The origin of the avian germ line and transgenesis in birds.

The origin of the germ cell lineage in vertebrates is a fundamental question that has preoccupied developmental biologists. Recent work on the origin of the avian germ line has extended and clarified our understanding of the temporal and spatial segregation of primordial germ cells (PGC) during prestreak stages of development. The germ cells first appear at Stage X (Eyal-Giladi and Kochav, 1976) in the ventral surface of the area pellucida in a scattered pattern among polyingressing cells. Subsequently, the PGC gradually translocate from the epiblast to the hypoblast. The entire process appears to be dependent upon the maintenance of an organized area pellucida. Little is known about the regulatory events governing germ cell emergence during this period; however, the culture of dispersed blastodermal cells on a mouse fibroblast feeder layer can compensate for a disorganized area pellucida and offers an in vitro system to examine the molecular basis of germ cell development. Such basic information is valuable for current approaches towards the production of transgenic poultry with targeted changes to the genome through the use of avian embryonic stem cells or primordial germ cells. Refinement of the culture of primordial germ cells or their precursors should allow academic and industrial research laboratories to answer significant biological questions and to improve the genetic potential of commercial poultry stocks. A better understanding of the biology of avian primordial germ cells during early embryo development can only enhance this process.