The germline is unique in mammals as it is the only cell lineage that undergoes mitosis, meiosis, and differentiation. The lineage is comprised of an array of developmentally distinct stages from the germinal stem cell to the terminally differentiated sperm or oocyte. Elucidation of the biology of germline development as well as germinal stem cell maintenance or differentiation would be of great impact to both the biomedical as well as agricultural communities. For example, improvements in assisted reproduction (in humans) and cloning (in animals) technologies are dependent on advancing current in vitro systems towards more efficient oocyte development. Similarly, the development of methodologies to maintain germinal stem cell systems for male or female instances of spontaneous or drug/toxin-induced infertility in humans or, in the case of agricultural animals, for alternative modes of cryopreservation to maintain germplasm or to improve transgenic technologies for the enhancement of production traits and the generation of animal models with nutraceutical worth, are all important.