Starfish oocytes form intracellular ice at unusually high temperatures

On crystallization of water confined in liposomes and cryoprotective action of DMSO

In this work, the phase behavior of cryoprotective mixtures based on dimethyl sulfoxide (DMSO) mixed with a lipid bilayer consisting of dimyristoyl phosphatidylcholine (DMPC) was studied. This system represented a model of a biological cell and its membrane. The aim of the work was to clarify the origin of the cryoprotective action of low-concentrated mixtures (1-10 vol%) DMSO in water, representing mixtures used in cryopreservation in cell therapy. The combination of experimental techniques of differential scanning calorimetry (DSC) and positron annihilation lifetime spectroscopy (PALS) allowed a study of crystallization behavior of water confined in liposomes imitating the intracellular environment. The ability of liposomes to show the fundamental aspects of water phase behavior seen during freezing of biological cells was proved. The presence of an amorphous freeze-concentrated phase of DMSO in the frozen state was confirmed and its possible crystallization into the DMSO trihydrate and ice during thawing was demonstrated. Correlation between the critical temperature range for the loss of cell viability during slow thawing and the temperatures of freeze-concentrated phase crystallization was found. Based on this finding, possible mechanisms of DMSO cryoprotection are discussed with support brought by results for the studied model system. Quantification of the ice phase fraction in the frozen mixtures revealed that even low concentrations of DMSO can induce a considerable decrease in the amount of ice present.

Amphibian Assisted Reproductive Technologies: Moving from Technology to Application

Amphibians have experienced a catastrophic decline since the 1980s driven by disease, habitat loss, and impacts of invasive species and face ongoing threats from climate change. About 40% of extant amphibians are under threat of extinction and about 200 species have disappeared completely. Reproductive technologies and biobanking of cryopreserved materials offer technologies that could increase the efficiency and effectiveness of conservation programs involving management of captive breeding and wild populations through reduced costs, better genetic management and reduced risk of species extinctions. However, there are relatively few examples of applications of these technologies in practice in on-the-ground conservation programs, and no example that we know of where genetic diversity has been restored to a threatened amphibian species in captive breeding or in wild populations using cryopreserved genetic material. This gap in the application of technology to conservation programs needs to be addressed if assisted reproductive technologies (ARTs) and biobanking are to realise their potential in amphibian conservation. We review successful technologies including non-invasive gamete collection, IVF and sperm cryopreservation that work well enough to be applied to many current conservation programs. We consider new advances in technology (vitrification and laser warming) of cryopreservation of aquatic embryos of fish and some marine invertebrates that may help us to overcome factors limiting amphibian oocyte and embryo cryopreservation. Finally, we address two case studies that illustrate the urgent need and the opportunity to implement immediately ARTs, cryopreservation and biobanking to amphibian conservation. These are (1) managing the biosecurity (disease risk) of the frogs of New Guinea which are currently free of chytridiomycosis, but are at high risk (2) the Sehuencas water frog of Bolivia, which until recently had only one known surviving male.

Global diversity and phylogeny of the Asteroidea (Echinodermata)

Members of the Asteroidea (phylum Echinodermata), popularly known as starfish or sea stars, are ecologically important and diverse members of marine ecosystems in all of the world's oceans. We present a comprehensive overview of diversity and phylogeny as they have figured into the evolution of the Asteroidea from Paleozoic to the living fauna. Living post-Paleozoic asteroids, the Neoasteroidea, are morphologically separate from those in the Paleozoic. Early Paleozoic asteroid faunas were diverse and displayed morphology that foreshadowed later living taxa. Preservation presents significant difficulties, but fossil occurrence and current accounts suggests a diverse Paleozoic fauna, which underwent extinction around the Permian-Triassic interval was followed by re-diversification of at least one surviving lineage. Ongoing phylogenetic classification debates include the status of the Paxillosida and the Concentricycloidea. Fossil and molecular evidence has been and continues to be part of the ongoing evolution of asteroid phylogenetic research. The modern lineages of asteroids include the Valvatacea, the Forcipulatacea, the Spinlosida, and the Velatida. We present an overview of diversity in these taxa, as well as brief notes on broader significance, ecology, and functional morphology of each. Although much asteroid taxonomy is stable, many new taxa remain to be discovered with many new species currently awaiting description. The Goniasteridae is currently one of the most diverse families within the Asteroidea. New data from molecular phylogenetics and the advent of global biodiversity databases, such as the World Asteroidea Database (http://www.marinespecies.org/Asteroidea/) present important new springboards for understanding the global biodiversity and evolution of asteroids.

The temperature of intracellular ice formation in mouse oocytes vs. the unfrozen fraction at that temperature

We have previously reported [Cryobiology 51 (2005) 29-53] that intracellular ice formation (IIF) in mouse oocytes suspended in various concentrations of glycerol and ethylene glycol (EG) occurs at temperatures where the percentage of unfrozen water is about 6% and 12%, respectively, even though the IIF temperatures varied from -14 to -41 degrees C. However, because of the way the solutions were prepared, the concentrations of salt and glycerol or EG in that unfrozen fraction at IIF were also rather tightly grouped. The experiments reported in the present paper were designed to separate the effects of the unfrozen fraction at IIF from that of the solute concentration in the unfrozen fraction. This separation makes use of two facts. One is that the concentration of solutes in the residual liquid at a given subzero temperature is fixed regardless of their concentration in the initial unfrozen solution. However, second, the fraction unfrozen at a given temperature is dependent on the initial solute concentration. Experimentally, oocytes were suspended in solutions of glycerol/buffered saline and EG/buffered saline of varying total solute concentration with the restriction that the mass ratios of glycerol and EG to salts are held constant. The oocytes were then cooled rapidly enough (20 degrees C/min) to avoid significant osmotic shrinkage, and the temperature at which IIF occurred was noted. When this is done, we find, as previously that the fraction of water remaining unfrozen at the temperature of IIF remains nearly constant at 5-8% for both glycerol and EG even though the IIF temperatures vary from -14 to -50 degrees C. But unlike the previous results, the salt and CPA concentrations in the unfrozen fraction vary by a factor of three. The present procedure for preparing the solutions produces a potentially complicating factor; namely, the cell volumes vary substantially prior to freezing: substantially greater than isotonic in some solutions; substantially smaller in others. However, the data in toto demonstrate that cell volume is not a determining factor in the IIF temperature.

Extra- and intra-cellular ice formation in Stage I and II Xenopus laevis oocytes

We are currently investigating factors that influence intracellular ice formation (IIF) in mouse oocytes and oocytes of the frog Xenopus. A major reason for choosing these two species is that while their eggs normally do not possess aquaporin channels in their plasma membranes, these channels can be made to express. We wish to see whether IIF is affected by the presence of these channels. The present Xenopus study deals with control eggs not expressing aquaporins. The main factor studied has been the effect of a cryoprotective agent [ethylene glycol (EG) or glycerol] and its concentration. The general procedure was to (a) cool the oocytes on a cryostage to slightly below the temperatures at which extracellular ice formation occurs, (b) warm them to just below the melting point, and (c) then re-cool them to -50 degrees C at 10 degrees C/min. In the majority of cases, IIF occurs well into step (c), but a sizeable minority undergo IIF in steps (a) or (b). The former group we refer to as low-temperature flashers; the latter as high-temperature flashers. IIF is manifested as abrupt blackening of the egg, which we refer to as "flashing." Observations on the Linkam cryostage are restricted to Stage I and II oocytes, which have diameters of 200 300 microm. In the absence of a cryoprotective agent, that is in frog Ringers, the mean flash temperature for the low-temperature freezers is -11.4 degrees C, although a sizeable percentage flash at temperatures much closer to that of the EIF (-3.9 degrees C). When EG is present, the flash temperature for the low-temperatures freezers drops significantly to approximately -20 degrees C for EG concentrations ranging from 0.5 to 1.5 M. The presence of 1.5 M glycerol also substantially reduces the IIF temperature of the low-temperature freezers; namely, to -29 degrees C, but 0.5 and 1 M glycerol exert little or no effect. The IIF temperatures observed using the Linkam cryostage agree well with those estimated by calorimetry [F.W. Kleinhans, J.F. Guenther, D.M. Roberts, P. Mazur, Analysis of intracellular ice nucleation in Xenopus oocytes by differential scanning calorimetry, Cryobiology 52 (2006) 128-138]. The IIF temperatures in Xenopus are substantially higher than those observed in mouse oocytes [P. Mazur, S. Seki, I.L. Pinn, F.W. Kleinhans, K. Edashige, Extra- and intracellular ice formation in mouse oocytes, Cryobiology 51 (2005) 29-53]. Perhaps that is a reflection of their much larger size.

Analysis of intracellular ice nucleation in Xenopus oocytes by differential scanning calorimetry

Intracellular ice formation (IIF) plays a central role in cell damage during cryopreservation. We are investigating the factors which trigger IIF in Xenopus oocytes, with and without aquaporin water channels. Here, we report differential scanning calorimeter studies of Xenopus control oocytes which do not express aquaporins. Stage I to VI oocytes (which increase progressively in size) were investigated with emphasis on stage I and II because they are translucent and can also be studied under the cryomicroscope. Measurements were made in 1, 1.5, and 2M ethylene glycol (EG) in frog Ringers plus SnoMax. A multistep freezing protocol was used in which the samples were cooled until extracellular ice formation (EIF) occurred, partially remelted, slowly recooled through the EIF temperature, and then rapidly (10 degrees C/min) cooled. EIF in the 1, 1.5, and 2M EG occurred at -6.4, -7.8, and -8.9 degrees C, respectively. Freezing exotherms of individual stage I-VI oocytes were readily visible. A general trend was observed in which the IIF temperature of the early stage oocytes (I-III) was well below T(EIF) while the later stages (IV-VI) froze at temperatures much closer to T(EIF). Thus, in 1.5M EG, T(IIF) was -21.1, -25, and -26.6 degrees C in stages I-III, but was -17 and -8.5 degrees C for stage IV and V-VI. Concurrently, the percentage of oocytes in which IIF was observed fell dramatically from a high of 40 to 72% in early stages (I-III) to a low of only 7% in stage V-VI because, particularly in the later stages, IIF was hidden in the EIF exotherm. We conclude that early stage oocytes are a good model system in which to investigate modulators of IIF, but that late stage oocytes are damaged during EIF and infrequently supercool.

Coral larvae conservation: Physiology and reproduction

Coral species throughout the world's oceans are facing severe environmental pressures. We are interested in conserving coral larvae by means of cryopreservation, but little is known about their cellular physiology or cryobiology. These experiments examined cryoprotectant toxicity, dry weight, water and cryoprotectant permeability using cold and radiolabeled glycerol, spontaneous ice nucleation temperatures, chilling sensitivity, and settlement of coral larvae. Our two test species of coral larvae, Pocillopora damicornis (lace coral), and Fungia scutaria (mushroom coral) demonstrated a wide tolerance to cryoprotectants. Computer-aided morphometry determined that F. scutaria larvae were smaller than P. damicornis larvae. The average dry weight for P. damicornis was 24.5%, while that for F. scutaria was 17%, yielding osmotically inactive volumes (V(b)) of 0.22 and 0.15, respectively. The larvae from both species demonstrated radiolabeled glycerol uptake over time, suggesting they were permeable to the glycerol. Parameter fitting of the F. scutaria larvae data yielded a water permeability 2 microm/min/atm and a cryoprotectant permeability = 2.3 x 10(-4) cm/min while modeling indicated that glycerol reached 90% of final concentration in the larvae within 25 min. The spontaneous ice nucleation temperature for F. scutaria larvae in filtered seawater was -37.8+/-1.4 degrees C. However, when F. scutaria larvae were chilled from room temperature to -11 degrees C at various rates, they exhibited 100% mortality. When instantly cooled from room temperature to test temperatures, they showed damage below 10 degrees C. These data suggest that they are sensitive to both the rate of chilling and the absolute temperature, and indicate that vitrification may be the only means to successfully cryopreserve these organisms. Without prior cryopreservation, both species of coral settled under laboratory conditions.

Extra- and intracellular ice formation in mouse oocytes

The occurrence of intracellular ice formation (IIF) during freezing, or the lack there of, is the single most important factor determining whether or not cells survive cryopreservation. One important determinant of IIF is the temperature at which a supercooled cell nucleates. To avoid intracellular ice formation, the cell must be cooled slowly enough so that osmotic dehydration eliminates nearly all cell supercooling before reaching that temperature. This report is concerned with factors that determine the nucleation temperature in mouse oocytes. Chief among these is the concentration of cryoprotective additive (here, glycerol or ethylene glycol). The temperature for IIF decreases from -14 degrees C in buffered isotonic saline (PBS) to -41 degrees C in 1M glycerol/PBS and 1.5M ethylene glycol/PBS. The latter rapidly permeates the oocyte; the former does not. The initial extracellular freezing at -3.9 to -7.8 degrees C, depending on the CPA concentration, deforms the cell. In PBS that deformation often leads to IIF; in CPA it does not. The oocytes are surrounded by a zona pellucida. That structure appears to impede the growth of external ice through it, but not to block it. In most cases, IIF is characterized by an abrupt blackening or flashing during cooling. But in some cases, especially with dezonated oocytes, a pale brown veil abruptly forms during cooling followed by slower blackening during warming. Above -30 degrees C, flashing occurs in a fraction of a second. Below -30 degrees C, it commonly occurs much more slowly. We have observed instances where flashing is accompanied by the abrupt ejection of cytoplasm. During freezing, cells lie in unfrozen channels between the growing external ice. From phase diagram data, we have computed the fraction of water and solution that remains unfrozen at the observed flash temperatures and the concentrations of salt and CPA in those channels. The results are somewhat ambiguous as to which of these characteristics best correlates with IIF.

Cryopreservation of starfish oocytes

Research from many laboratories over the past several decades indicates that invertebrate oocytes and eggs are extraordinarily difficult to freeze. Since starfish oocytes, eggs, and embryos are an important cell and developmental biology model system, there is great interest to cryopreserve these cells. Previous starfish oocyte cryopreservation studies using slow cooling protocols revealed that these cells are highly sensitive to osmotic stress and form intracellular ice at very high sub-zero temperatures, suggesting that common freezing methodologies may not prove useful. We report here that a short exposure to 1.5 M Me2SO/1 M trehalose in hypotonic salt solution followed by ultra-rapid cooling to cryogenic temperatures allows starfish oocytes to be cryopreserved with the average survival rate of 34% when normalized to control oocytes that were exposed to CPA, but not frozen. On average, 51% of the oocytes in 77% of the batches of frozen oocytes underwent meiotic maturation in response to the starfish maturation hormone, 1-methyladenine. In one experiment, eggs developing from thawed oocytes were capable of being fertilized and two developed into embryos. These data suggests that successful cryopreservation of starfish oocytes is possible, but will need further refinement to increase the numbers of fully competent embryos.

High ice nucleation temperature of zebrafish embryos: slow-freezing is not an option

Although fish embryos have been used in a number of slow-freezing cryopreservation experiments, they have never been successfully cryopreserved. In part this is because little is known about whether ice forms within the embryo during the slow-freezing dehydration process. Therefore, we examined the temperature of intraembryonic ice formation (T(IIF)) and the temperature of extraembryonic ice formation (T(EIF)), using a cryomicroscope. We used both unmodified zebrafish embryos and those with water channels (aquaporin-3 or AQP3) inserted into their membranes to increase permeability to water and cryoprotectants, examined at 100% epiboly to the 6-somite stage. In these experiments we examined: (1) the spontaneous freezing of (external) solutions; (2) the spontaneous freezing of solutions containing embryos; (3) the effect of preloading the embryos with cryoprotectants on T(IIF); (4) whether preloading the embryos with cryoprotectant helps in survival after nucleating events in the solution; and (5) the damaging effects of extracellular nucleation events versus solution toxicity on the embryos. The solutes alone (embryo medium--EM, sucrose culture medium, 1 M propylene glycol in EM, and 1 M propylene glycol in a sucrose culture medium) froze at -14.9 +/- 1.1, -17.0 +/- 0.3, -17.8 +/- 1.0, and -17.7 +/- 1.4, respectively. There was no difference amongst these means (P > 0.05), thus adding cryoprotectant did not significantly lower the nucleation point. Adding embryos (preloaded with cryoprotectant or not) did not change the basic freezing characteristics of these solutes. In all these experiments, (T(EIF)) equaled (T(IIF)), and there was no difference in the freezing point of the solutions with or without the embryos (P > 0.05). Additionally, there was no difference in the freezing characteristics of embryos with and without aquaporins (P > 0.05). The formation of intraembryonic ice was lethal to the zebrafish embryos in all cases. But this lethal outcome was not related to solution injury effects, because 88-98% of embryos survived when exposed to a higher solute concentration with no ice present. Taken together, these data suggest that slow-freezing is not a suitable option for zebrafish embryos. The mechanism of this high temperature nucleation event in zebrafish embryos is still unknown.