Characterization of a major permeability barrier in the zebrafish embryo

Physiology and cryosensitivity of coral endosymbiotic algae (Symbiodinium)

Coral throughout the world are under threat. To save coral via cryopreservation methods, the Symbiodinium algae that live within many coral cells must also be considered. Coral juvenile must often take up these important cells from their surrounding water and when adult coral bleach, they lose their endosymbiotic algae and will die if they are not regained. The focus of this paper was to understand some of the cryo-physiology of the endosymbiotic algae, Symbiodinium, living within three species of Hawaiian coral, Fungia scutaria, Porites compressa and Pocillopora damicornis in Kaneohe Bay, Hawaii. Although cryopreservation of algae is common, the successful cryopreservation of these important coral endosymbionts is not common, and these species are often maintained in live serial cultures within stock centers worldwide. Freshly-extracted Symbiodinium were exposed to cryobiologically appropriate physiological stresses and their viability assessed with a Pulse Amplitude Fluorometer. Stresses included sensitivity to chilling temperatures, osmotic stress, and toxic effects of various concentrations and types of cryoprotectants (i.e., dimethyl sulfoxide, propylene glycol, glycerol and methanol). To determine the water and cryoprotectant permeabilities of Symbiodinium, uptake of radio-labeled glycerol and heavy water (D(2)O) were measured. The three different Symbiodinium subtypes studied demonstrated remarkable similarities in their morphology, sensitivity to cryoprotectants and permeability characteristics; however, they differed greatly in their sensitivity to hypo- and hyposmotic challenges and sensitivity to chilling, suggesting that standard slow freezing cryopreservation may not work well for all Symbiodinium. An appendix describes our H(2)O:D(2)O water exchange experiments and compares the diffusionally determined permeability with the two parameter model osmotic permeability.

Current status of sperm cryopreservation in biomedical research fish models: zebrafish, medaka, and Xiphophorus

Aquarium fishes are becoming increasingly important because of their value in biomedical research and the ornamental fish trade, and because many have become threatened or endangered in the wild. This review summarizes the current status of sperm cryopreservation in three fishes widely used in biomedical research: zebrafish, medaka, and live-bearing fishes of the genus Xiphophorus, and will focus on the needs and opportunities for future research and application of cryopreservation in aquarium fish. First, we summarize the basic biological characteristics regarding natural habitat, testis structure, spermatogenesis, sperm morphology, and sperm physiology. Second, we compare protocol development of sperm cryopreservation. Third, we emphasize the importance of artificial fertilization in sperm cryopreservation to evaluate the viability of thawed sperm. We conclude with a look to future research directions for sperm cryopreservation and the application of this technique in aquarium species.

Cryogenic preservation of embryos ofProchilodus lineatus(Valenciennes, 1836) (Characiforme; Prochilodontidae)

While the freezing techniques of mammal embryos have been providing promising results, the cryopreservation of teleostean eggs and embryos have remained unsuccessful up to now. Therefore, this work aimed to develop a procedure of cryogenic preservation of embryos ofProchilodus lineatusand to observe, at both structural and ultrastructural levels, the morphological alterations that took place after the application of freezing/thawing techniques. The embryos at the morula stage could not tolerate exposure to the cryoprotectants ethylene glycol monomethyl ether, propylene glycol monomethyl ether, methanol, dimethyl sulphoxide and propylene glycol, presenting 100% of mortality. Embryos at the 4- to 6-somites stage tolerated exposure to propylene glycol and dimethyl sulphoxide, and the results revealed no significant differences (α = 0.05) regarding survival from both treatments. None of the freezing, thawing and hydration protocols was effective on preserving embryo viability. The ultrastructural analyses of frozen and thawed embryos showed that cells from ectoderm, somites, notochord and endoderm were structurally intact, with well preserved nuclei and mitochondria. The yolk globules were able to tolerate the freezing process, but the yolk syncytial layer was unorganized, displaying an electron-dense and compacted appearance, collapsed reticules, nuclei with modified chromatin and ruptures on the plasmatic membrane at the contact zone with endoderm. It might be concluded that the procedures tested for freezing were unable to avoid the formation of intracellular ice crystals, leading to drastic morphological modifications and makingP. lineatusembryos unviable.

Cryoprotective effects of antifreeze proteins delivered into zebrafish embryos

Fish embryo cryopreservation, which is useful in aquaculture or biodiversity conservation, is still far from being achieved. Structural barriers reduce the entrance of cryoprotectants into embryo compartments. Previous studies demonstrated a better ability for freezing in Arctic species which naturally express antifreeze proteins (AFPs). In this study, AFPs were delivered in early zebrafish embryos by incubation in media containing protein. Their cryoprotective effects were then analyzed. Chilling sensitivity was evaluated at 4 degrees C and -10 degrees C. Survival rates significantly increased in embryos incorporating AFPI and kept at -10 degrees C. To analyze their effects on cryopreservation, 5-somite embryos were vitrified. Incorporation of AFPI reduced the percentage of embryos that collapsed at thawing (14.2% of AFPI-treated embryos and 48.9% of controls). Cellular damage caused by vitrification was assessed after thawing by cell dissociation and further analysis of cell survival in culture (SYBR-14/IP labeling). The percentage of viable cells at thawing ranged from 25 to 50%, considered incompatible with embryo development. Cells recovered from frozen-control embryos did not survive in culture. However, the incorporation of AFPs allowed survival similar to that of cells recovered from non-frozen embryos. Blastomere cryopreservation trials incorporating AFPI in the extender also demonstrated a significant increase in viability after freezing. Our findings demonstrated that delivery of AFPs into zebrafish embryos by incubation in media containing protein at early stages is a simple and harmless method that increases cryoprotection of the cellular compartment. This beneficial effect is also noticed in blastomeres, encouraging their use in further protocols for embryo cryopreservation.

Biophysics of zebrafish (Danio rerio) sperm

In the past two decades, laboratories around the world have produced thousands of mutant, transgenic, and wild-type zebrafish lines for biomedical research. Although slow-freezing cryopreservation of zebrafish sperm has been available for 30 years, current protocols lack standardization and yield inconsistent post-thaw fertilization rates. Cell cryopreservation cannot be improved without basic physiological knowledge, which was lacking for zebrafish sperm. The first goal was to define basic cryobiological values for wild-type zebrafish sperm and to evaluate how modern physiological methods could aid in developing improved cryopreservation protocols. Coulter counting methods measured an osmotically inactive water fraction (Vb) of 0.37+/-0.02 (SEM), an isosmotic cell volume (V(o)) of 12.1+/-0.2 microm(3) (SEM), a water permeability (L(p)) in 10% dimethyl sulfoxide of 0.021+/-0.001(SEM)microm/min/atm, and a cryoprotectant permeability (P(s)) of 0.10+/-0.01 (SEM)x10(-3)cm/min. Fourier transform infrared spectroscopy indicated that sperm membranes frozen without cryoprotectant showed damage and lipid reorganization, while those exposed to 10% glycerol demonstrated decreased lipid phase transition temperatures, which would stabilize the cells during cooling. The second goal was to determine the practicality and viability of shipping cooled zebrafish sperm overnight through the mail. Flow cytometry demonstrated that chilled fresh sperm can be maintained at 92% viability for 24h at 0 degrees C, suggesting that it can be shipped and exchanged between laboratories. Additional methods will be necessary to analyze and improve cryopreservation techniques and post-thaw fertility of zebrafish sperm. The present study is a first step to explore such techniques.

Group II intron-based gene targeting reactions in eukaryotes

BACKGROUND

Mobile group II introns insert site-specifically into DNA target sites by a mechanism termed retrohoming in which the excised intron RNA reverse splices into a DNA strand and is reverse transcribed by the intron-encoded protein. Retrohoming is mediated by a ribonucleoprotein particle that contains the intron-encoded protein and excised intron RNA, with target specificity determined largely by base pairing of the intron RNA to the DNA target sequence. This feature enabled the development of mobile group II introns into bacterial gene targeting vectors ("targetrons") with programmable target specificity. Thus far, however, efficient group II intron-based gene targeting reactions have not been demonstrated in eukaryotes.

METHODOLOGY/PRINCIPAL FINDINGS

By using a plasmid-based Xenopus laevis oocyte microinjection assay, we show that group II intron RNPs can integrate efficiently into target DNAs in a eukaryotic nucleus, but the reaction is limited by low Mg(2+) concentrations. By supplying additional Mg(2+), site-specific integration occurs in up to 38% of plasmid target sites. The integration products isolated from X. laevis nuclei are sensitive to restriction enzymes specific for double-stranded DNA, indicating second-strand synthesis via host enzymes. We also show that group II intron RNPs containing either lariat or linear intron RNA can introduce a double-strand break into a plasmid target site, thereby stimulating homologous recombination with a co-transformed DNA fragment at frequencies up to 4.8% of target sites. Chromatinization of the target DNA inhibits both types of targeting reactions, presumably by impeding RNP access. However, by using similar RNP microinjection methods, we show efficient Mg(2+)-dependent group II intron integration into plasmid target sites in zebrafish (Danio rerio) embryos and into plasmid and chromosomal target sites in Drosophila melanogster embryos, indicating that DNA replication can mitigate effects of chromatinization.

CONCLUSIONS/SIGNIFICANCE

Our results provide an experimental foundation for the development of group II intron-based gene targeting methods for higher organisms.

The Sensitivity and Reproducibility of the Zebrafish (Danio rerio) Embryo Test for the Screening of Waste Water Quality and for Testing the Toxicity of Chemicals

The sensitivity of the zebrafish embryo test, a test proposed for routine waste water control, was compared with the acute fish toxicity test, in the determination of six types of waste water and ten different chemicals. The waste water was sampled from the following industrial processes: paper and cardboard production, hide tanning, metal galvanisation, carcass treatment and utilisation, and sewage treatment. The chemicals tested were: dimethylacetamide, dimethylsulphoxide, cadmium chloride, cyclohexane, hydroquinone, mercuric chloride, nickel chloride, nonylphenol, resmethrin and sodium nitrite. For many of the test substances, the zebrafish embryo test and the acute fish toxicity test results showed high correlations. However, there were certain environmentally-relevant substances for which the results of the zebrafish embryo test and the acute fish toxicity test differed significantly, up to 10,000-fold (Hg2+ > 150fold difference; NO2- > 300-fold; Cd2+ > 200-fold; resmethrin > 10,000-fold). For the investigated waste water samples and chemicals, the survival rate of the zebrafish embryos showed high variations between different egg samples, within the range of the EC50 concentration. Subsequently, 5–6 parallel assays were deemed to be the appropriate number necessary for the precise evaluation of the toxicity of the test substances. Also, it was found that the sensitivities of different ontogenetic stages to chemical exposure differed greatly. During the first 12 hours after fertilisation (4-cell stage to the 5-somite stage), the embryos reacted most sensitively to test substance exposure, whereas the later ontogenetic stages showed only slight or no response, indicating that the test is most sensitive during the first 24 hours post-fertilisation.

Antifreeze protein gene expression in winter flounder pre-hatch embryos: implications for cryopreservation

Cryopreservation of fish embryos has proven to be an elusive goal. Two reasons for this lack of success are their high chilling sensitivity and the formation of ice crystals while in the frozen state or during the thawing process. Antifreeze proteins (AFP) that protect marine teleost fishes from freezing in subzero waters have been shown to be capable of inhibiting ice recrystallization and protecting cell membranes from cold induced damage. Therefore they have the potential to improve the success of embryo cryopreservation. A recent study demonstrated that vitrified winter flounder embryos continued to show developmental changes following thaw [V. Robles, E. Cabrita, G.L. Fletcher, M.A. Shears, M.J. King, M.P. Herráez, Vitrification assays with embryos from a cold tolerant sub-arctic fish species, Theriogenology 64 (2005) 1633-1646]. Since winter flounder produce AFP it was hypothesized that these proteins, if present in the embryos, could have contributed to this progressive step towards success. Winter flounder produce three species of type 1 AFP: a small liver type, a large "hyperactive" liver type and a skin type. This study was conducted to determine which, if any, of these AFP genes was being expressed in pre-hatch winter flounder embryos. There was no evidence of AFP activity in freshly fertilized embryos. However, low levels of AFP activity were found in embryos at 4, 8, and 11 days post-fertilization. Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses of the AFP mRNA isolated from the embryos revealed the expression of seven different skin type AFP genes that translated into four distinct AFP. Neither of the liver type AFP genes was expressed in the embryos.

Incorporation of antifreeze proteins into zebrafish embryos by a non-invasive method

The cryopreservation of fish embryos is a challenge because of their structure, with multiple compartments and permeability barriers, and their high chilling sensitivity. Vitrification at advanced developmental stages is considered to be the more promising option. Nevertheless, all reported attempts have failed. Previous studies demonstrated a better ability for freezing in species that naturally express antifreeze proteins (AFPs). These proteins have been delivered into other fish embryos using time-consuming techniques like microinjection. In the present study, the introduction of FITC labelled AFPs was assayed in zebrafish embryos at early developmental stages (from 2-cell to high blastula stage), before the formation of the yolk syncytial layer, by an easy and non-invasive method and evaluated by fluorescence and confocal microscopy. Incubation with AFPs at 128-cell or high blastula stage provides incorporation of the protein in 50-90% of embryos without affecting hatching. Incubation in media containing protein is a simple, harmless and effective method which makes it possible to treat several embryos at the same time. AFPs remain located in derivatives from marginal blastomeres: the yolk syncytial layer, the most cryosensitive and impermeable barrier, and different digestive organs. Our findings demonstrate that delivery of AFP type I and AFP type III into zebrafish embryos by incubation in media containing protein is a simple and harmless method that may improve cryoprotection of the cellular compartment.

An alternative method for delivering exogenous material into developing zebrafish embryos

Non-invasive manipulation of multicellular systems is important for medical and biological research. The ability to introduce, remove, or modify molecules in the intracellular environment is pivotal to our understanding of cellular structure and function. Herein, we report on an alternative method for introducing foreign material into developing embryos using the application of femtosecond (fs) laser pulses. When intense fs laser pulses are focused to a sub-micron spot, transient pores are formed, providing a transport pathway for the delivery of exogenous material into embryonic cells. In this study, zebrafish embryos were used as a model system to demonstrate the non-invasiveness of this applied delivery tool. Utilizing optically induced transient pores chorionated and dechorionated zebrafish embryos were successfully loaded with a fluorescent reporter molecule (fluorescein isothiocyanate), Streptavidin-conjugated quantum dots or DNA (Simian-CMV-EGFP). Pore formation was independent of the targeted location, with both blastomere-yolk interface and blastomere pores competent for delivery. Long-term survival of laser manipulated embryos to pec-fin stage was 89% and 100% for dechorionated and chorionated embryos, respectively. To our knowledge, this is the first report of DNA delivery into zebrafish embryos utilizing fs laser pulses.

Structural and ultrastructural characteristics of the yolk syncytial layer inProchilodus lineatus(Valenciennes, 1836) (Teleostei; Prochilodontidae)

The yolk syncytial layer (YSL) has been regarded as one of the main obstacles for a successful cryopreservation of fish embryos. The purpose of this study was to identify and characterize the YSL inProchilodus lineatus, a fish species found in southeastern Brazil and considered a very important fishery resource. Embryos were obtained through artificial breeding by hormonal induction. After fertilization, the eggs were incubated in vertical incubators with a controlled temperature (28 °C). Embryos were collected in several periods of development up to hatching and then fixed with 2% glutaraldehyde and 4% paraformaldehyde in 0.1 M sodium phosphate buffer (pH 7.3). Morphological analyses were carried out under either light, transmission or scanning electron microscopy. The formation of the YSL inP. lineatusembryos starts at the end of the cleavage stage (morula), mainly at the margin of the blastoderm, and develops along the embryo finally covering the entire yolk mass (late gastrula) and producing a distinct intermediate zone between the yolk and the endodermal cells. The YSL was characterized by the presence of microvilli on the contact region with the yolk endoderm. A cytoplasmic mass, full of mitochondria, vacuoles, ribosomes, endomembrane nets and euchromatic nuclei, indicated a high metabolic activity. This layer is shown as an interface between the yolk and the embryo cells that, besides sustaining and separating the yolk, acts as a structure that makes it available for the embryo. The structural analyses identified no possible barriers to cryoprotectant penetration.

The permeability to water and cryoprotectants of immature and mature oocytes in the zebrafish (Danio rerio)

To identify a stage feasible for the cryopreservation of zebrafish oocytes, we investigated the permeability to water and cryoprotectants of immature (stage III) and mature (stage V) oocytes. The permeability to water (microm/min/atm) of immature oocytes at 25 degrees C (0.37) was significantly higher than that of mature oocytes (0.10). The permeability (x10(-3)cm/min) of immature oocytes to ethylene glycol, propylene glycol, and Me(2)SO (1.49-3.03) at 25 degrees C was substantially higher than that of mature oocytes approximately 0. The permeability of immature oocytes to glycerol was also high (1.75), although the permeability could not be measured in mature oocytes. Immature oocytes would be more suitable than mature oocytes for conservation of the zebrafish.

Exogenous Expression of Rat Aquaporin-3 Enhances Permeability to Water and Cryoprotectants of Immature Oocytes in the Zebrafish (Danio rerio)

Movement of water and cryoprotectants through the plasma membrane needs to be accelerated for successful cryopreservation of zebrafish oocytes/embryos, which are much larger than their mammalian counterparts. Aquaporin-3 is a water/solute channel that can transport not only water but also various cryoprotectants. In this study, we attempted to increase the permeability of immature zebrafish oocytes at stage III to water and cryoprotectants by exogenous expression of rat aquaporin-3. Immature zebrafish oocytes were injected with rat aquaporin-3 cRNA and cultured for 5-12 h. Permeability to water and cryoprotectants was then determined based on changes in the volumes of the oocytes in a hypertonic sucrose solution and various cryoprotectant solutions at 25 C. The permeability to water of the aquaporin-3 cRNA-injected oocytes was three times higher than that of intact and water-injected oocytes. The permeability of the aquaporin-3 cRNA-injected oocytes to ethylene glycol, glycerol, propylene glycol, and DMSO was also 2-4 times higher than that of intact oocytes. Thus, the permeability of immature zebrafish oocytes to water and cryoprotectants was enhanced by exogenous expression of aquaporin-3. Cryopreservation of teleost oocytes may be realized through a further increase in permeability.

Expression of aquaporin-3 improves the permeability to water and cryoprotectants of immature oocytes in the medaka (Oryzias latipes)

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes and embryos. Several efforts have been made to facilitate the movement of water and cryoprotectants across the plasma membrane of fish oocytes/embryos because of their large size. Aquaporin-3 is a water/solute channel that can also transport various cryoprotectants. In this study, we tried to improve the permeability of immature medaka (Oryzias latipes) oocytes to water and cryoprotectants by artificially expressing aquaporin-3. The oocytes were injected with aquaporin-3 cRNA and cultured for 6-7 h. Then, hydraulic conductivity (L(P)) and cryoprotectant permeability (P(S)) were determined from volume changes in a hypertonic sucrose solution and various cryoprotectant solutions, respectively, at 25 degrees C. The L(P) value of the cRNA-injected oocytes was 0.22+/-0.04 microm/min/atm, nearly twice larger than that of intact or water-injected oocytes (0.14+/-0.02 and 0.14+/-0.03 microm/min/atm, respectively). P(S) values of intact oocytes for ethylene glycol, propylene glycol, and DMSO were 1.36+/-0.34, 1.97+/-0.20, and 1.17+/-0.52 x 10(-3) cm/min, respectively. The permeability to glycerol could not be calculated because oocytes remained shrunken in the glycerol solution. On the other hand, cRNA-injected oocytes had significantly higher P(S) values (glycerol, 2.20+/-1.29; ethylene glycol, 2.98+/-0.36; propylene glycol, 3.93+/-1.70; DMSO, 3.11+/-0.74 x 10(-3) cm/min) than intact oocytes. When cRNA-injected oocytes were cultured for 12-14 h, 51% matured to the metaphase II stage, and 43% of the matured oocytes were fertilized and hatched following in vitro fertilization and 14 days of culture. Thus, the permeability of medaka oocytes to water and cryoprotectants was improved by the artificial expression of aquaporin-3, and the oocytes retained the ability to develop to term.

Uptake and elimination of ethanol by young zebrafish embryos

Among animal models being explored to understand ethanol-induced teratogenesis, the zebrafish (Danio rerio) is attracting attention because its embryonic development is well characterized and readily visualized. Despite the potential of the zebrafish embryo in research on developmental anomalies produced by ethanol exposure, little is known about the relationship between embryonic ethanol content and the nature/severity of ethanol-mediated deficits. Here, using gas chromatography and radiometry of labeled ethanol carbon, we examine accumulation and clearance of ethanol by dechorionated zebrafish embryos during blastulation/gastrulation. Our data indicate that: (a) rates of uptake and loss of ethanol are directly proportional to the extra-/intra-embryonic ethanol concentration gradient and (b) ethanol in the water fraction of embryos reaches near equimolarity with ethanol in the exposure medium. It appears that, within a wide range of exposure concentrations, embryonic ethanol content can be predicted accurately according to exposure time. Furthermore, it appears that embryonic ethanol can be adjusted rapidly to and maintained at a targeted concentration.

Japanese flounder (Paralichthys olivaceus) embryos are difficult to cryopreserve by vitrification

The first successful cryopreservation of fish embryos was reported in the Japanese flounder by vitrification [Chen and Tian, Theriogenology, 63, 1207-1219, 2005]. Since very high concentrations of cryoprotectants are needed for vitrification and fish embryos have a large volume, Japanese flounder embryos must have low sensitivity to cryoprotectant toxicity and high permeability to water and cryoprotectants. So, we investigated the sensitivity and the permeability of Japanese flounder embryos. In addition, we assessed the survival of flounder embryos after vitrification with solutions containing methanol and propylene glycol, following Chen and Tian's report. The embryos were relatively insensitive to the toxicity of individual cryoprotectants at lower concentrations, especially methanol and propylene glycol as their report. Although their permeability to water and cryoprotectants could not be measured from volume changes in cryoprotectant solutions, the embryos appeared to be permeable to methanol but less permeable to DMSO, ethylene glycol, and propylene glycol. Although vitrification solutions containing methanol and propylene glycol, which were used in Chen and Tian's report, were toxic to embryos, a small proportion of embryos did survived. However, when vitrified with the vitrification solutions, no embryos survived after warming. The embryos became opaque during cooling with liquid nitrogen, indicating the formation of intracellular ice during cooling. When embryos had been kept in vitrification solutions for 60 min after being treated with the vitrification solution, some remained transparent during cooling, but became opaque during warming. This suggests that dehydration and/or permeation by cryoprotectants were insufficient for vitrification of the embryos even after they had been over-treated with the vitrification solutions. Thus, Chen and Tian's cryopreservation method lacks general application to Japanese flounder embryos.

Understanding dioxin developmental toxicity using the zebrafish model

Zebrafish (Danio rerio) have advantages over mammals as an animal model for investigating developmental toxicity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a persistent global contaminant, is the most comprehensively studied developmental toxicant in zebrafish. The hallmark responses of TCDD developmental toxicity manifested in zebrafish larvae include edema, anemia, hemorrhage, and ischemia associated with arrested growth and development. Heart and vasculature development and function are severely impaired, and jaw malformations occur secondary to inhibited chondrogenesis. The swim bladder fails to inflate, and the switch from embryonic to adult erythropoiesis is blocked. This profile of developmental toxicity responses, commonly referred to as "blue sac syndrome" because the edematous yolk sac appears blue, is observed in the larval form of all freshwater fish species exposed to TCDD at the embryonic stage of development. Components of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AHR/ARNT) signaling pathway in zebrafish have been identified and functionally characterized. Their role in mediating TCDD toxicity has been determined using morpholinos to specifically knockdown the translation of zfAHR1, zfAHR2, zfARNT1, and zfARNT2 mRNAs, respectively, and a line of zfARNT2 null mutant zebrafish has provided further insight. These studies have shown that zfAHR2 and zfARNT1 mediate TCDD developmental toxicity. In addition, the growing use of molecular and genomic tools for research on zebrafish have led to advances in our understanding of the mechanism of TCDD developmental toxicity at the molecular level, including the recent finding that toxicity is not mediated by increased cytochrome P4501A (zfCYP1A) expression.

Avian Genetic Resource Banking: Can Fish Embryos Yield Any Clues for Bird Embryos?

Cryopreservation of avian germplasm is becoming better understood and more commonly practiced. However, one area that would be of great benefit for genome resource banking is the preservation of avian embryos. Little is know about the cryobiology of avian embryos, and they have never been successfully cryopreserved. However, it is likely that they share many of the challenges of other yolk-filled multicompartmental embryos. For example, the fish embryo has 1) a large overall size, resulting in a low surface-to-volume ratio, which retards water and cryoprotectant efflux/influx; 2) large-sized cells, such as the yolk, which could increase the likelihood of membrane disruption by intracellular ice formation; 3) compartments, such as the blastoderm and yolk, with differing permeability properties; and 4) susceptibility to chilling injury. Both the avian and fish systems share many physical and anatomical properties, and it is predicted that some of the same permeability barriers would exist in both as well. Although the systems are similar, some of the goals, and thus the practices, to protect the genome may be quite different. One of these major goals in avian developmental biology is to produce chicken:chicken transgenic animals, especially those with germ line transmission. Producing efficient germ line transmissions and being able to cryopreserve these transmissions would be extremely beneficial to both basic and agricultural science. This could be accomplished through the cryopreservation of embryonic gonadal tissue followed by grafting into a host. The gonadal/tail-graft system would provide an advantage for cryopreservation because it is small (in comparison with the whole embryo), has fairly uniform tissue, and contains the essential primordial germ line cells capable of recreating the genetic line of interest. Moreover, because the chicken is such a robust model for most other avian species, the cryopreservation of the gonadal/tail-graft may potentially open up similar treatments for other commercially important species.

Vitrification assays with embryos from a cold tolerant sub-arctic fish species

Pseudopleuronectes americanus is a Northern teleost species that produces antifreeze proteins (AFPs) to protect them from freezing during the winter. These AFPs bind to ice crystals to inhibit their growth, and they also protect cell membranes at low temperatures. In this study, vitrification trials were done with fish embryos at three different developmental stages, using two different protocols for incorporating the vitrifying solutions. Toxicity of the cryoprotectants and permeability to dimethyl sulfoxide were analyzed. Embryos were vitrified in 0.5 ml straws by direct immersion in liquid nitrogen, and their morphology and development analyzed following thaw. The embryos responded well to vitrification as evidenced by the high percentage that exhibited good morphology following thaw. Although none of the embryos hatched, a small percentage (0.92%) of them showed active movements within the chorion and continued to develop for a number of days following thaw. This is the first record of post-thaw development of vitrified fish embryos.

Toxicity and protective efficiency of cryoprotectants to flounder (Paralichthys olivaceus) embryos

With the purpose of finding an ideal cryoprotectant or combination of cryoprotectants in a suitable concentration for flounder (Paralichthys olivaceus) embryo cryopreservation, we tested the toxicities, at culture temperature (16 degrees C), of five most commonly used cryoprotectants-dimethyl sulfoxide (Me2SO), glycerol, methanol (MeOH), 1,2-propylene glycol (PG) and ethylene glycol (EG). In addition, cryoprotective efficiency to flounder embryos of individual and combined cryoprotectants were tested at -15 degrees C for 60 min. Five different concentrations of each of the five cryoprotectants and 20 different combinations of these cryoprotectants were tested for their protective efficiency. The results showed that the toxicity to flounder embryos of the five cryoprotectants are in the following sequence: PG < MeOH < Me2SO < glycerol < EG (P < 0.05); whereas the protective efficiency of each cryoprotectant, at -15 degrees C for a period of 60 min, are in the following sequence: PG > Me2SO approximately MeOH approximately glycerol > EG (greater symbols mean P < 0.05, and approximate symbols mean P > 0.05). Methanol combined with any one of the other cryoprotectants gave the best protection, while ethylene glycol combined with any one of the other cryoprotectants gave the poorest protection at -15 degrees C. Toxicity effect was concentration dependent with the lowest concentration being the least toxic for all five cryoprotectants at 16 degrees C. For PG, MeOH and glycerol, 20% solutions gave the best protection at -15 degrees C; whereas a 15% solution of Me2SO, and a 10% solution of EG, gave the best protection at -15 degrees C.

Cryopreservation of flounder (Paralichthys olivaceus) embryos by vitrification

Conventional cryopreservation of complex teleost embryos has been unsuccessful, possibly because their large size (1-7 mm diameter), multi-compartmental structure and low water permeability lead to intracellular ice formation and chilling injury. To overcome these obstacles, we have developed a vitrification procedure for cryopreservation of flounder (Paralichthys olivaceus) embryos. In initial toxicity tests, propylene glycol (PG) and methanol (MeOH) were less toxic to embryos than dimethylformamide (DMF) or dimethyl sulfoxide (Me2SO), whereas ethylene glycol (EG) and glycerol (Gly) were toxic to all tested embryos. Embryos between four-somite and tail bud stages were more tolerant to vitrifying solutions than embryos in other developmental stages. Four vitrifying solutions (FVS1-FVS4) were prepared by combining a basic saline solution (BS2) and cryoprotectants PG and MeOH in different proportions (FVS1: 67, 20 and 13%; FVS2: 60, 24 and 16%; FVS3: 55, 27 and 18%; FVS4: 50, 30 and 20% of BS2, PG and MeOH, respectively). Their impact on flounder embryos was then compared. FVS1 produced the highest survival rate; whereas deformation rate was highest for FVS4. Five-step equilibration of embryos in FVS2 resulted in higher survival rates than equilibration in 4, 3, 2 or 1 steps. Flounder embryos varying from the 14-somite to the pre-hatching stage were cryopreserved in the four vitrifying solutions in liquid nitrogen for 1-7 h. From eight experiments, 20 viable thawed embryos were recovered from 292 cryopreserved embryos. Fourteen larvae with normal morphology hatched successfully from the 20 surviving frozen-thawed embryos from five experiments. Embryos at the tail bud stage exhibited greater tolerance to vitrification than embryos at other stages. These results establish that cryopreservation of flounder embryos by vitrification is possible. The technology has many potential applications in teleost germplasm resource conservation.

Water- and cryoprotectant-permeability of mature and immature oocytes in the medaka (Oryzias latipes)

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes/embryos. To identify a stage feasible for the cryopreservation of teleost oocytes, we investigated the permeability to water and various cryoprotectants of medaka (Oryzias latipes) oocytes at the germinal vesicle (GV) and metaphase II (MII) stages. In sucrose solutions, the volume changes were greater in GV oocytes than MII oocytes. Estimated values for osmotically inactive volume were 0.41 for GV oocytes and 0.74 for MII oocytes. Water-permeability (microm/min/atm) at 25 degrees C was higher in GV oocytes (0.13+/-0.01) than MII oocytes (0.06+/-0.01). The permeability of MII oocytes to various cryoprotectants (glycerol, propylene glycol, ethylene glycol, and DMSO) was quite low because the oocytes remained shrunken during 2 h of exposure in the cryoprotectant solutions at 25 degrees C. When the chorion of MII oocytes was removed, the volume change was not affected, except in DMSO solution, where dechorionated oocytes shrunk and then regained their volume slowly; the P(DMSO) value was estimated to be 0.14+/-0.01x10(-3) cm/min. On the other hand, the permeability of GV oocytes to cryoprotectants were markedly high, the P(s) values (x10(-3) cm/min) for propylene glycol, ethylene glycol, and DMSO being 2.21+/-0.29, 1.36+/-0.18, and 1.19+/-0.01, respectively. However, the permeability to glycerol was too low to be estimated, because GV oocytes remained shrunken after 2 h of exposure in glycerol solution. These results suggest that, during maturation, medaka oocytes become less permeable to water and to small neutral solutes, probably by acquiring resistance to hypotonic conditions before being spawned in fresh water. Since such changes would make it difficult to cryopreserve mature oocytes, immature oocytes would be more suitable for the cryopreservation of teleosts.

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.

Developmental expression of aquaporin-3 in zebrafish embryos (Danio rerio)

Fish embryos have never been successfully cryopreserved because of the low permeability of cryoprotectants into the yolk. Recently, we used aquaporin-3 fused with a green fluorescent protein (AQP3GFP) to modify the zebrafish embryo, and demonstrated that the pores functioned physiologically. This increased the water and cryoprotectant permeability of the membranes. We have continued our work on AQP3-modified embryos and here we report their developmental expression of AQP3, the success of various culture media on their survival and development, and their reproductive success. The AQP3GFP expression begins within 30 m after the mRNA AQP3GFP injection into the yolk of the 1- to 4-cell embryo. This expression is distributed in the membranes throughout the blastoderm and the yolk syncytial layer within 24 h. It diminishes after 96 h. We found no difference in the survival or normal development of embryos from AQP3GFP or wild-type adults. Additionally, zebrafish embryos did not require special culture medium to survive after AQP3GFP modification. In fact, they survived best in embryo medium (ca. 40 mOsm). Embryos reared entirely in embryo medium had a higher percent survival and a higher percent normal development than those exposed to a high osmolality sucrose culture medium (ca. 330 mOsm). The mechanism whereby these embryos can maintain their internal osmolality in a hypoosmotic solution with water channels in their membranes is unknown.

Effect of different cryoprotectants and vitrificant solutions on the hatching rate of turbot embryos (Scophthalmus maximus)

Vitrification could provide a promising tool for the cryopreservation of fish embryos. However, in order to achieve a vitrifiable medium, a high concentration of permeable cryoprotectants must be employed, and the incorporation of high molecular weight compounds should also be considered. The toxicity of these permeable and non-permeable agents has to be assessed, particularly when high concentrations are required. In the present study, permeable and non-permeable cryoprotectant toxicity was determined in turbot embryos at two development stages (F stage-tail bud and G stage-tail bud free). Embryos treated with pronase (2mg/ml, 10 min at 22 degrees C) were incubated in dimethyl sulfoxide (Me2SO), methanol (Meth.) or ethylene glycol (EG) in concentrations ranging from 0.5 to 6M for periods of 10 or 30 min, and in 5, 10, and 15% polyvinylpyrrolidone (PVP), 10, 15, and 20% sucrose or 0.1, 1, and 2% X-1000 for 2 min. The embryos were then washed well and incubated in seawater until hatching. The toxicity of permeable cryoprotectants increased with concentration and exposure time. There were no significant differences between permeable cryoprotectants. However, embryos tolerated higher concentrations of Me2SO than other cryoprotectants. Exposure to permeable cryoprotectants did not affect the hatching rate except at G stage with X-1000 treatment and 20% sucrose. Taking into account the cryoprotectant toxicity and the vitrification ability of cryoprotectant mixtures, three vitrification solutions (V1, V2, and V3), and one protocol for stepwise incorporation were designed. The tested solutions contained 5M Me2SO+2M Meth+1M EG plus 5% PVP, 10% sucrose or 2% X-1000. The hatching rate of embryos that had been exposed to the the vitrification solutions was analyzed and no significant differences were noticed compared with the controls. Our results demonstrate that turbot embryos can be subject to this cryoprotectant protocol without deleterious effect on the hatching rate.

Dimethyl sulfoxide influx in turbot embryos exposed to a vitrification protocol

The particular characteristics of fish embryos require the development of specific methods for cryopreservation. One of the main obstacles is related to the presence of membranes and compartments with different water and cryoprotectant permeability. To assess dimethyl sulfoxide (Me2SO4) permeability, we exposed turbot embryos (Scophthalmus maximus) at F stage (tail bud) to the cryoprotectant solutions used in a vitrification protocol and then evaluated the Me2SO4 content inside the embryo using high-performance liquid chromatography (HPLC). The Me2SO4 influx was analyzed in normal embryos and in embryos treated with pronase (2mg/ml) in order to increase chorion permeability. The evaluation was made after each step of cryoprotectant incorporation and removal. Three embryo compartments were distinguished: the perivitelline space (PVS), the yolk sac (YS) and the cellular compartment (CC), and the relative volumes of each, estimated using stereoscopic microscopy imaging, were 11.37, 81.23 and 7.40%, respectively. The Me2SO4 concentration inside the embryos was calculated based on their entrance into one, two or three compartments. Results suggest high entrance of Me2SO4 into the PVS and a low concentration of this cryoprotectant inside the other compartments. Pronase did not significantly increase Me2SO4 influx, but facilitated its elimination during the washing steps.

Altering fish embryos with aquaporin-3: an essential step toward successful cryopreservation

Fish populations are globally threatened by overharvesting and habitat degradation. The ability to bank fish embryos by cryopreservation could be crucial for preserving species diversity, for aquaculture (allowing circannual fish farming), and for managing fish models used in human biomedical research. However, no nonmammalian embryo has ever been successfully cryopreserved. For fish, low membrane permeability prevents cryoprotectants from entering the yolk to prevent cryodamage. Here, we present evidence of a membrane mechanism hindering cryopreservation of fish and propose a novel solution to this obstacle. Zebrafish (Danio rerio) embryos have rectifying membranes that allow water to leave but not to reenter readily. This feature may be an evolutionary trait that allows freshwater embryos to grow in hypoosmotic environments without osmoregulatory organs. However, this trait may also prevent successful fish embryo cryopreservation because both water and cryoprotectants must move into and out of cells. As a solution, we injected zebrafish embryos with mRNA for the aquaporin-3 water channel protein and demonstrated increased membrane permeability to water and to a cryoprotectant. Modeling indicates that sufficient cryoprotectant enters aquaporin-3-expressing zebrafish embryos to allow cryopreservation.

Cryopreservation of chum salmon blastomeres by the straw method

In order to preserve genetic resources of chum salmon, Oncorhynchus keta, optimum conditions for cryopreservation of isolated blastomeres were investigated. Survival rates under various conditions were compared: the nature and the concentration of cryoprotectants before and after freezing, the seeding temperature, and the developmental stages of donor embryos. Isolated blastomeres immersed for 30 min in Eagle's MEM containing both a cryoprotectant and 10% fetal bovine serum (FBS) at 10 degrees C were transferred into a straw and frozen at 1 degrees C/min to -30 degrees C by a programmable freezer before being plunged into liquid nitrogen. Ice seeding was carried out at -5 to -15 degrees C. Frozen blastomeres were thawed in water at 15 degrees C. Blastomeres cryopreserved with MEM containing 10% dimethyl sulfoxide (Me(2)SO) and 10% FBS (10% Me(2)SO/MEM10) showed higher survival rates than those cryopreserved with MEM containing 10% FBS and 10% glycerol, ethyleneglycol, 1, 2-propanediol, or sucrose. Blastomeres treated with 10% Me(2)SO/MEM10 showed higher survival rates than those treated with MEM containing only 10% Me(2)SO. Blastomeres seeded above -10 degrees C showed higher survival rates than non-seeded ones. Frozen blastomeres at advanced stages demonstrated high survival rates. Blastomeres cryopreserved under optimum conditions showed survival rates of 59.3+/-2.8%. These results indicate that 10% Me(2)SO/MEM10 is a suitable cryoprotectant medium to cryopreserve chum salmon blastomeres, that seeding should be carried out above -10 degrees C on pre-freezing, and that blastomeres at the blastula stage should be used as material.

Overcoming a permeability barrier by microinjecting cryoprotectants into zebrafish embryos (Brachydanio rerio)

The goal of this research was to examine the developmental effects on zebrafish embryos (Brachydanio rerio) when cryoprotectants were directly microinjected into the yolk. Our objectives were to: (i) determine the final concentration of propylene glycol (PG) and dimethyl sulfoxide (Me(2)SO) that the embryos could tolerate without causing teratogenic effects; (ii) determine if the toxicity of Me(2)SO could be reduced by the simultaneous presence of various proportions of amides; and (iii) examine whether this intracellular cryoprotectant incorporation could reduce the cryodamage to the yolk syncytial layer (YSL) after vitrification trials. The rationale for conducting these microinjection experiments was to overcome the permeability barrier of the YSL. Intracellular PG produced better survival than Me(2)SO (P < 0.05). Embryos tolerated both 10- and 30-nl microinjections of PG, yielding final concentrations of 2.3 and 5.0 M within the yolk, resulting in 70 +/- 3 and 35 +/- 4% survival at day 5, respectively. In similar experiments with Me(2)SO, survival was lower than PG at 60 +/- 4 and 14 +/- 4% at 2.4 and 5.2 M. Unlike other cellular systems, the presence of amides, specifically acetamide or formamide, did not reduce the toxicity of Me(2)SO in zebrafish embryos (P > 0.05). During vitrification trials, we estimated a 25% dehydration of the yolk, yielding an effective PG concentration of 5.9 M. However, the incorporation of this vitrifiable concentration of PG was not sufficient to improve the postthaw morphology of the YSL (P > 0.05). Clearly, other factors need to be examined in establishing a successful vitrification protocol for zebrafish embryos.

The effect of partial removal of yolk on the chilling sensitivity of zebrafish (Danio rerio) embryos

The effect of partial removal of yolk on the survival of zebrafish embryos and the chilling sensitivity of yolk-reduced embryos were investigated at several stages of embryo development. Dechorionated embryos were punctured with a sharp microneedle and approximately 50 to 75% of yolk content was released following multiple punctures. The survival of yolk-reduced embryos was found to be stage dependent. Only 7.9% of 26-somite (24 h) embryos survived, whereas 56.7% of prim-6 (27 h), 62.4% of prim-15 (34 h), and 81.3% of high-pec (49 h) embryos survived after partial removal of yolk. For chilling sensitivity studies the yolk-reduced embryos at high-pec stage were cultured in embryo medium for 2, 6, or 24 h to allow embryo recovery before they were chilled at 0 degrees C for 6 h. No significant differences (P > 0.05) were seen in normalized survivals between control and yolk-reduced embryos following a 2- or 6-h recovery period. However, when the recovery period was extended to 24 h, the yolk-reduced embryos showed significant (P < 0.05) higher survival than that of chilled controls and the significance was more pronounced (P < 0.01) after a longer period (10 h) of chilling. Similar results were also obtained with embryos at prim-6 stage. These results indicated that after partial removal of yolk, zebrafish embryos at post-prim-6 stage can survive well and their sensitivity to chilling can be reduced. This may have significant implications in alleviating certain difficulties confronting the cryopreservation of fish embryos. Copyright 1999 Academic Press.

Membrane permeability modeling: Kedem-Katchalsky vs a two-parameter formalism

The analysis of experiments for the purpose of determining cell membrane permeability parameters is often done using the Kedem-Katchalsky (KK) formalism (1958). In this formalism, three parameters, the hydraulic conductivity (Lp), the solute permeability (Ps), and a reflection coefficient (final sigma), are used to characterize the membrane. Sigma was introduced to characterize flux interactions when water and solute (cryoprotectant) cross the membrane through a common channel. However, the recent discovery and characterization of water channels (aquaporins) in biological membranes reveals that aquaporins are highly selective for water and do not typically cotransport cryoprotectants. In this circumstance, sigma is a superfluous parameter, as pointed out by Kedem and Katchalsky. When sigma is unneeded, a two-parameter model (2P) utilizing only Lp and Ps is sufficient, simpler to implement, and less prone to spurious results. In this paper we demonstrate that the 2P and KK formalism yield essentially the same result (Lp and Ps) when cotransporting channels are absent. This demonstration is accomplished using simulation techniques to compare the transport response of a model cell using a KK or 2P formalism. Sigma is often misunderstood, even when its use is appropriate. It is discussed extensively here and several simulations are used to illustrate and clarify its meaning. We also discuss the phenomenological nature of transport parameters in many experiments, especially when both bilayer and channel transport are present.