Freezing mammalian embryos: A review of the techniques

A 100-Year Review: Reproductive technologies in dairy science

Reproductive technology revolutionized dairy production during the past century. Artificial insemination was first successfully applied to cattle in the early 1900s. The next major developments involved semen extenders, invention of the electroejaculator, progeny testing, addition of antibiotics to semen during the 1930s and 1940s, and the major discovery of sperm cryopreservation with glycerol in 1949. The 1950s and 1960s were particularly productive with the development of protocols for the superovulation of cattle with both pregnant mare serum gonadotrophin/equine chorionic gonadotrophin and FSH, the first successful bovine embryo transfer, the discovery of sperm capacitation, the birth of rabbits after in vitro fertilization, and the development of insulated liquid nitrogen tanks. Improved semen extenders and the replacement of glass ampules with plastic semen straws followed. Some of the most noteworthy developments in the 1970s included the initial successes with in vitro culture of embryos, calves born after chromosomal sexing as embryos, embryo splitting resulting in the birth of twins, and development of computer-assisted semen analysis. The 1980s brought flow cytometric separation of X- and Y-bearing sperm, in vitro fertilization leading to the birth of live calves, clones produced by nuclear transfer from embryonic cells, and ovum pick-up via ultrasound-guided follicular aspiration. The 20th century ended with the birth of calves produced from AI with sexed semen, sheep and cattle clones produced by nuclear transfer from adult somatic cell nuclei, and the birth of transgenic cloned calves. The 21st century has seen the introduction of perhaps the most powerful biotechnology since the development of artificial insemination and cryopreservation. Quick, inexpensive genomic analysis via the use of single nucleotide polymorphism genotyping chips is revolutionizing the cattle breeding industry. Now, with the introduction of genome editing technology, the changes are becoming almost too rapid to fully digest.

Forty years of embryo transfer in cattle: a review focusing on the journal Theriogenology, the growth of the industry in North America, and personal reminisces

After the first successful transfer of mammalian embryos in 1890, it was approximately 60 years before significant progress was reported in the basic technology of embryo transfer (ET) in cattle. Starting in the early 1970s, technology had progressed sufficiently to support the founding of commercial ET programs in several countries. Today, well-established and reliable techniques involving superovulation, embryo recovery and transfer, cryopreservation, and IVF are utilized worldwide in hundreds, if not thousands, of commercial businesses located in many countries. The mean number of embryos produced via superovulation has changed little in 40 years, but there have been improvements in synchrony and hormonal protocols. Cryopreservation of in vivo-derived embryos is a reliable procedure, but improvements are needed for biopsied and in vitro-derived embryos. High pregnancy rates are achieved when good quality embryos are transferred into suitable recipients and low pregnancy rates are often owing to problems in recipient management and not technology per se. In the future, unanticipated disease outbreaks and the ever-changing economics of cattle and milk prices will continue to influence the ET industry. The issue of abnormal pregnancies involving in vitro embryos has not been satisfactorily resolved and the involvement of abnormal epigenetics associate with this technology merits continued research. Last, genomic testing of bovine embryos is likely to be available in the foreseeable future. This may markedly decrease the number of embryos that are actually transferred and stimulate the evolution of more sophisticated ET businesses.

Changes in the freezing protocol for human zygotes alter embryonic development and pregnancy rates

OBJECTIVE

To determine the effect of various freezing protocols on postthaw development and pregnancy rates resulting from transfer of human zygotes.

DESIGN

Prospective study.

SETTING

Tertiary care center.

PATIENT(S)

Couples undergoing assisted reproductive technology (ART) procedures who wished to have their excess zygotes cryopreserved.

INTERVENTION(S)

We cryopreserved zygotes with one of three protocols.

MAIN OUTCOME MEASURE(S)

Post-thaw survival and development of the zygotes as well as pregnancy rate after transfer of these zygotes.

RESULT(S)

A 3-minute hold time after seeding, followed by a final preplunging temperature of -180 degrees C, resulted in a clinical pregnancy rate of 28.6%. In contrast, a 15-minute postseed hold time and a -30 degrees C final chamber temperature resulted in a 37.3% clinical pregnancy rate. When we combined the protocols to provide a 15-minute postseed holding time and a -180 degrees C before plunging into liquid nitrogen, we achieved a 69.6% clinical pregnancy rate.

CONCLUSION(S)

By increasing the postseeding hold time and decreasing the temperature of the freezing chamber before plunging the zygotes into liquid nitrogen, significant improvements can be made in postthaw development and pregnancy rates.

Commercialization of animal biotechnology

Commercialization of animal biotechnology is a wide-ranging topic for discussion. In this paper, we will attempt to review embryo transfer (ET) and related technologies that relate to food-producing mammals. A brief review of the history of advances in biotechnology will provide a glimpse to present and future applications. Commercialization of animal biotechnology is presently taking two pathways. The first application involves the use of animals for biomedical purposes. Very few companies have developed all of the core competencies and intellectual properties to complete the bridge from lab bench to product. The second pathway of application is for the production of animals used for food. Artificial insemination (AI), embryo transfer, in vitro fertilization (IVF), cloning, transgenics, and genomics all are components of the toolbox for present and future applications. Individually, these are powerful tools capable of providing significant improvements in productivity. Combinations of these technologies coupled with information systems and data analysis, will provide even more significant change in the next decade. Any strategies for the commercial application of animal biotechnology must include a careful review of regulatory and social concerns. Careful review of industry infrastructure is also important. Our colleagues in plant biotechnology have helped highlight some of these pitfalls and provide us with a retrospective review. In summary, today we have core competencies that provide a wealth of opportunities for the members of this society, commercial companies, producers, and the general population. Successful commercialization will benefit all of the above stakeholders.

Review: dairy cattle reproductive physiology research and management--past progress and future prospects

Artificial insemination developed as the solution for two important problems in the dairy cattle industry during the past 50 yr: 1) the need for genetic improvement and 2) the elimination of costly venereal diseases. Cooperation among researchers, extension workers, veterinarians, dairy producers, and emerging AI organizations in pooling their expertise, was instrumental to the remarkably rapid development of AI. The cooperation of universities, government, and producers to fund teams of reproductive specialists to collaborate and transfer findings quickly to potential users was a major component of this successful venture. Money invested in these experiments was estimated to have returned about $100 for each $1 invested. Successful freezing of sperm led to the development of the field of cryobiology, and AI paved the way for embryo transfer. The development of ultrasound equipment; various types of rapid hormone assays; prostaglandins, progestogens, and GnRH; and computerization made possible various alternative management plans for controlling reproduction. Multidisciplinary, multigeographical teams that gather basic needed information have the potential for making excellent progress. As herd size increases, new programs for efficient reproductive management and for identifying needed research through computer modeling are a must. Sexed embryos from elite cows and bulls will be used selectively. When embryonic stem cell technology becomes practical, it will revolutionize cattle breeding.

Freezing bovine embryos with a portable liquid nitrogen freezer requiring no external seeding

One-hundred and twenty excellent morula to blastocyst stage bovine embryos were obtained nonsurgically from superovulated Holstein heifers. Completely portable, nonelectric (manual) liquid nitrogen (LN) freezers combined with simplified freezing curves utilizing self-seeding were compared to a programmable LN freezer (Planner-R204) following the conventional freezing rate for freezing embryos. Seeding was self induced in ampules at -6.8 degrees C and at -5.5 degrees C in straws in the manual freezers. Glycerol was used as the cryoprotectant at 1.5 M concentration. Post-thaw appearance, fluorscein diacetate testing (FDA), and growth after 12 and 24 h incubation were used as indicators of embryo viability. There were no significant differences between embryos frozen in the two types of freezers in terms of the viability tests used. Pregnancy rates resulting from transfer of embryos frozen in the two types of freezers will be determined in subsequent field trials. The manual LN freezers used in this study are capable of successfully freezing bovine embryos. The simplified nature of these freezers and the freezing procedures used with them greatly decreases the complexity and expense of freezing embryos.

Evaluation of cryopreservation techniques for bovine embryos

A total of 228 embryos was nonsurgically collected from superovulated cows and dehydrated in dimethyl sulfoxide (DMSO) or glycerol by a three-step procedure or a (T.I.T.) timed interval titration procedure. Embryos were loaded in straws, frozen by cooling to -6.0 degrees C at 1.0 degrees C/min, and seeded, followed by cooling to -30 degrees C at 0.3 degrees C/min and to -38 degrees C at 0.1 degrees C/min. At this time the straws were plunged into liquid nitrogen at -195 degrees C. Embryos were thawed in a 27 degrees C or 37 degrees C water bath and rehydrated by a six-step, three-step (sucrose) or one-step (sucrose) procedure. This yielded a 2 x 2 x 2 x 3 factorial treatment structure. Survival was based on development after 12 h in in vitro culture. The only significant single factor affecting survival was the initial quality grade of the embryo. Grades 1 and 2 embryos survived more often than Grade 3 embryos (P < 0.05). Using DMSO as the cryoprotectant resulted in better scores for the post dehydration to post thawing interval (P = 0.02). For both intervals, post dehydration to post thawing and post thawing to post rehydration, the previous quality grade was significant in determining the subsequent quality grade (P < 0.01). At each step of the freeze-thaw process, the embryos became progressively less morphologically intact.

Effect of varying glycerol and sucrose concentration combinations on embryo survival rate in a one-step cryoprotectant removal from frozen-thawed ovine embryos

Two hundred fifty-one ovine embryos were frozen in different levels of glycerol (1.0, 1.4, 2.0 or 2.8M) and thawed into one of four sucrose levels (0, 0.25, 1.0 or 2.0M) to determine the optimal glycerol-sucrose combination for one-step, in-straw thawing. Sucrose was toxic at low glycerol levels and mandatory at high levels. The 1.0M sucrose level with either 1.4 or 2.0M glycerol was optimal for one-step cryoprotectant removal.

The effect of freezer type, cryoprotectant, and processing methods on viability of frozen embryos

Two hundred forty excellent-quality blastocysts flushed from 53 superovulated Holstein heifers were frozen by 1 of 16 procedures in a 2 x 2 x 2 x 2 factorial design. The main effects included a simple, inexpensive, portable mechanical freezing unit instead of a programmable Liquid Nitrogen (LN) freezer for freezing bovine embryos, cryoprotective agents dimethyl sulfoxide (DMSO) and glycerol, addition rates of the cryoprotectants and freezing rates. Embryo viability was assessed morphologically and by fluorescein diacetate (FDA) evaluation. Neither the type of freezer, the cryoprotectant nor the rate of cryoprotectant addition affected embryo viability. Embryo survival after 12 h of incubation was higher (P < 0.05) using a conventional freezing rate than a two-step method (37.2 vs 16.5%). The correlation coefficient between viability evaluation methods (morphology vs FDA) was influenced by cryoprotectant and embryo processing methods and ranged from -0.13 to +0.70. This study indicates that more simplified embryo freezing equipment and handling procedures may provide protection equal to that of more complicated, expensive equipment and more time-consuming methods. Economical on-farm embryo freezing is feasible.

Preliminary studies on bovine embryo survival following short-term storage at 4°C

A total of 113 non-surgically collected bovine embryos, 5-8 days of age, were stored for 48 hours at 4 degrees C in a modified phosphate-buffered saline solution (PBS). Following storage, embryos were cultured for 8-12 hours at 37 degrees C, and those which were morphologically normal were transferred to synchronized recipients by several methods designed to achieve twin pregnancies. Embryos which were collected and transferred on the same day served as controls. Of 113 embryos stored, 47 (42%) appeared to be transferable after the brief culture period. There was a marked breed effect on viability after refrigeration, with Hereford embryos surviving significantly better than Angus embryos (71% vs. 12%, respectively, p < .001). Post-transfer embryo survival of stored and control embryos, based on actual calvings, was 34 and 48 percent, respectively, a difference which was not significant (p=0.3). A marked difference in pregnancy rate following non-surgical transfer by 2 different technicians was noted (50% vs. 21.7%, respectively).

Freezing mouse blastocysts. The influence of the preparations prior to freezing on the survival rate of the blastocysts

A good survival rate in culturing mouse blastocysts can be obtained in Ovum Culture Medium, enriched with 20 per cent inactivated Foetal Bovine Serum or Sheep Serum under air. The transfer of fresh blastocysts gives the best results if the recipients are on day 3 of the pseudo-pregnancy, but with 20 hours' cultured blastocysts it is better to use recipients on day 4. Exposure to 1.5 M DMSO has no harmful effect, provided that the DMSO is added at 5 degrees C in 6 steps and is removed, again in 6 steps, at 35 degrees C. The crystallization of the medium containing the embryos at -5 degrees C to -6 degrees C doet not appear te have a harmful influence on culture results of the blastocysts.

Development and viability of frozen-thawed cattle embryos

Embryos recovered 7 to 8 days after estrus were frozen from -7 to -30 degrees C at 0.3 degrees C/min, from -30 to -33 degrees C at 0.1 degrees C/min, and then plunged into liquid nitrogen. They were thawed in a 25 degrees C waterbath. In a preliminary study, 15 of 18 embryos continued to develop during the 24-hour culture post-thaw in either Ham's F-10 or modified Dulbecco's phosphate buffered saline (PBS). In the main study, 5 of 20 embryos developed to 60-day pregnancies when embryos were transferred within 5 hours after thawing. The incidence of extended estrous cycles (pregnancy or presumed embryonic mortality) was 10 of 14, when the zona pellucida was intact after thawing, and 0 of 6, when it was ruptured or absent (P<.05). Embryos cultured in PBS tended to develop more readily than those in Ham's F-10 (15 of 20 vs 9 of 20, respectively, P reverse similar.1). Quality of the embryos, at recovery from the donor and after thawing, affected development in culture (19 of 27 embryos excellent at recovery developed vs 5 of 13 poor to very good, P reverse similar.1; 23 of 33 embryos good to excellent after thawing developed vs 1 of 7 poor, P<.05). The proportion of pyknotic nuclei in embryos which were cultured ranged from 18 to 100%. The pregnancy rate from embryos which were cultured was low (2 of 20). Thirty percent of frozen and thawed embryos had damaged zonae pellucidae. The study showed that: the pregnancy rate from frozen embryos was approximately half that achieved with unfrozen embryos; culturing embryos for 24 hours before transfer was not beneficial; the PBS culture system appears to be the system of choice for assessing embryo viability in vitro .