Germ cells are set aside during early development and, in many organisms (including Drosophila melanogaster, Caenorhabditis elegans and Xenopus laevis), they form in a unique cytoplasm, termed the germ plasm. The germ plasm is synthesized during oogenesis, and the initial polarization of the oocyte is likely to determine where the germ plasm will form within the egg cell. Although we do not know how the fate of germ cells is specified in any organism, recent genetic analysis in Drosophila has identified the TGF-α homolog gurken as the signal involved in the initial polarization of the oocyte. These results imply that the limiting steps in the assembly of the germ plasm are localization of the OSK RNA and regulated synthesis of the OSK protein, encoded by oskar, which are components of the germ plasm.
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