The Questions of Developmental Biology

Glycogen granules and lipochondrial inclusions store the carbohydrate and lipid
components of the yolk, respectively.
Most eggs are highly asymmetrical, and it is during oogenesis that the animal-vegetal
axis of the egg is specified. Danilchik and Gerhart (1987) have shown that although the
concentration of yolk increases nearly tenfold as one moves from the animal to the vegetal poles
of the mature Xenopus egg, vitellogenin uptake is uniform around the surface of the oocyte. What
varies is its movement within the oocyte, and this depends on where the yolk proteins enter.
When yolk platelets are formed in the future animal hemisphere, they move inward toward the
center of the cell. Vegetal yolk platelets, however, do not actively move, but remain at the
periphery of the cell for long periods of time, enlarging as they stay there. They are slowly
displaced from the cortex as new yolk platelets come in from the surface. As a result of this
differential intracellular transport, the amount of yolk steadily increases in the vegetal
hemisphere, until the vegetal half of a mature Xenopus oocyte contains nearly 75% of the yolk
(Figure 19.22B-E). The mechanism of this translocation remains unknown.
As the yolk is being deposited, the organelles also become arranged asymmetrically. The
cortical granules begin to form from the Golgi apparatus; they are originally scattered randomly
through the oocyte cytoplasm, but later migrate to the periphery of the cell. The mitochondria
replicate at this time, dividing to form millions of mitochondria that will be apportioned to the
different blastomeres during cleavage. (In Xenopus, new mitochondria will not be formed until
after gastrulation is initiated.) As vitellogenesis nears an end, the oocyte cytoplasm becomes
stratified. The cortical granules, mitochondria, and pigment granules are found at the periphery of
the cell, within the actin-rich oocyte cortex. Within the inner cytoplasm, distinct gradients
emerge. While the yolk platelets become more heavily concentrated at the vegetal pole of the
oocyte, the glycogen granules, ribosomes, lipid vesicles, and endoplasmic reticulum are found
toward the animal pole. Even specific mRNAs stored in the cytoplasm become localized to
certain regions of the oocyte.
While the precise mechanisms for establishing these gradients remain unknown,
studies using inhibitors have shown that the cytoskeleton is critically important in
localizing specific RNAs and morphogenetic factors. There seem to be two pathways for
gettting mRNAs into the vegetal cortex (Forristall et al. 1995; Kloc and Etkin 1995, Kloc et
al. 1998). The first pathway moves messages such as those encoding the Vg1 protein,
which are initially present throughout the oocyte, into the vegetal cortex in a two-step
process (Yisraeli et al. 1990). In the first phase, microtubules are needed to bring Vg1
mRNA into the vegetal hemisphere. In the second phase, microfilaments are responsible
for anchoring the Vg1 message to the cortex. The portion of the Vg1 mRNA that binds to
these cytoskeletal elements resides in its 3´ untranslated region.