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Insight from the Sea Urchin
How a spiky invertebrate helps unlock secrets of genes, reproduction, and cancer
The fact that urchin eggs divide sychronously is one of the things that makes them so valuable for research. Watch a movie of many urchin eggs dividing at once.

Uncovering the role of chromosomes

The observations of Hertwig and others made it clear that reproduction involved the donation of a nucleus from both a male and a female. But what did that mean in terms of inheritance? Scientists of the time knew that offspring inherited traits via some substance that carried parental information, and they believed this substance was found in the nucleus. Seeing two nuclei fuse implied that each made a contribution to inheritance. But what specific role did each play?

In 1901, German biologist Theodor Boveri began to answer this question. By that time, Boveri had discovered that mature egg cells have half as many chromosomes—complex coiled structures containing DNA, found inside the nucleus-as did other cells, and that egg and sperm donate equal numbers of chromosomes. Around this same time, Boveri also became familiar with the work of Gregor Mendel, a Belgian monk who, fifty years earlier, had noted patterns of heredity that emerge over generations, but whose findings had gone largely unnoticed.

Boveri performed several ingenious experiments using sea urchin embryos to demonstrate two new points: a fertilized egg must have a specific number of chromosomes to develop normally, and each individual chromosome contributes different qualities.

These findings, combined with what Boveri knew from Mendel about patterns of inheritance, led him to a grand conclusion: chromosomes are the carriers of inheritance. This is something we take for granted today, but just over a century ago, it was a major discovery.

One becomes two, two becomes four—and so on

    What makes urchins great
    models for research?

  • A developing urchin embryo is somewhat transparent, letting researchers see what's happening inside.
  • Cells in developing embryos divide at the same time, and a group of fertilized eggs also tend to divide synchronously.
  • It's easy to obtain and fertilize thousands of urchin eggs at once.
  • Having thousands of eggs at once makes it possible to isolate substances that occur only in very small quantities in each urchin.
Due in part to Boveri's work, scientists had a better understanding of the mechanisms behind fertilization and inheritance. But as is common in science, these timeless discoveries prompted still more questions. Once an egg cell is fertilized, what makes it start dividing? How does each new cell get the right chromosomes? Again, research on the urchin would lead to more answers.

In order to tackle such questions, researchers must capture chemical "snapshots" at different points during the process of cell division. By comparing these chemical profiles, they can see the biochemical changes that occur as a cell divides. But to get large enough amounts of these chemicals to study in the first place, scientists need a lot of embryos going about their business, all doing the same thing at the same time. Urchin embryos, it turns out, divide synchronously in two ways: cells in the same egg tend to divide at the same time, and a group of eggs tend to undergo cell division at the same rate. These synchronies make urchins perfect subjects for studying questions of cell division.

Shortly after World War II, American biologist Daniel Mazia and Japanese biologist Katsuma Dan, took advantage of the urchin's tendency to divide in synchrony to collect a sufficient quantity of cells and isolate the structures that pull sets of chromosomes apart. Identifying these bits and pieces, now known as the mitotic apparatus, was a major accomplishment. Today, researchers still strive to understand the subtleties of how the microtubules and proteins of the apparatus work to assure that new cells have the right complement of chromosomes.

Next: Knowing the why behind the how. »