Embryo transfer: The next 100 years

Translating Basic Research to Animal Agriculture

Procedures to maintain viability of mammalian gametes and embryos in vitro, including cryopreservation, have been exceedingly valuable for my research over the past 55 years. Keeping sperm viable in vitro enables artificial insemination, which, when combined with selective breeding, often is the most effective approach to making rapid genetic change in a population. Superovulation and embryo transfer constitute a parallel approach for amplifying reproduction of female mammals. More recent developments include sexing of semen, in vitro fertilization, cloning by nuclear transfer, and genetic modification of germline cells, tools that are enabled by artificial insemination and/or embryo transfer for implementation. I have been fortunate in being able to contribute to the development of many of the above techniques, and to use them for research and applications for improving animal agriculture. Others have built on this work to circumvent human infertility, assist reproduction of companion animals, and rescue endangered species. It also has been a privilege to teach, mentor, and be mentored in this area. Resulting worldwide friendships have enriched me personally and professionally. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Assisted reproduction with gametes and embryos: what research is needed and fundable?

Principles for selecting future research projects include interests of investigators, fundability, potential applications, ethical considerations, being able to formulate testable hypotheses and choosing the best models, including selection of the most appropriate species. The following 10 areas of assisted reproduction seem especially appropriate for further research: efficacious capacitation of bovine spermatozoa in vitro; improved in vitro bovine oocyte maturation; decreasing variability and increasing efficacy of bovine superovulation; improved fertility of sexed semen; improving equine IVF; improving cryopreservation of rooster spermatozoa; understanding differences between males in success of sperm cryopreservation and reasons for success in competitive fertilisation; mechanisms of reprogramming somatic cell nuclei after nuclear transfer; regulation of differentiation of ovarian primordial follicles; and means by which spermatozoa maintain fertility during storage in the epididymis. Issues are species specific for several of these topics, in most cases because the biology is species specific.

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.

ASAS Centennial Paper: Future research in physiology and endocrinology

Over the next quarter century in North America, the following eventualities are likely for physiology and endocrinology research with agricultural animals. 1) Total funding adjusted for inflation will change little but will come less from public sources, and most of that will be in the context of human health. Much of the privately funded research will be herd specific and remain proprietary. 2) The numbers of MS, PhD, and postdoctoral students probably will decrease, but research in the context of credentialing will remain important. 3) Resources such as expanded databases in genomics and proteomics, and remarkable new tools such as small inhibitory RNA will continue to become available, likely at a faster rate than in the previous 25 yr. 4) The huge amounts of data from production agriculture will make agricultural animals ideal models for some kinds of basic research, such as studying fetal programming, resulting in synergy with more applied research. Most of these experimental animals will be in private production herds and flocks, even when work is publicly funded. 5) The trend toward more interdisciplinary research will continue, especially considering interactions among reproduction, health, nutrition, selective breeding, management factors, and societal concerns; reductionist research probing deeper into cellular and molecular mechanisms will remain important, as will whole-animal approaches. 6) Agricultural animals are a product of evolution plus selective breeding. Insights drawn from the former will aid progress in the latter. One focus of research in physiology and endocrinology will be understanding heterosis, inbreeding depression, and epigenetic effects as it becomes possible to manipulate and identify the allelic structure of individual animals. 7) Additional insightful concepts will evolve that will simplify thinking in some respects, such as the maternal to embryonic shift in transcribed RNA in early embryos; however, animal biology will turn out to be even more complex than most of us currently imagine.

The current status and future of commercial embryo transfer in cattle

A commercially viable cattle embryo transfer (ET) industry was established in North America during the early 1970s, approximately 80 years after the first successful embryo transfer was reported in a mammal. Initially, techniques for recovering and transferring cattle embryos were exclusively surgical. However, by the late 1970s, most embryos were recovered and transferred nonsurgically. Successful cryopreservation of embryos was widespread by the early 1980s, followed by the introduction of embryo splitting, in vitro procedures, direct transfer of frozen embryos and sexing of embryos. The wide spread adoption of ethylene glycol as a cryoprotectant has simplified the thaw-transfer procedures for frozen embryos. The number of embryos recovered annually has not grown appreciably over the last 10 years in North America and Europe; however, there has been significant growth of commercial ET in South America. Within North America, ET activity has been relatively constant in Holstein cattle, whereas there has been a large ET increase in the Angus breed and a concomitant ET decrease in some other beef breeds. Although a number of new technologies have been adopted within the ET industry in the last decade, the basic procedure of superovulation of donor cattle has undergone little improvement over the last 20 years. The export-import of frozen cattle embryos has become a well-established industry, governed by specific health regulations. The international movement of embryos is subject to sudden and dramatic disturbances, as exemplified by the 2001 outbreak of foot and mouth disease in Great Britain. It is probable that there will be an increased influence of animal rights issues on the ET industry in the future. Several companies in North America are currently commercially producing cloned cattle. The sexing of bovine semen with the use of flow cytometry is extremely accurate and moderate pregnancy rates in heifers have been achieved in field trials, but sexed semen currently is available in only a few countries and on an extremely limited basis. As of yet, all programs involving the production of transgenic cattle are experimental in nature.

Embryo transfer and embryonic capsules in the bobcat (Lynx rufus)

Bobcats (Lynx rufus) (n=22) were used to test a surgical embryo transfer protocol for wild felines. Five blastocysts were collected 8-14 days post-initial copulation (PIC). Translucent capsule-like structures were recovered at 12 days PIC and are the first report of such a structure in a felid. Endometrial fibrosis was observed in one cat but, in general, post-surgical fibrosis of the uterus did not seem to impede ova or embryo transport. One embryo underwent cryopreservation and this embryo plus two other transferrable embryos were placed in recipient cats during the course of the study. No pregnancies were maintained; but one non-cryopreserved embryo was detected by ultrasound examination at 2 weeks post-transfer. This study provides valuable groundwork for future studies and warrants optimism for continued research in this area.

Effect of feeding and body condition score on multiple ovulation and embryo production in zebu cows

The aim of this study was to standardize the feeding regimen and the body condition score (BCS) for maximum superovulatory responses in indigenous zebu cows. Ten regularly cycling 5-8-year-old dry cows, weighing 176--260 kg with BCS 2.5--4.5 were divided into two equal groups at random. The groups were maintained on either a good-nutrition or a high-nutrition diet. The feedstuffs were analysed by proximate feed analysis and the metabolizable energy content was estimated. After 3 months feeding, individual cows were injected (i.m) with 1500 IU pregnant mare's serum gonadotrophin (PMSG) at day 10 or day 11 of the oestrous cycle (day of oestrus = day 0). Alfaprostol (6 mg) was injected (i.m) 48 h after the injection of PMSG to induce oestrus. At day 6 or day 7 (day of insemination = day 1), the contents of individual uterine horns were flushed with 150-200 ml of phosphate-buffered saline + 0.2% bovine serum albumin using a two-way Foley catheter. The embryos were identified, evaluated and graded as excellent, good, fair or poor under a stereomicroscope. For the good- and high-nutrition diets, the daily intake of green grass, straw, concentrate, dry matter, crude protein and estimated metabolizable energy by individual cows were 5 and 6 kg, 3 and 3 kg, 1.5 and 3.5 kg, 4.87 and 6.82 kg, 0.39 and 0.74 kg, and 39.60 and 59.12 MJ, respectively. The protein content was 8 and 11% in the good- and high-nutrition diets, respectively. The two groups of cows on different nutritional diets differed significantly with regard to body weight, body condition score and number of palpated corpora lutea (p < 0.01). For cows on the good-nutrition diet, the median number of recovered embryos and transferable quality embryos were three and two, respectively. The recovery rate of embryos was 79.30% of palpated corpora lutea. Cows on the high-nutrition diet did not yield any embryos. The indigenous zebu cows fed on the good-nutrition diet with BCS 2.5-3 were considered suitable for the induction of superovulation, the cows on the high-nutrition diet with BCS 4-4.5 were unsatisfactory and were more prone to cyst formation in the ovaries.

Challenges to dairy cattle management: genetic considerations

Several new technologies will be used to produce continued genetic change in dairy cattle. These technologies are categorized broadly as 1) improved modeling, selection, and evaluation methods; 2) use of new and improved reproductive technologies; 3) new developments in molecular genetics; and 4) new developments in immunogenetics. Improvements in evaluation will continue as computers become faster and have more storage capabilities. Improved mathematical models that more nearly describe the biology of lactation will maximize estimation of genetic differences and reduce residuals. New reproductive technology could allow reduction of generation intervals two- to fivefold compared with present generation intervals and, combined with genetic markers, could markedly accelerate progress. Health problems in dairy cattle are expected to increase as production increases. Thus, selection for decreased incidences of health disorders will be needed, probably by selection of sires with improved general immunocompetence. Research is in the early stages of application of techniques of molecular genetics to animal breeding. Early uses will allow detection and alleviation of genetic defects. Eventually, marker genes that directly affect production and metabolic pathways that also affect production will be subjected to selection. The ability to foresee new and potentially useful techniques will be determined by scientific advancement of areas in which researchers are engaged; thus, accurate prediction far into the future cannot be expected.

Characterization of in vitro growth of bovine preantral ovarian follicles: A preliminary study

Preantral follicles were mechanically extracted from bovine ovaries collected at slaughter. On average 90 early growing follicles were collected per ovary. The follicles were cultured for 7 days in Dulbecco's modified Eagle medium F-12 nutrient mixture + 10% fetal calf serum + 10% newborn calf serum. The individual follicular diameter and the follicular DNA content were recorded at the start and the end of culture. The follicular DNA content was estimated by a microfluorometric method using the fluorochrome, 4'-6 diamidino-2-phenylindol-2HCl (DAPI). After culture, 86% of the follicles looked morphologically normal. Of the surviving follicles, 73% showed an increase in diameter varying between 5 and 30 mum, and the follicular DNA content increased from 1 to 72% under the same culture conditions. These results indicate that bovine preantral follicles can survive in vitro and even grow in a single medium.

Current status and potential of embryo transfer and reproductive technology in dairy cattle

Significant use of embryo transfer in dairy cattle commenced with the introduction of nonsurgical embryo recovery in the mid-1970s and developed with the use of nonsurgical transfers in the late 1970s. Numbers of registered Holstein calves from embryo transfer doubled yearly through 1980, after which the rate of increase slowed; the total reached nearly 19,000 calves in 1990. However, the efficacy of superovulation procedures and commercial success rates of transferred fresh embryos have not improved the past 10 to 15 yr. Fertilization rates in superovulated donors remain low. Although embryo-splitting techniques were perfected in the early 1980s, they are not used widely. A practical, commercial embryo-sexing procedure remains unavailable. Recent significant improvement is apparent in the technology of ultrasound-guided oocyte collection and in vitro oocyte maturation, fertilization, and embryo culture. In the future, this technology may be used in conjunction with sperm separated by sex with a flow cytometer. Modest numbers of embryo clones have been produced in several commercial programs via nuclear transfer techniques. However, the efficiency of gene transfer experiments involving ova of cattle and other domestic species has been low. Recently, DNA probe technology has begun to provide genotype information for cattle and will ultimately be applied to embryos.