Molecular Mechanisms of Germ Line Cell Determination in Animals

Abstract

The principles of the formation of germ line cells are reviewed. Germ line cells are separated from other embryonic cells during early embryogenesis. In some groups of animals, germ cell precursors are induced by surrounding cells. In most taxa, however, cells that have inherited certain maternal determinants (the so-called germ plasm) become primordial germ cells. Germ plasm usually contains mitochondria, electron-dense granules with complex structure, and maternal proteins and RNAs necessary to form germ line cells. In Xenopus, the mitochondrial cloud is the source of material for germ plasm; it also specifically binds maternal RNAs involved in the formation of primordial germ cells and transports them to the vegetal pole. The cis-acting elements that are responsible for the transport of these RNAs are usually located in the 3′-untranslated region of RNA, and their function is mediated by binding with trans-acting protein factors. In addition to specific localization of macromolecules in germ plasm, the special status of germ line cells is ensured by degradation of RNA and the protein components of germ plasm in somatic cells, total suppression of transcription in primordial germ line cells until advanced stages of embryogenesis, and specific regulation of RNA translation in somatic and germ line cells. Experimental data on the characteristics of the germes maternal RNA, which is a novel component of the Xenopus germ plasm, and the protein that it encodes are briefly discussed.