Molecular mechanisms of metazoan oocyte development

The gametes are responsible for passing all the genetic information from one generation to another, giving rise to all the tissues in a developing animal, and ultimately guaranteeing the survival of species. The formation of germ-line stem cells occurs during early development in all animals. The differentiation of these pluripotent cells into mature gametes provides a continuous supply of sperms and eggs during adult life. Many aspects of germ-line development are conserved across species. For example, in most metazoans, female primordial germ cells (PGCs) migrate from an extragonadal site of origin to reach the somatic gonad and to produce oocytes. After a mitotic proliferative stage, the primary oocytes enter meiosis. In most animal species this process is arrested during prophase, and is completed only in response to intercellular signaling or fertilization, which trigger oocyte meiotic maturation. After the arrest, the oocyte synthesizes and stores a large amount of mRNAs that will be translated only during re-entry into the meiotic division both to promote oocyte maturation and early embryonic development. Translational control is obtained through a complex regulation carried out by different but highly conserved molecular mechanisms. Here we review the basic principles that underlie oocyte development, focusing on analogies and differences among the main model organisms.