Mount Holyoke College

Sea Urchin Embryos:
Skeletons and Polarized Light


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(Two movies on this page)

These videos were made by Alexandria Younossi, a student in Bio 305 (fall 1999). Sea urchin eggs were fertilized and the embryos raised in seawater until the gastrulae were swimming around in the dish. She filmed the embryos using polarized light. The skeletal rods are made of magnesium calcite, and are birefringent. This means that the mineral interacts with polarized light to change the orientation light as it passes through the material. The rest of the embryo is not birefringent: if you look at it through crossed polarizing filters, it appears dark--no light gets through. The spicules (made of the birefringent calcite), however, are seen to glow when viewed through crossed polarizing filters. Note that the two halves of the skeleton are not exactly aligned optically--one half can be glowing with the other half dark, and then they change appearance as the embryo reorients. (The reasons that the background is not black, and that you can see some of the tissues of the embryo, are twofold: first, the polarizing filters are not exactly crossed, and second, we used fairly inexpensive polarizing plastic to modify our microscope.)

The movie above is of older embryos, at a higher magnification. The larvae had reached the pluteus larval stage. The word pluteus (plutei= plural) comes from the German word for easel. While filming the plutei, Alexandria rotated the polarizing filter. When the background is dark, the two polarizing filters are at cross angles, and the skeletal rods are most visible. When the polarizer is rotated so that the background is light, much more of the actual embryonic tissues are visible (ectoderm, gut, etc.)