Studying Cell Biology
Preface | Introduction to Problem Solving | Problem Sets | Acknowledgments

To understand the egg’s response to sperm contact, let’s focus on the egg cortex, a region that lies just beneath the plasma membrane, as illustrated in the figure above. The cortex is filled with many small vesicles of uniform size called, not surprisingly, cortical granules; these lie adjacent to and almost touching the plasma membrane.

Once you become familiar with this cortical view, turn to the next figure by placing your cursor on top of the image to examine the same field of view with two difference sets of optics. On the right, differential interference contrast microscopy produces an “optical slice” of a layer of cortical granules just beneath the plasma membrane; on the left, a fluorescence microscope illustrates the same optical slice. The egg is suspended in sea water containing a water-soluble dye that becomes fluorescent in a lipid environment (but which is non-fluorescent in an aqueous one). In this initial frame none of the granules fluoresce, although the overlying plasma membrane (which is out of the plane of focus) is fluorescent.

Click on the image or on this text to play the movie. As the video runs, watch the behavior of the cortical granules, especially those enclosed by green circles: the frame interval is 0.5 sec. Try stepping the video one frame at a time, to characterize the behavior of these four granules, and then consider the questions in the side-bar.

[You may obtain more information about signaling and cortical granules mechanisms in sea urchin fertilization in Terasaki, M. 1998. Mol. Biol. Cell 9:1609-1612. PMID:9658156 (PubMed ID number)]

Questions 1. Describe carefully what happens to the 4 granules, correlating the DIC and fluorescent images. 2. How might the two images be causally related: i.e., how might the change in the left-hand image produce the change in the right-hand image? 3. How might you test your hypothesis? <<Previous Problem