EMBRYO TRANSFER TECHNOLOGY IN ANIMALS

May 26, 2022

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EMBRYO TRANSFER TECHNOLOGY IN ANIMALS

With the ever increasing demand of the milk and milk industry, technology has also grown to a level of excellence for meeting the requirements of such demands. With the advancement of science and technology, newer and newer approaches are being made for the production of such livestock that would be able to compensate such growing demands. Among such production technologies in vitro maturation and embryo transfer technology (ETT) may be said to have taken an upper hand and thus paving a way for the development of the ‘Animal Husbandry’ sector.

In animal husbandry, breeding and selection for special phenotypic characteristics or superior performances has been conducted by the humans for many years leading to the development of several artificial reproductive technologies (ART). For the veterinarian ARTs like artificial insemination, ETT including superovulation and oestrus synchronization, in vitro production and manipulation of embryos including production of transgenic embryos and chimera, oocyte recovery and oocyte transfer. In this regard cryopreservation of oocytes and embryos plays a vital role in production of offspring using such technologies.

Embryo transfer technology is the most advance technique in animal biotechnological research, and it has attained such perfection that it can now be used on a commercial scale. ETT is the transfer of embryos from one female (donor) to another (recipient) and is a useful tool when it is desirable to separate fetal and maternal genetic effects. Such procedures are useful under some circumstances to produce offsprings from females which able to produce fertile gametes but are not able to carry a pregnancy to term. Interspecies ETT may provide an avenue for propagation of endangered species and lessen the risk of new disease organism being introduced into the herd by bringing in embryos that develop to term in-uteri of females already in the herd.

A female animal has a natural ovulation pattern. But in some animals, particularly in cattle, sheep and horses, there is a physiologically low ovulation rate. Thus to increase the ovulation rate of the females of such species a treatment is intended by the use exogenous gonadotropic hormones. Such treatment is called as superovulation and is intended to increase the ovulation rate as well as the number of available oocyte in the female donor animal without disrupting the physiological and endocrinological processes associated with oocyte maturation, ovulation, fertilization and embryonic development.

The different hormones used for superovulation are pregnant mare serum gonadotropins (PMSG), follicle stimulating hormone (FSH) and crude pituitary extract containing both FSH and luteinizing hormone (LH). In a variable dose rate in some animals use of these hormones between 3-5 days prior to oestrus leads to a considerable increase in the ovulation rate, but in some species this response is unpredictable due to refractile nature of the species. In addition to the health and physiological status of the animal, factors like genetic variability, variable potency of commercially prepared hormones etc. are also responsible for increasing or decreasing the ovulation rate. The use of prostaglandin (PGF) and its synthetic analog in the luteal phase of oestrus cycle for synchronization of oestrus in animals has largely helped to overcome the variable superovulatory response in animals. But repeated injection of the hormone (in large scale superovulatory programme) should be avoided in order to prevent the development of antibody in the donor female.

After the superovulatory regime, the donor animal shows normal oestrus and at this time she should be allowed to breed naturally or, inseminated by artificial method. The ovulated animal usually shows ovulation for an extended period of time, and thus she should be inseminated twice 12-24 hours after signs of oestrus are shown.

Following superovulation, the embryos are recovered from the donor animals using surgical (like midventral laparotomy) or, non-surgical technique. In small animals like sheep and goat, surgical method is the method of choice, but in large animals both the methods are equally effective. However, considering the pre and post operative preparations, care, treatment and expenditures, non- surgical method is the method of choice in large animals under farm conditions, when collection is desired on commercial basis. In different animals, there is difference in the time of migration of the fertilized ova from the fallopian tube to the uterus of the donor animal. Thus the exact time of embryo collection is different in different animal. For a successful embryo collection from a donor animal, knowledge of the reproductive cycle and its associated terms/conditions should be well known by the collector.

The embryo from the donor animal is then collected (by an expert) in a collection media and after it’s appropriate processing, the embryo is transferred to the recipient animal. Before transferring of the embryo, the recipient too, like the donor is synchronized with exogenous drugs like PGF2a. One important point to note here is that certain requirements with regards to parity, age, fertility,

temperament etc. of the recipient animal is to be considered before the embryo is transferred into her body. If the embryo is to be transported to a long distance (before transferring it into the recipient), or, if it is to be transferred later, then cryopreservation (-196oC) technique is used taking into consideration the other prevailing factors and conditions. The transfer of the isolated embryo in the uterus of rabbit for transportation is also possible.

After successful transfer of the embryo into the recipient’s body, the embryo is allowed to develop to term after which the young animal is delivered either normally or through surgical method.

With the advancement in science and technology, now-a-days, it is also possible to know the sex of the embryo isolated from the donor animal. This pre- sexing is done either by cytogenic or by immunogenic method. Splitting of the embryo at certain stage of its development is also now possible, which (the splitted embryo) when transferred to the recipient mother produces identical twins in the recipient mother.

In addition to these, expression of a desired characteristic into the developing embryo can also be done by the production of transgenic animals, where the specific gene of interest is injected in the nucleus of the embryo isolated from the donor animal. Such embryos when develops to term in the recipient’s uterus, the offspring expresses the character of the gene transferred. Thus ETT has emerged out to be a useful technique in order to preserve important genetic material in outbreak of serious diseases, for conservation of germplasm, to introduce new breeds with desired traits, for commercial breeding purpose, for movement of livestock at the embryo level, for research purposes etc.