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Oocytes may remain in the ovary for months or years in a quiescent state from which they may be aroused within seconds by fertilization or activation . As a result of activation, the cortex of the sea urchin egg undergoes a radical reorganization that transforms it from a quiescent structure to one that undergoes rapid microvillar formation (2, 3) and membrane recycling (14) . Initiation of this recycling starts about 1 .5 min after activation and involves rapid formation of endocytic vesicles whose contents can later be detected deep within the cytoplasm (5) . The major burst of endocytosis lasts for 3-5 min, after which coated pits and vesicles continue to form at a considerably slower rate . Concomitant with the passage of coated vesicles from the cortex to the interior of the egg is transport ofelectron-lucent vesicles, which we call clear vesicles, from the egg interior to the cortex . These particles, which are probably pigment granules in eggs of Arbacia punctulata, are also found in Strongylocentrotus purpuratus eggs (see Fig. 1 for examples in unfertilized eggs and Fig. 2 for examples in fertilized eggs) . This second type of transport is essentially over by 10-20 min in both species, thus overlapping the inward transport of vesicles derived from coated vesicles and indicating that simultaneous bidirectional transport to and from the cortex occurs naturally . Transport of pigment granules from the interior of the egg to the surface is inhibited by cytochalasin B but not by colchicine (13), although it is not known whether transport of endocytic-derived vesicles and/or clear vesicles is also inhibited . It is known, however, that not all pigment granules are transported to the egg cortex (10-20% remain within the cytoplasm) and that these granules move into the asters at first cleavage, a process inhibited by colchicine . Thus, at least two cytoskeletal elements, actin filaments and microtubules, can participate in the differential transport of a given class of cytoplasmic organelle to different locations within the cell .

Initiation of two-way cortical traffic after fertilization in sea urchin eggs.