Have of the fully-formed organism. Many of these

Have you ever wondered how different parts of a developing embryo know what kind of tissue to become? What prevents two heads from developing on the same embryo, or for that matter, what ensures that a head develops at all? A small group of cells called Spemann’s organizer determines the fates of the cells around them.

Early Experiments in Embryology

Before the 1880’s, the field of developmental biology had mainly been descriptive.

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Researchers would observe developing embryos and describe the changes that they saw. Now, a lot can be learned using this technique, but several developmental biologists recognized that a lot more could be learned by experimentation and manipulation of embryos.Wilhelm Roux was an early proponent of embryo manipulation. In 1888, he published results that showed if one cell of a two-cell stage amphibian embryo was killed, only half of the animal would develop.

However, at about the same time, another biologist named Hans Driesch was performing similar experiments with sea urchin embryos. He was able to separate the two cells of the embryos and keep them both alive. In 1892, he published results showing that each of the separated cells were capable of forming a complete (but half-sized) sea urchin embryo on their own.These two different experiments raised more questions than they answered. The scientific community wasn’t sure what to make of the conflicting results.

Some believed Roux’s results and argued that this was proof that even at the two-cell stage each cell had a predetermined fate and was destined to become a specific set of tissues and organs. Many of these scientists felt that since Driesch’s experiments were done in sea urchin embryos, the results weren’t representative of what happened in vertebrates.However, there were many scientists who put more credence in Driesch’s experiments. They believed that this was evidence that each cell in the early embryo was capable of generating all of the organs and tissues of the fully-formed organism. Many of these scientists wondered if Roux’s method of killing one cell in the embryo was somehow affecting the remaining cell and its development.Then along came Hans Spemann.

Using very fine hair from a human baby, he was able to separate the cells of amphibian embryos at the two-cell stage and keep them alive by tying a knot between them. The results of this experiment were very surprising. In some cases, both of the separated cells produced fully-formed, half-sized embryos, but in others, one cell would form a complete half-sized embryo, and the other would form only the ventral half of the embryo.

The Three Major Body Axes

Now, you may not be too familiar with the different body axes, but the dorsoventral axis comes into play here, so let’s take a minute to talk about it and the other body axes. The dorsoventral axis organizes the body from back to belly.

All vertebrates, and many other animals, have a dorsal side, which is the back side of the animal. Opposite the dorsal side is the ventral side, or underbelly side, of the animal. A second axis, called the anteroposterior axis, organizes the body from head to tail. The anterior end of the animal is located at its head. Opposite the anterior end is the posterior end, which is located at the tip of the animal’s tail, or where the tail would be if that particular animal doesn’t have one. The third major body axis is the left-right axis, which organizes the body from left to right.

Spemann’s Organizer

Okay, now that we’re all familiar with the different body axes, let’s get back to Spemann’s experiments, which on the surface might not make a whole lot of sense. After all, what good is the experiment if sometimes both cells could form a complete embryo, but other times one of the cells formed only the ventral parts of the embryo?Fortunately, Spemann was very observant, and he noticed that if the first cleavage separated the future dorsal and ventral sides of the zygote, the cell from the dorsal side could form a complete embryo, and the cell from the ventral side produced only the ventral portion of the embryo. However, if the first cleavage left both cells with part of the dorsal side of the zygote, then both cells could form a complete embryo.Spemann interpreted this result to mean that the dorsal part of the zygote contained some sort of ‘organizer’ which could direct the development of other cells in the embryo. He and his graduate student, Hilde Mangold, began performing a series of grafting experiments. Using gastrulation-stage amphibian embryos with different levels of pigmentation, they transplanted a chunk of cells from the dorsal lip, or the dorsal side of the lip of the gastrula, of one embryo to the opposite side of a second embryo.

Amazingly, the transplanted dorsal lip induced a second set of body axes and structures in the embryo. Not only that, but the difference in pigmentation between the graft and the host embryo allowed Spemann and Mangold to demonstrate that the graft was directing cells from the host embryo to change their fates and become structures and tissues they would not have become without the transplanted organizer.Because of its ability to direct the differentiation of cells around it and organize body structures and axes, the dorsal lip of the amphibian gastrula is now called Spemann’s organizer. Now you may remember that gastrulation in mammals happens differently than it does in amphibians. As a result, mammals don’t have a dorsal lip, but they still have an organization center that has the same functions as Spemann’s organizer, which is called the node.

You may also hear the term Hensen’s node, which refers to the equivalent of Spemann’s organizer in birds.

Lesson Summary

Let’s review. All vertebrates, and many other animals, have three main body axes. The dorsoventral axis organizes the body from back to belly. The dorsal side is the back side of the animal.

Opposite the dorsal side is the ventral side, or underbelly side, of the animal. The anteroposterior axis organizes the body from head to tail. The anterior end of the animal is located at its head.

Opposite the anterior end is the posterior end, which is located at the tip of the animal’s tail, or where the tail would be if that particular animal doesn’t have one. The third major body axis is the left-right axis, which organizes the body from left to right.In the 1880’s, developmental biologists began doing experiments that manipulated developing embryos in different ways to try to understand how things like cell differentiation and body patterning were controlled. Among these early experiments, the work of Hans Spemann stood out from the rest.

He was able to separate the cells of amphibian embryos at the two-cell stage without killing either cell.In these experiments, he observed that if the first cleavage separated the future dorsal and ventral sides of the zygote, the cell from the dorsal side could form a complete embryo, and the cell from the ventral side produced only the ventral portion of the embryo. However, if the first cleavage left both cells with part of the dorsal side of the zygote, then both cells would form a complete embryo.

Spemann interpreted these results to mean that the dorsal part of the zygote contained some sort of ‘organizer’ which could direct the development of the other cells in the embryo. He and his graduate student, Hilde Mangold, then used gastrulation-stage amphibian embryos with different levels of pigmentation in a series of grafting experiments. They transplanted a chunk of cells from the dorsal lip, or the dorsal side of the lip of the gastrula, of one embryo to the opposite side of a second embryo.Amazingly, the transplanted dorsal lip induced a second set of body axes and structures in the embryo. These experiments resulted in the discovery of Spemann’s organizer, or the dorsal lip of an amphibian gastrula which is capable of organizing the structures of the developing embryo. Eventually, the bird equivalent of Spemann’s organizer was found and named Hensen’s node.

This was followed by the discovery of the mammalian equivalent, which was simply named the node.

Lesson Objectives

After watching this lesson, you should be able to:

  • Explain the conflicting results of Wilhelm Roux’s and Hans Driesch’s embryo experiments
  • Define the dorsoventral axis, the anteroposterior axis and the left-right axis
  • Summarize Hans Spemann’s embryo experiments and their results
  • Describe the function of Spemann’s organizer
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