Unscrambling the oocyte and the egg: clarifying terminology of the female gamete in mammals.

Most reproductive biologists who study female gametes will agree with the sixteenth century anatomist William Harvey's doctrine: "Ex Ovo Omnia." This phrase, which literally translates to "everything from the egg," recognizes the centrality of the egg in animal development. Eggs are most impressive cells, capable of supporting development of an entirely new organism following fertilization or parthenogenetic activation. Not so uniformly embraced in the field of reproductive biology is the nomenclature used to refer to the female germ cell (Wessel, 2009). What is an oocyte? What is an egg? Are these terms the same, different, interchangeable? Here we provide functional definitions of the oocyte and egg, and how they can be used in the context of mammalian gamete biology and beyond.