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

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 of electron-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 Strongy-locentrotus purpuratus eggs (see Fig. 1 for examples in unfer-tilized 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. Allen and Rowe (1) showed that Arbacia eggs in capillaries could be fertilized from one side, resulting in a partially activated egg as indicated by cortical granule breakdown and formation of a partial fertilization membrane. They also found that pigment granules moved to the cortex only in the region in which cortical granule breakdown occurred. Since cortical granule breakdown is rapidly followed by formation of bundles of actin filaments in microvilli that extend deep into …