Embryonic stem cells (ESCs) were isolated in the early 1980s from mouse and in the late 1990s from primate and human. These cells present the unique property of self-renewal and the ability to generate differentiated progeny in all embryonic lineages both in vitro and in vivo. The mESCs (mouse embryonic stem cells) can contribute to both somatic and germinal lineages once re-injected into a recipient embryo at the blastocyst stage. In avian species, chicken embryonic stem cells (cESCs) have been isolated from the in vitro culture of early chicken blastodermal cells (cBCs) taken from stage X embryo (EG&K) These cESCs can be maintained under specific culture conditions and have been characterized on the basis of their morphology, biochemical features, in vitro differentiation potentialities and in vivo morphogenetic properties. The relationship between these cESCs and some of the chicken germ cells identified and grown under specific culture conditions are still under debate, in particular with the identification of the Cvh gene as a key factor for germ cell determination. Moreover, by cloning the avian homologue of the Oct4 mammalian gene, we have demonstrated that this gene, as well as the chicken Nanog gene, was involved in the characterization and maintenance of the chicken pluripotency. These first steps toward the understanding of pluripotency control in a non-mammalian species opens the way for the development and characterization of putative new cell types such as chicken EpiSC and raises the question of the existence of reprogramming in avian species. These different points are discussed.Key words: blastoderm, chicken, germ cells, Nanog, Oct4, stem cells.
Totipotency and pluripotency in animal cellsPioneer works carried out both in invertebrates (sea urchin) and vertebrates (newt) demonstrated as early as the last decade of the 19th century the existence of totipotent and pluripotent cells in early embryos. Indeed, Driesch (1891) demonstrated that a small sea urchin blastoderm dice will develop and form two independent animals whose size will be smaller than the parental one. This phenomenon is known as blastomere totipotency. Spemann replicated these results in 1902 with newt blastomeres taken at a two-cell stage and demonstrated that a newt larva could be split in two equal parts at 4, 8 or 16 blastomeres and regenerate two complete embryos. At this stage, the blastomeres are considered as pluripotent, that is, they are able to participate in the development of different embryonic tissues but not to reconstitute autonomously a whole organism.The development of mammalian embryos takes place exclusively in the mother's body. After fertilization, the egg will enter into division and form a small cluster of cells called the morula. It will be compacted and the most outer cells will form the trophectoderm annexes, which will give rise to extra-embryonic tissues while the inner cell mass (ICM), derived from the inner part of the morula will form the future embryo. During this expansion process a cavity will b...