The hard cell(s) of avian transgenesis.

After 25 years, the search for the avian cell that can be cultured indefinitely, genetically modified, and clonally derived while retaining its ability to enter the germline has ended. van de Lavoir et al. [2006a, Nature 441:766-769] have defined the conditions for culture and genetic modification of primordial germ cells (PGCs) and shown that these cells are transmitted at high rates through the germline. The advent of this technology provides the ability to introduce transgenes of any size and to make site-specific changes to the genome. Although PGCs are committed to the germline, they can be induced into somatically committed embryonic germ (EG) cells by changing the culture conditions. EG cells resemble embryonic stem (ES) cells that are also committed to the somatic lineages (van de Lavoir 2006b, Mech Dev 123:31-41). These cell-based systems facilitate insertion of larger transgenes that provide high level, developmentally regulated and tissue-specific expression in transgenic chimeras and their offspring. Following introduction of a transgene, high-grade somatic chimeras can be made with ES and EG cells within 4 weeks and 4 months respectively, allowing quick assessment of the transgenic phenotype. Following introduction of a tansgene into PGCs, high-grade germline chimeras can be made within 8-9 weeks and the high rate of germline transmission of G0 chimeras produces a large cohort of transgenic chicks in 16-17 weeks. PGC, EG and ES cells can be grown in conventional laboratory settings and small flocks of recipient birds or third-party vendors can supply recipient embryos to make somatic and/or germline chimeras. In general, animal management is routine although some specialized equipment and technical skill is required to incubate chimeras in surrogate shells.