Various techniques have been developed and refined to obtain a large number of offspring from genetically superior animals or obtain offspring from infertile (or subfertile) animals.
These techniques include: artificial insemination, cryopreservation (freezing) of gametes or embryos, induction of multiple ovulations, embryo transfer, in vitro fertilization, sex determination of sperm or embryos, nuclear transfer, cloning, etc.
Artificial Insemination and Cryopreservation
Artificial insemination (AI) has been used to obtain offspring from genetically superior males for more than 200 years. Improvements in methods to cryopreserve (freeze) and store semen have made AI accessible to more livestock producers. In the same manner as cryopreservation of semen, embryo freezing allowed for the global commercialization of animals with high genetic qualities. Semen from bulls is especially amenable to freezing and long-term storage. In the dairy industry, where large numbers of dairy cows are managed intensely, AI is simple, economical, and successful. More than 60 percent of dairy cows in the United States are bred by AI. However, the situation is different for beef cattle, where breeding populations are usually maintained on range or pasture conditions. In the United States beef industry, AI accounts for less than 5 percent of inseminations. For reasons that are not yet well understood, it is more difficult to freeze and store semen from other livestock species, including horses, pigs, and poultry, than it is to freeze cattle semen. NIFA has supported research projects to understand the physiological processes associated with cryopreservation of semen or embryos and to develop improved methods of cryopreservation for gametes (eggs and sperm) and embryos from several livestock species.
Multiple Ovulation and Embryo Transfer
Development of embryo transfer technology allows producers to obtain multiple progeny from genetically superior females. Depending on the species, fertilized embryos can be recovered from females (also called embryo donors) of superior genetic merit by surgical or nonsurgical techniques. The genetically superior embryos are then transferred to females (also called embryo recipients) of lesser genetic merit. In cattle and horses, efficient techniques recover fertilized embryos without surgery, but only one or sometimes two embryos are produced during each normal reproductive cycle. In swine and sheep, embryos must be recovered by surgical techniques. To increase the number of embryos that can be recovered from genetically superior females, the embryo donor is treated with a hormone regimen to induce multiple ovulations, or superovulation. NIFA has supported basic and applied research to enhance the efficiency of superovulation and embryo transfer in livestock species.
In Vitro Fertilization
As an alternative to collecting embryos from donor animals, methods have been developed recently to produce embryos in vitro (in the laboratory). The methods are also called in vitro embryo production. Immature oocytes (female eggs) can be obtained from ovaries of infertile or aged females, or from regular embryo donors (described above). Ovum (egg) pick up is a nonsurgical technique that uses ultrasound and a guided needle to aspirate immature oocytes from the ovaries. Once the immature oocytes have been removed from the ovary, they are matured, fertilized, and cultured in vitro for up to seven days until they develop to a stage that is suitable for transfer or freezing. NIFA has supported a number of basic research projects to understand the physiological mechanisms associated with production of embryos in vitro.
Sex Determination of Sperm or Embryos
The beef industry in the United States prefers male calves, which tend to have higher body weights and higher feed efficiency (compared to female or heifer calves) when placed in feedlots for the growing and finishing stages of meat production. In contrast, the dairy industry prefers heifer calves, which will ultimately produce offspring and milk for human consumption. Thus, methods are needed to determine the sex of sperm or embryos so producers can control the sex of the offspring of their livestock. Using a specific dye that binds to DNA (the Hoechst 33342 stain) and a flow cytometer/cell sorter, the DNA content of individual sperm is measured. In cattle, the X-bearing sperm contain 3.8 percent more DNA than the Y-bearing sperm. In mammals, the presence of a Y chromosome (and one X chromosome) determines that the individual will be a male. Female mammals contain 2 X chromosomes. Although the process to sort the X and Y bearing sperm is slow (approximately 10 million live sperm of each sex can be obtained per hour—this is about the number of live sperm required for one conventional dose of frozen semen for artificial insemination), this procedure determines the sex with higher than 95 percent accuracy.
Nuclear Transfer or Cloning
Since the mid 1980s, technology has been developed to transfer the nucleus from either a blastomere (cells from early, and presumably undifferentiated cleavage stage embryos) or a somatic cell (fibroblast, skin, heart, nerve, or other body cell) to an enucleated oocyte (unfertilized female egg cell with the nucleus removed). This “nuclear transfer” produces multiple copies of animals that are themselves nearly identical copies of other animals (transgenic animals, genetically superior animals, or animals that produce high quantities of milk or have some other desirable trait, etc.). This process is also referred to as cloning. To date, somatic cell nuclear transfer has been used to clone cattle, sheep, pigs, goats, horses, mules, cats, rabbits, rats, and mice.
The technique involves culturing somatic cells from an appropriate tissue (fibroblasts) from the animal to be cloned. Nuclei from the cultured somatic cells are then microinjected into an enucleated oocyte obtained from another individual of the same or a closely related species. Through a process that is not yet understood, the nucleus from the somatic cell is reprogrammed to a pattern of gene expression suitable for directing normal development of the embryo. After further culture and development in vitro, the embryos are transferred to a recipient female and ultimately result in the birth of live offspring. The success rate for propagating animals by nuclear transfer is often less than 10 percent and depends on many factors, including the species, source of the recipient ova, cell type of the donor nuclei, treatment of donor cells prior to nuclear transfer, the techniques used for nuclear transfer, etc. NIFA has supported research projects to obtain a better understanding of the basic cellular mechanisms associated with nuclear reprogramming.