[SagarikaGhosh]

A transgenic animal contains in its genome a gene or genes introduced by one or the other technique of transfection. The gene introduced by transfection is called transgene. In animals, transfection species the introduction of a DNA segment, either naked or integrated into a vector, into an animal cell. The same phenomenon is known as transformation in all other organism.

Transfection may be transient or permanent (stable). In transient transfection, the introduced gene is gradually lost from the daughter cells of transfected cells. But in case of stable transfection, the introduced gene is retained and expressed in all the cells derived from the transfected cells. Since most of the animal vectors are unstable, i.e., are gradually lost, in the extra chromosomal state, stable transfections are ordinarily due to the integration of introduced gene into the cell genome.

Transgenics have been produced in a variety of animal species, e.g., mice (including rats), rabbits, swine, sheep, goat, cattle, poultry, fish, amphibians, insects and nematodes. However, these activities are so far limited to experimental stages, and production technologies based on transgenic animals have not yet reached commercial applications. Below are a few examples of transgenic animals:

1. Transgenic Mice

Mouse is the most preferred mammal for studies on gene transfers due to its many favorable features like short estrous cycle and gestation period, relatively short generation time, production of several offspring per pregnancy (i.e. litter), convenient in vitro fertilization, successful culture of embryos in vitro at least for a period of time, production and maintenance of ES cell lines, availability of a diverse array of genetic stocks, etc. As a result, the techniques for gene transfer and transgenic production have been developed using mice as models; subsequently, these approaches have been modified to adapt them to other animal species.

2. Transgenic Rabbits

Rabbits are quite promising for gene farming or molecular farming, which aims at the production of recoverable quantities of pharmaceutically or biologically important proteins encoded by transgenes. Transgenic animals used for this purpose are popularly called bioreactors. Generally, the transgenes are designed to be expressed in mammary tissue so that their protein products are secreted in milk. Consequently, the protein product is easily harvested with milk from which it is conveniently purified. The following human genes encoding valuable proteins have been separately transferred into rabbits; interleukin 2, growth hormone, tissue plasminogen activator, antitrypsin etc. In addition, bovine lactalbumin has also been expressed. These genes were expressed in the mammary tissues and their proteins were harvested from milk.