In many bilaterians, animal–vegetal polarity, gastrulation
and the establishment of antero-posterior polarity are
closely related. In most deuterostomes and many protostomes,
animal–vegetal polarity results from an
asymmetric association between the oocyte and neighboring
ovarian cells and is manifested in early embryology by
spatially restricted maternal mRNA transcripts and/or proteins.
There are, however, important exceptions: an
animal–vegetal pole relationship is irrelevant for centrolecithal
oocytes (e.g. in Drosophila), and localized
maternal messages play only an inconspicuous role in patterning
the early embryos of placental mammals [2].