With the global population projected to exceed 10 billion people by the year 2050, the demand for animal‑derived proteins such as meat and milk also is expected to significantly increase. To meet that demand, we must figure out how to improve the efficiency of livestock production by developing critical, new management strategies and technologies.
Fertility, or reproductive efficiency, is one of the most important economic factors in livestock production; however, reproduction remains relatively inefficient in most livestock species. Nearly half of all beef cows and well over half of all dairy cows will not become pregnant within the first month after they are bred.
One of my laboratory’s primary research goals is to understand what differentiates an embryo that can establish pregnancy from one that cannot. In collaboration with scientists at the University of Florida, we sought to determine whether the quantity of certain genes that are important for development differs between embryos that are able to establish pregnancy and those that fail. Not only did our experiment help distinguish genetic differences between viable and nonviable embryos, we also were able to identify the specific genes that influence embryo development.
The critical first weeks
Approximately 60–80% of pregnancy loss in cattle occurs within the first three weeks of gestation. During this time, several important developmental events must occur for pregnancy to continue.
In the first seven days following fertilization, the embryo, which starts out with only two cells, must undergo a series of cell divisions that result in an embryo with about 150 to 200 cells. In addition to cell growth, at this stage the embryo must undergo its first differentiation, which results in two distinct cell types, those that will develop into the placenta and those that will develop into the fetus.
At the end of the first week of development, the embryo must hatch from its protective shell and undergo a period of significant elongation. Around day 12 of gestation, the cattle embryo is a tiny, football-shaped structure approximately 1–2 mm in length, but by day 16, the embryo transforms into a string-like structure that can measure up to 60 cm in length. This elongation during the second week of gestation is important for future development of the placenta and is necessary for what we call “maternal recognition of pregnancy.” Near day 16 of gestation, the embryo must secrete a factor that lets the mother know it is viable and present in the uterus, so she does not return to estrus, or heat.
As the embryo enters the third week of gestation, placental development is now fully underway, and the placenta starts to attach to the lining of the uterus. Attachment is critical for continued pregnancy and fetal development because it allows the mother to share vital nutrients with her developing offspring.
Factors that determine success or failure
Although we know that the vast majority of pregnancy failures occur within the first few weeks of gestation, it can be difficult to determine the cause. In cattle, pregnancy cannot accurately be diagnosed until the start of the fourth week of gestation, and earlier pregnancy failures are difficult to assess.
One important factor of many that affect the establishment and maintenance of pregnancy is genetics. The egg from the dam and the sperm from the sire both contribute genes that regulate early embryo development and are necessary for survival. Another important factor is the maternal environment, including the oviduct and uterus, two components of the female reproductive system that secrete factors to support early embryo development. These factors include various classes of molecules, such as growth factors, hormones and metabolites. But even when the oviduct and uterus provide a favorable environment for embryo survival, the embryo itself must be able to appropriately respond to maternal signals.
Genes associated with embryo survival
In our experiment, we stimulated a group of virgin heifers with hormones to make them ovulate multiple eggs that could be fertilized using artificial insemination then develop into embryos. One week after insemination, embryos were retrieved from the uteri of the heifer donors and split in half using a microblade system. One half of the embryo was immediately transferred to a surrogate heifer (one half per surrogate), and the other half was frozen for gene expression analysis.
We diagnosed pregnancies in the heifer surrogates at days 30 and 60 of gestation and analyzed the global gene expression patterns of the corresponding frozen embryo halves. We compared the embryos that successfully established and maintained a pregnancy with those that did not. By analyzing the data, we determined that 155 genes were differentially expressed between the two experimental groups, with 73 genes upregulated and 82 genes downregulated in embryos that successfully established and maintained pregnancy compared to embryos that failed to establish pregnancy.
Interestingly, many of the genes that were more abundant in the viable embryos were receptors for various regulatory molecules secreted by the oviduct or the uterus during early pregnancy. Our results suggest that embryos are more likely to survive if they are better able to respond to maternal signals that support their development.
We also found that many of the genes that were less abundant in viable embryos were linked to metabolic activity. The most competent embryos were less metabolically active, indicating that increased metabolic activity is associated with reduced viability.
The future of gene editing
Moving forward, my laboratory will work to utilize functional genomics tools, such as gene editing, to explain the importance of some of the genes that were differentially regulated.
Gene editing has been used for many years in research that uses rodent models, but only recently has it been used effectively in studies of large animal species, such as livestock. It can be used to insert, delete or modify specific genes related to embryo viability, to more effectively determine their roles in development and survival.
We hope that the findings from our research will enable us to develop applied approaches and technologies that cattle producers can use to improve reproductive efficiency in their herds and meet the future supply demands of our growing global population.
By Jeremy Block, Assistant Professor, Department of Animal Science, (307) 766-3429, jeremy.block@uwyo.edu
Reprinted from the 2022 issue of Reflections, the College of Agriculture, Life Sciences and Natural Resources annual research magazine.
