CYTOSKELETON AND EGG POLARITY

The eggs of most Metazoa are highly asymmetrical. A typical metazoan e gg is spherical or spheroidal in shape, but has animal and vegetal pol es which are easy to distinguish. Egg polarity is thought to be a prim itive feature, acquired by a common ancestor of all living Metazoa. Pr esumably this precursor was a two-cell type organism, comprising a som a and a germ line. The alleged ancestor developed by a simple, two-way cell differentiation process driven by factors localized in egg cytop lasm. Egg polarity probably arose as a consequence of the basic asymme try imposed on all animal cells by the position of the centrosome with respect to the nucleus. It is proposed that in oocytes of the earlies t Metazoa the centrosome-nucleus pair induced a bipolar organization, defining an animal-vegetal axis. The animal pole is the site near whic h the cell nucleus lies, and where the polar bodies are emitted. The v egetal pole forms on the side of the cell defined by the initial posit ion of the centrosome. It is further argued that a cytoskeleton-based localization system was used to concentrate a special type of macromol ecules near the vegetal pole of the oocyte. These macromolecules becom e sequestered by a fraction of embryonic cells, causing them to differ entiate along the germ line.