Comparative development of avian primordial germ cells and production of germ line chimeras

Primordial germ cells (PGCs) are the precursors of gametes and serve as the vehicle for genetic modification of successive generations. Despite their importance, the molecular and cellular components involved in the establishment of the avian germ line have yet to be defined. This article presents a current and comparative view of avian germ cell development from its origin in the cleavage stage embryo until sexual differentiation. Recent advances have enabled the identification of the germ cell lineage at very early stages of development and for an extended period of embryogenesis. In the preoviposition embryo, germ cell origin can be examined using an antibody to germ line-specific chicken vasa homologue (CVH) protein. The protein is found in the cleavage furrows and gradually is included into blastomeres that give rise to PGCs or their precursors. At oviposition germ cells can be identified in the central area of the blastoderm and subsequent morphogenetic movements passively displace them to the anterior of the embryo. PGCs then undergo a transitory period of circulation through the blood vessels, followed by an active migration through the mesenchyme to the developing gonad. The interaction of PGCs with the extracellular matrix, somatic cells and other germ cells is described. A timeline of markers expressed on germ cells from cleavage until spermatic! and oocyte formation is also provided. The review concludes with an evaluation of various approaches in the production of germ line chimeras. In general, the identification of factors controlling the origin, migration and proliferation of PGCs will provide information essential for the application of avian germ cell biology for the genetic modification of poultry.