dobrinsky of the Minitube International Center for Biotechnology, and included a robotic system currently under development.
It should be emphasized, however, that major elements of cryopreservation and rederivation depend on human technical skills involved in surgery, micropipetting, micromanipulation, microscopy, and the like. It is highly dubious that many of these skills can be replaced by machines. Consequently, a major route to achieving higher throughput may be to maximize the skills and experience of the technicians.
The second element, managerial decisions, is more complex. They involve the following fact: a relatively low and unpredictable percentage of the cryopreserved germplasm will be rederived in a given year, and the rederivation of most will never be requested. John Critser reported in his talk that, at the Missouri mouse repository, there is only one request or less per year for rederivation from 90% of the cryopreserved strains. Experience at the Jackson Laboratory is similar. For example, last year, they received requests for the rederivation of 441 (8%) of their 5500 cryopreserved mouse strains. The strategy followed depends critically on the confidence limits that one imposes on the ability to rederive a strain from cryopreserved germplasm. If, for example, one imposes 99% confidence limits on cryopreserved embryos, then it will probably be mandatory to test the ability to rederive a representative of every strain preserved. This testing would effectively become part of the front-end costs, and, although it would add greatly to these costs, it provides a very high probability that banked embryos or sperm from a strain can yield living offspring. Indeed, the Jackson Laboratory does not consider a strain successfully banked unless and until live young have been produced from a sample of the cryopreserved germplasm.
Alternatively, one could do no front-end testing on cryopreserved samples, and instead rely on redundancy to achieve high confidence limits. By redundancy we mean that, if frozen-thawed embryos fail to yield viable offspring, one could then turn to rederivation via frozen-thawed sperm. If that fails, then one could apply ICSI. The advantage of this approach is that it would impose the costs only on the relatively small fraction of strains for which rederivation was actually required. Of course, these rederivation costs could be high. Critser stated that, if the strain were mutant in a number of genes, it might take as many as 10 backcross generations to rederive the important characteristics of the strain from sperm alone.
Another question related both to confidence limits and inventory is, how many samples of a line should be cryopreserved? The answer, of course, impacts the front-end costs. The magnitude of the impact depends on whether or not aliquots of the cryopreserved material are tested for their ability to yield viable offspring.
Similar considerations apply to other aspects of the process such as disease control. Leila Riley (University of Missouri) stated that 80%–90% of mice coming into the Missouri repository carry at least one disease. The chief one is parvovirus (65% of semen samples; 27% of embryos). Does one, therefore, assume that the embryos/sperm in all incoming mice are infected, and reduce the infectious load in all by washing or treatment with monoclonal antibodies? Doing so, of course, would add significantly to the front-end costs. Alternatively, or in addition, does one test the disease status of the thawed germplasm of those strains that are being rederived, a cost that will add materially to the back-end costs? Furthermore, what actions can be taken if the thawed material contains infectious agents? Although primarily intended to address disease transmission via gametes and embryos of the large domestic species (cattle, swine, sheep, goats), a manual prepared by the International Embryo Transfer Society [87] and sanctioned by several international agencies provides detailed instructions to reduce or prevent transmission of infectious agents. That manual makes the point that the zona pellucida protects embryos from microbial and viral diseases; consequently, embryos become the vehicle of choice for cryopreservation when diseases may be an issue.
Another important consideration is, who should decide on the managerial strategy? One option is that the two or several repositories should be or act as competing commercial entities, and that each determines its own strategies. The client would then choose depending on how they assess performance and cost. The alternative is that some external agency decides on the strategy and imposes it on the commercial entities. The latter is probably the situation with respect to producers of, say, flu vaccine, but we favor the former. There is also the possibility of some intermediate course of action.
Still another matter to consider is, how should developments to enhance high throughput be funded? Should it be solely by the repositories themselves, as in pharmaceutical development? Or should it be funded in whole or in part by a federal agency, such as NIH? If the latter, one option is individual or institution-initiated grants through NIH or other funding agencies. Another option is through federally funded small business initiatives. If the research at repositories is supported by publicly supported grants, we firmly believe that findings made under auspices of those grants should be in the public domain.
The above discussion of high throughput applies to cryopreserved mouse germplasm in central repositories. This still leaves a number of problems to resolve. First, there are likely to be individual laboratories that wish to cryopreserve the germplasm of their own mouse strains rather than use centralized repositories. In this case, it is important that there exist a manual that includes annotated, up-to-date cryopreservation procedures. We say "annotated" because a simple listing of procedures does not distinguish between critical steps and less stringent ones. For example, if mouse embryos or sperm have been vitrified, it is critical that those actually handling the straws understand that, until the final thawing for recovery, the straw contents must never be allowed to warm above about –130°C (the temperature at which glassy water crystallizes), as, for example, during inspection or transfer. Even a few seconds above –100°C can destroy the contents. On the other hand, if they have been preserved by classical slow freezing to –36°C to –65°C, and then stored in liquid nitrogen, a few hours at, for example, –70°C is not likely to damage them. We specify "an up-to-date manual" because we have seen too many cases where so-called standard methods are actually 20 yr out of date.
Second, as indicated, in most animal species used for research other than the mouse, the problem in germplasm cryopreservation is more a matter of achieving modest throughput than it is about achieving high throughput. The specific nature of the problem depends on the species. However, the need for cryopreservation in most of these species is as great as that in the mouse:
How then to proceed with the germplasm preservation of these various species? Since the need is great and urgent, there seems no alternative but to proceed with the currently available imperfect or flawed methods. However, in parallel, research clearly needs to go forward to reduce, eliminate, or bypass the imperfections and flaws. Whether this is done by supporting investigator-initiated research or by requests from funding agencies for applications in specific areas, and how these funding agencies should balance fundamental vs. empirical approaches, is a matter of judgment and the quality and innovativeness of individual proposals, as well as the degree of faith in the power of the scientific process.
Correspondence: 1Peter Mazur, 10515 Research Dr., Suite 300/10, Knoxville, TN 37932-2575. FAX: 865 974 8027; e-mail: pmazur{at}utk.edu
Received: 7 July 2007.
First decision: 30 July 2007.
Accepted: 26 September 2007.
REFERENCES
