![]() We provide evidence that the primitive streak is not a conserved feature in amniote development and that the mammalian and avian primitive streaks have evolved independently through different supracellular mechanisms that led to their morphological emergence. In this Review, we offer a phylogenetic and ontogenetic overview of the primitive streak and its role in mediating amniote gastrulation, and we discuss the implications of embryonic stem cell–based models of early mammalian embryogenesis on the function of this iconic structure. Interpretation of developmental features observed in these in vitro models requires proper understanding of animal gastrulation in general and of the amniote primitive streak in particular. As such, many countries have established a legal limit of 14 days for the in vitro culture of fertilized human eggs-this is known as the “14-day rule.” In recent years, pluripotent stem cells have become a promising in vitro model for studying the cellular and molecular mechanisms associated with early human development. The appearance of the primitive streak breaks the radial symmetry of the epiblast (a sheet of epithelialized pluripotent cells) and has been suggested to symbolize the emergence of human individuality. In humans, the primitive streak forms ~14 days after fertilization. ![]() Human development also follows this pattern. In many amniote embryos (such as those of reptiles, birds, and mammals), gastrulation has been associated with a transient structure called the primitive streak. These cells are transformed into the recognizable outline of an organism through the process of gastrulation, which endows them with lineage and spatial identities in the context of an emerging coordinate system. ![]() Pluripotent cells are generated by embryonic divisions that occur shortly after fertilization. ![]()
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