About the Cover
Cover
Mature eggs of the surf clam Spisula solidissima are arrested in prophase of the first meiotic division. When the eggs are activated by sperm or KCl, the first meiotic division is completed, the second meiotic division follows, and then (with sperm) embryonic development ensues. As in all meioses, the cleavages are very eccentric, and each produces a small polar body. Because very large numbers of Spisula eggs can be activated simultaneously, and thus form their polar bodies in near synchrony, these eggs are an excellent model with which to study, not only the usual embryonic cell division, but also polar body formation-an example of extremely asymmetrical cytokinesis.
In this issue of The Biological Bulletin (pp. 192-193), Rafal Pielak, Valeriya Gaysinskaya, and William D. Cohen report on the organization of F-actin and microtubules in meiotic stages that immediately precede the formation of polar bodies in Spisula eggs. The movements and locations of these structures were revealed by confocal fluorescence microscopy after appropriate staining (F-actin, red-orange; microtubules, green; chromosomes, blue-violet). Four images from the report-set upon a background of diagrammatic surf clams-appear on the cover (see scale bars in Fig. 1, p. 193).
At about 13 min after activation (23C), the metaphase spindle of the first meiotic division is already fully formed and eccentrically positioned; it then moves toward the cell surface. In the upper left image on the cover, microtubules of the peripheral aster curve outward along the F-actin-containing cortex, away from a microtubule-poor central region. At about 20 min post-activation, with the aster diminishing, the chromosomes are now arranged in anaphase (upper-right image), and a "bulls-eye" F-actin ring-the cytokinetic ring-appears on the cortex (side view, upper right; computer-generated face view, lower left). Finally, at about 26 min post-activation, the peripheral nucleus and its remaining centrosomal material enter the F-actin-poor center of the ring to produce the first polar body (lower-right image).
These stages include critical activities-particularly, docking of the spindle with the cell cortex, and signaling to generate the cytokinetic contractile ring-that occur in all sexually reproducing animals by mechanisms yet unknown. But note that, at metaphase, the pattern and dimensions of the contact between the astral rays and the egg cortex approximate those of the F-actin ring at anaphase. This correspondence suggests that generation of the contractile ring is triggered by signals from the astral microtubule-cortex contact.
Rafal Pielak and Valeriya Gaysinskaya were summer 2003 research interns in Hunter College-Howard Hughes Medical Institute (HHMI) Undergraduate Biological Science Education Program at the Marine Biological Laboratory. The surf clam pattern on the cover was designed by William D. Cohen. The cover was designed by Beth Liles (Marine Biological Laboratory).
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Copyright © 2005 by the Marine Biological Laboratory.
