Germ Cell, Stem Cell, and Genomic Modification in Birds

Primordial germ cells (PGCs), the precursors of germ cells, form during early embryonic development. To date, studies on the PGCs of many species have investigated specification, proliferation, and differentiation, and have created various applications using model animal systems. Avian species are optimal vertebrates in which to examine germ cell development during embryogenesis, because of their oviparity. Earlier studies on PGCs focused principally on the locations and numbers of such cells. To identify PGCs in early stage embryos, PGC-specific markers including the periodic acid-Schiff (PAS) reagent [1,2] and anti-stage specific embryonic antigen-1 (SSEA-1) antibody [3], were used. During early embryonic development, chicken PGCs exhibit a unique migratory activity (toward the genital ridges) [4]. PGCs in the central zone of the area pellucida move toward and gather in the anterior region of the embryo, the germinal crescent of Hamburg and Hamilton stage 4 (HH 4). Subsequently, PGCs invade blood vessels, and migrate therein, settling in the genital ridges, where they finally differentiate into functional gametes [5]. The unique migration pathway of chicken PGCs (through the circulation) is unlike that of mammalian PGCs (which migrate through the dorsal mesentery [6]), and makes it possible to isolate, manipulate, and re-transplant such cells.