Biotechnology: Its Application to Animals

Biotechnology in Animal Reproduction

George E. Seidel, Jr., professor of physiology and biophysics, Animal Reproduction Laboratory, Colorado State University, Fort Collins.

Gametes, Embryos, and Fetuses

Animals pass on their genetic characteristics to the next generation via cells called gametes, sperm for the male and eggs or oocytes for the female. An embryo is formed when an oocyte is fertilized by a sperm, and development of a new animal begins.

About every 20 hours, embryonic cells duplicate their genes and divide, progressing through the 2, 4, 8, and 16-cell stages, etc. The embryo floats freely in the lumen of the female reproductive tract for the first 1 to 4 weeks depending on the species, and then it attaches to the lining of the uterus, a process called implantation. The embryo is termed a fetus when recognizable organs form such as the brain and heart.

The sperm, the oocyte before fertilization, and the embryo before implantation all can be removed from the reproductive tract for various biotechnological purposes without damaging them. This has led to the development of such techniques as artificial insemination, in vitro (outside the body) fertilization, and embryo transfer, which is the replacement of the embryo into the female reproductive tract for gestation to term.

Availability of Gametes and Embryos

Nature has gone to great lengths to insure that animals reproduce. For example, each male of most farm animal species produces trillions of sperm each year, yet under natural conditions sires only a few dozen offspring per year. Fertilization of each ovum requires only one sperm. Female farm animals usually produce a few offspring (with swine a few litters) in their lifetime, yet their ovaries contain hundreds of thousands of oocytes. The unused oocytes degenerate within the ovaries at a rate of several dozen each day.

One major principle of biotechnology is to take advantage of the huge numbers of sperm and oocytes from genetically superior animals that ordinarily would be wasted by degeneration of oocytes and loss of sperm in urine.

Characteristics of Gametes and Embryos

The oocyte is the largest cell in the body, but a microscope still is required to observe it because it is only 1/200 of an inch in diameter. The sperm is one of the smallest cells in the body, about 1/6000 of an inch in diameter. By the end of the first week of development, the embryo grows to more than 100 cells, but it remains about the same size as the oocyte at fertilization. Thus, the embryonic cells get smaller and smaller during the first cell divisions.

Gametes and embryos are quite resilient to manipulation in vitro provided that a proper environment is maintained. Sperm are kept in vitro for up to several days, and we are slowly learning to keep embryos healthy in vitro throughout the pre-implantation period.

Biotechnology Techniques for Gametes and Embryos Recovery and Transfer. Sperm are collected with a device called an artificial vagina. Depending on the species and other factors, oocytes and embryos are collected and transferred either by minor surgical intervention or by nonsurgical procedures. They are recovered by irrigating the reproductive tract with a medium consisting primarily of salt and water.

Females usually are treated with hormones to cause superovulation, so that more mature oocytes or embryos can be harvested than is normal for the species. Superovulation can be used to amplify reproductive rates of valuable animals because the embryos are transferred to less valuable females for gestation. Embryo transfer is done similarly to artificial insemination; that is, a catheter is inserted into the lumen of the female reproductive tract, and the embryo is expelled in a few drops of medium.

Genetically identical twin calves were produced by splitting a single embryo.

Cryopreservation. One of the most useful biotechnological procedures for both sperm and embryos is cryopreservation. Cooling to the temperature of liquid nitrogen ( 320 F) is done in a medium containing chemicals called cryoprotectants. Sperm and embryos can be kept in suspended animation at this temperature for decades, and then thawed, resulting in normal offspring. Storage may be successful at this temperature for hundreds, if not thousands, of years.

Cryopreservation provides great flexibility for various applications. Semen can be stored, eliminating the need to have males in proximity when semen is used. Semen and embryos can be sent from country to country inexpensively.

Strains of animals that are no longer of economic importance can be kept frozen at low cost for a future genetic resource, and animals that have been dead for years can become genetic parents.