Polarity in the mammalian egg

When you get immersed in the Drosophila literature, it's easy to lose sight of an important fact: flies are weird. Honestly, every species has unique properties, so it's not that we wouldn't be saying exactly the same thing about some other species if it had become the dominant experimental model system, but it's good to constantly remind ourselves of the diversity we see in biology.

One of the most noticeable properties of early fly development is the existence of an extensive maternal contribution to the embryo's organization. Mother Fly goes to a fair amount of effort to stamp her eggs with molecular labels that say, "This End Up". Is this a common strategy? Well, yes—lots of animals give their progeny a boost in reliability of development by incorporating an asymmetric distribution of informational macromolecules. There seem to be some exceptions, though, and they happen to be of particular interest to us, because they are us: mammals. Mammalian ova lack any overt asymmetries, and their early cleavages are simple and equal, producing a clump of cells called a morula with no discernible up, down, left, right, back, or front. So how does a young mammalian zygote figure out which end is supposed to be the head and which the tail?

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