Germ line, stem cells, and epigenetic reprogramming.

The germ cell lineage has the unique attribute of generating the totipotent state. Development of blastocysts from the totipotent zygote results in the establishment of pluripotent primitive ectoderm cells in the inner cell mass of blastocysts, which subsequently develop into epiblast cells in postimplantation embryos. The germ cell lineage in mice originates from these pluripotent epiblast cells of postimplantation embryos in response to specific signals. Pluripotent stem cells and unipotent germ cells share some fundamental properties despite significant phenotypic differences between them. Additionally, early primordial germ cells can be induced to undergo dedifferentiation into pluripotent embryonic germ cells. Investigations on the relationship between germ cells and pluripotent stem cells may further elucidate the nature of the pluripotent state. Furthermore, comprehensive epigenetic reprogramming of the genome in early germ cells, including extensive erasure of epigenetic modifications, is a critical step toward establishment of totipotency. The mechanisms involved may be relevant for gaining insight into events that lead to reprogramming of somatic cells into pluripotent stem cells.