Several ways to generate asymmetry in a developing embryo. (a) Intrinsic asymmetry reflects the existing distribution of cellular components, especially mRNA and protein. The asymmetries are either inherent to the developing oocyte or created during fertilization. (b) Extrinsic asymmetry is generated by cell-cell signaling. Although asymmetry is not necessarily created in any given cell, the cell-cell signals alter the fate of a cell or a group of cells in the embryo, contributing to embryonic asymmetry. The signals can involve direct cell-cell contacts or the action of a secreted, diffusible signal.