Transgenic chickens.

The development of transgenic chicken technology has lagged far behind that of mammalian species. Two reasons for this are that only a one-cell-stage oocyte can be obtained from a sacrificed hen and that the yolk prevents high-magnification microscopic observation of oocytes. Recently, several new methods have been developed that will enable the successful establishment of transgenic chickens. Retroviral vectors are used in many cases because of their ability to incorporate transgenes into host cell chromosomes in a highly efficient manner. These viral vectors are injected directly into the embryos, usually at the blastodermal stage. In some cases, primordial germ cells (PGCs) are infected in vitro and then implanted into recipient embryos. Methods that do not rely on retroviral vectors are also available for creating transgenic chickens. Long-term culture of PGCs permits the selection of stably transfected cells and implantation of the manipulated PGCs. In addition, embryonic stem (ES) cell systems are available; however, the induction of functional gametes from ES cells has not, to our knowledge, been successful. It is clear that recent developments suggest that chickens may be used as a valuable experimental genetic system.