Cell Differentiation: The Search For The Organizer (1984)

[Music] In the early spring, two salamanders begin their courtship and mate. Then responding to an ancient instinct, the female folds a leaf into an underwater cradle where she lays a fertilized egg. As salamander eggs grow into embryos, their early development parallels the growth of human embryos. So, we can learn more about ourselves by collecting amphibian eggs and watching them grow in the laboratory. The eggs will reveal a process common to all animals on Earth. As one-sellled eggs become complex individuals with billions of cells, each cell specialized to perform a specific function. The cell divides into two cells, the two into four. The embryo is developing. [Music] As cell division goes on, each of the cells looks much the same at first. [Music] Inside the developing embryo, an open space is forming, creating a hollow ball or [Music] blastula. As cell division continues, a dimple called the blasapore begins to form. as cells start migrating inward to line a newly developing cavity. As cells move in, some are destined to become embriionic muscle, some stomach and [Music] intestines. A last movement of streaming cells pushes in nearly all the yolk fils. Eventually, only a small plug of yolk cells remains on the surface of the developing embryo. This yolk plug area will become the embryo's posterior. The other end will be its head. If we look at the embryo from above, we will see its nervous system begin as two parallel ridges of tissue along its back. Gradually, the ridges fold over and fuse together, enclosing cells that will form the brain and spinal cord. Originally, the cells simply increased in number. Now, they're also becoming specialized or differentiated, forming specific tissues and organs. Scientists have devised many experiments to study differentiation more closely. To embryologist Hanspayon, the embryo was, in his words, a conversational partner who must be permitted to answer in his own language. Bayon's genius lay in asking the right questions. One question he asked was when do permanent differences appear among an embryo's cells. He wondered does it happen sometime during cell division or is it determined at the very beginning in the substance of the undivided egg? If that is so, is one half of the egg committed to building a head, gills, eyes, a brain, and the other to building only the rest of the [Music] salamander? Schamman found a unique way to determine if the halves of the eggs were already committed to these separate directions of development. First, he tied a strand of baby hair into a loop. Then, just before a salamander egg divided for the first time, he slipped the loop around it, dividing it vertically. [Music] [Music] reasoned that if the two halves of an egg are already differentiated, then one half might form a head while the other might form a [Music] tail. In fact, something different happened. By the end of the first day, both halves of the egg were growing separately, except where they shared a few cells within the loop of hair. Each side had developed its own blasapore and underwent cell migration. Soon the embryo developed two complete spinal cords. When the experiment was performed with the loop tied tightly, two separate complete embryos formed. When the hair was tied loosely, a secondary embryo formed that was connected at the location of loop. If each of the halves can form an embryo, then permanent differences must not begin until after an egg has divided, perhaps several times. Speman tried dividing the embryo after it had developed a blastoapore so that the blasapore was only in the dorsal half on the left here. He found that this half will form a spinal cord. It will continue to grow almost normally differentiating into complex organs and tissues. But the other half without a blast will remain a stunted unorganized mass that called a belly. Schamman concluded that the formation of a blasapore marks the moment when permanent differences start to appear among the embryo [Music] cells. Is it possible to determine the future of the cells that migrate through the blaspore? Another embryologist, Valter Folk, by staining parts of the developing embryo with harmless dyes, was actually able to follow the migrating cells. With a good microscope and a steady hand, we can repeat Folk's experiments. First, we'll prepare small bits of colored agar to act as tiny paint brushes. We place these bits of agar on the surface of an egg, one by one. [Music] Now the bits are removed but the stains [Music] remain. Dyed cells migrate normally into the blastapore unaffected by their tags of color. [Music] The cells seem especially active on the upper or dorsal lip of the blaster. [Music] A cross-section of the blastula would show its hollow interior, the blastoil. Above is a single layer of tissue called ectoerm. Tissue below is called endoderm formed of large yolk fil cells. Here on the lower right are some of our initial colored stains about to move into the blastapore. Stained cells continue to migrate through the blasapore. Inside, some cells are forming meoderm tissue that lines the roof of a new cavity called the aranton. This cavity will become the digestive tract. Now we add new areas of stain to the outer surface of the developing embryo, the ectoerm. [Music] As the yolk plug disappears, two ridges form along a thickened area of cells on the embryo's back. [Music] [Music] The ridges fold over and join, encasing the thickened area that will become nervous tissue. Over it, a thin transparent layer of other ectomal cells will form the embryo's skin. Underneath, dyed cells are still faintly visible. They are elongating, pushing their way forward inside the embryo's body to form the spinal cord and brain. On the basis of such observations, Walter Folk constructed what he called a fate map. It outlined several areas of cells on a blast surface and predicted their future development. Folks suggested that adult organs can in every case be traced to specific regions on an early embryo's surface. We can check this idea by watching the process backwards. [Music] began to suspect that one part of an embryo might induce other parts to develop in certain ways. And his assistant, Hilda Mangold, asked the question, what would happen if cells from one region of an embryo during its early development were transplanted to another? They found that when cells destined, for instance, to become nervous tissue were transplanted into a region of future epidermis, the cells became epidermal cells. But what would happen if the dorsal lip of a blast were cut out and moved? To recreate and mongold's experiment, we can prepare a salamander embryo that has just formed a blast. To prepare the cell for micro surgery, we need to peel away a thin membrane. This will allow us to get at the embryo's tender [Music] surface. In another dish, we have a second embryo ready. This one has been dyed blue. The membrane must also be removed from this cell. When a bit of its tissue is transplanted to the first embryo, the blue color will indicate the grafted cells. With a fine tungsten needle, we cut out a small group of cells from the blue embryo's dorsal lip. This will be our graft. On the undyed embryo, the graft's host, we cut out a matching group of future epidermal cells that are not close to its blast. [Music] Now the extra dorsal lip stained blue is slipped into place on its host surface. [Music] [Music] We can observe that the host embryo now has an extra dorsal lip. As the embryo recovers from micro surgery, cell migration begins at two different sites. Cells migrate inward through the embryo's blasapore and through the new blasapore that has formed under the grafted dorsal lip. Now there are two centers of organization. [Music] After 3 days growth, the embryo begins to form two spinal cords. Our embryo is becoming two organisms joined together. From this and other experiments, determined that the grafted dorsal lip induces cells around it to change their fate. Different structures are induced depending on where the transplant is made and what part of the dorsal lip is transplanted. Two heads may be formed or two tails or two hearts can form beating in delicate syncupation. At the dorsal lip of the blasapore, Schpamon had found the organizer, that group of cells that differentiates first and induces the differentiation of other cells around it. In part for this achievement, Speamean won a Nobel Prize in 1935. His and Pot's brilliant experiments opened a whole new field of investigation into one of the central problems of biology. How the cells of a fertilized egg are guided in fulfilling their differentiated destinies.