Transgenic chickens hold great promise in basic biological research and in industrial applications. A number of different strategies designed to allow the manipulation of the avian genome have been investigated with varying degrees of success. Infection of chick embryos with retroviral vectors has successfully produced transgenic chickens by exploiting the natural abilities of retroviruses to enter cells and integrate into the host chromosomes. However, technical and safety considerations limit the usefulness of retrovirus-mediated gene transfer, particularly in agricultural applications. Microinjection of DNA into the fertilized ovum has also successfully produced transgenic birds. Unfortunately, microinjection is a difficult procedure which is only rarely successful. The direct transfection of early embryos in ovo has been attempted, but only transient gene expression has been observed. Sperm-mediated gene transfer has been suggested, as DNA can associate with chicken sperm cells, however, no transgenic chickens with stably integrated transgenes have been produced by this method. Transgenesis via the use of chimeric intermediates constructed with primordial germ cells or blastodermal cells has been proposed to allow precisely designed gene targeting experiments to be carried out. While it is now possible to routinely produce chimeras using both primordial germ cells and blastodermal cells, long-term culture techniques which would permit the genetic modification of blastodermal or primordial germ cells by homologous recombination in vitro are not yet available. Overall, despite significant progress, much research is still required in order to establish practical, efficient and economical techniques for the production of transgenic chickens.
