Stem Cells

Editor's Notes

• #2: Read the lesson title aloud to students or have a volunteer read the title aloud.
• #3: Click to reveal each of the learning objectives. Ask students to think about how many cells the human body actually has. Allow four to five guesses. Then indicate who was closest and point out that the human body has one hundred trillion cells. Write out the number on the board: 100,000,000,000,000. Point out that a majority of the cells in their body are of a distinct cell type; they are specialized. Divide students into groups and challenge them to see how many different types of cells they can name. Compile the group lists on the board or chart paper. Remind students that all organisms began as a single cell. Ask: How do you think one cell gives rise to all these different specialized cells? Sample answer: As the single cell reproduces, different daughter cells undergo changes that allow them to perform specific functions. Tell students that by the end of the presentation, they will be able to describe the process by which cells become specialized and will be able to discuss the role of stem cells in the body. Distribute the lesson worksheet and instruct student to use the vocabulary chart to record new terms they come across throughout the presentation.
• #4: Remind students that each of us started life as a single cell, passed through a developmental stage called an embryo, continued to develop through childhood, and are continuing into adulthood. During the process of development, our cells, and those of any multicellular organism including this buttercup, become more specialized for particular functions. Ask a volunteer complete the sentence. Click to reveal the correct answer. Tell students: Differentiation is the process by which cells become specialized. Direct student attention to the photos. Explain that the three circular pictures are photomicrographs showing cells from the buttercup. Ask students to match the photos with their function. Once the class agrees, click to reveal the correct connecting lines.
• #5: Identify the animal in the diagram as the roundworm C. elegans, an animal well studied by biologists. Explain that scientists have been able to map out the fate of each cell from each cell division throughout the animal’s development from embryo to adult. Each and every time a new worm develops, the process is the same, resulting in 959 cells with precisely determined functions. Walk students through the diagram, emphasizing how the process of differentiation is predetermined for this species, always happening in the same way. This diagram offers an opportunity to review exponents. Help students calculate how many cells are in the embryonic worm when its nervous system begins to differentiate. Have a volunteer come to the board to write in the correct answer. Then have them determine, based on the equation for the eighth cell division, what the proper exponent should be to calculate the number of cells in the worm when the cuticle starts to differentiate. Have a volunteer come to the board to write in the complete equation. Click to reveal the calculations. Point out that scientists have learned a lot about cell differentiation by studying relatively simple organisms like the roundworm. Cell differentiation in mammals, however, is more complex, with a number of factors interacting to control the process. Misconception alert: Students may think that cells differentiate by passing on different hereditary information. Remind students that when cells divide, mitosis ensures each daughter cell receives a complete set of genetic information from its parent cell.
• #6: Explain that the original, unspecialized cells from which all cell types arise are called stem cells. Point out that how all possible cell types arise from a single cell—the zygote—is one the most important questions in biology! Ask a volunteer to come to the board to write in the term that completes the sentence. Click to reveal the correct answer. Explain that scientists use the term totipotent to describe the zygote. The term literally means “can do everything.” These cells can become any cell or tissue type anywhere in the body. The only cells in the developing organism that are truly totipotent are the original one-celled zygote and those cells produced by the first few cell divisions. Click to reveal the totipotent definition. Explain that after about four days of development, the zygote develops into a blastocyst, a hollow ball of cells. Click to reveal the diagram of the blastocyst. Point out that even at this very early stage of development, the cells have already become somewhat specialized. The outer layer of cells form tissues that will attach the embryo to its mother. The inner cell mass becomes the embryo itself. Click to reveal the definition of pluripotent. The cells of the inner cell mass are described as “pluripotent”—they become almost any cell or tissue type, except for those tissues that will surround the embryo.
• #7: Explain that after fertilization, the human embryo develops into a hollow ball of cells known as a blastocyst. The actual body of the embryo develops from the inner cell mass. These cells in the inner cell mass are embryonic stem cells. They can differentiate into virtually any cell type. Ask a volunteer to go to the board and point to or circle the portion of the blastocyst that represents embryonic stem cells. Click to reveal the circle surrounding the inner cell mass, label, and arrow. Explain that scientists are able to culture embryonic stem cells by removing the inner cell mass from blastocysts and growing the cells in a lab. Walk students through the figure to ensure they understand what the arrows represent. Ask: What does figure show happening to the inner cell mass of the blastocyst? Answer: The cells are transferred to a culture. Ask: What four cell types do these cells in culture become? Answer: neuron, fat cell, smooth muscle cell, macrophage Ask: Are the cells sitting in the Petri dish totipotent or pluripotent? Answer: pluripotent
• #8: Emphasize that it makes sense to find stem cells in the early stages of embryonic development, but stem cells can in fact also be found in an adult’s body. Ask students if they have heard anything about adult stem cells in news reports. Point out that while the adult body does have stem cells, these cells are neither totipotent nor pluripotent. Instead, they are described as multipotent. Click to reveal the definition of multipotent. Click to reveal each of the remaining bullet points as you explain the following. Explain that there are several regions in the adult body where stem cells are found. These stem cells give rise to a limited variety of cell types, mainly those found in tissues where the stem cells are produced. These adult stem cells produce cells for tissues such as blood and skin that have a limited life span and must be constantly replaced. Share with students that bone marrow transplants, which they may have heard about, are effectively stem cell transplants. Hematopoietic cells differentiate into white blood cells, red blood cells, and platelets. Bone marrow transplants can be used to treat patients with certain blood disorders that cause the patients to produce abnormal blood cells. Bone marrow transplants can also be used to treat cancer patients whose own marrow has been damaged by high doses of chemotherapy or radiation.
• #9: Explain that bone marrow transplants are just one type of stem cell therapy researchers have worked on. Researchers are looking for ways to repair damage to heart muscle from heart attacks, destruction of brain cells from strokes, and damage to nerve cells from spinal cord injuries. The diagram here shows one method currently being investigated to reverse the damage caused by a severe heart attack. Read through the steps shown in the diagram. Ask: Are the cells being transferred pluripotent or multipotent? How do you know? Answer: Multipotent; they are from adult tissues, not from an embryo in early stages of development. Ask: How would the fate of the stem cells change after they are moved from the bone marrow to the heart? Answer: They would become heart muscles rather than blood cells. Point out that similar techniques might be used to treat brain damage, regenerate nerves, or repair organs such as the liver and kidneys that have been damaged by chemicals or disease.
• #10: Ask students why research into therapies using adult stem cells is less controversial than research into therapies involving embryonic stem cells. Encourage them to consider how the issue of donor consent might play a role. Also guide them to see that both sides of the controversy over embryonic stem cell use are making an ethical case: Those arguing for using embryonic stem cells argue that not using the cells would prevent doctors researchers from saving lives, while many arguing against the use of these stem cells argue that they are protecting human embryonic life. To bring the discussion to a close, click to reveal the full statements for each bullet point.
• #11: Identify the scientist in the photo as researcher Shinya Yamanaka. Describe how in 2007 he was able to convert human fibroblast cells (cells that produce components of the body’s connective tissues) into cells that closely resemble embryonic stem cells. These cells are called induced pluripotent stem cells. Click to reveal the first bullet point. These cells may ultimately make it possible to tailor specific therapies to an individual by using that person’s own cells. Click to reveal the second bullet point. Ask students how these breakthroughs affect the debate over stem cell use. Lead a brief discussion about the possibilities. Be sure students come away from discussion with the understanding that further work on iPS cells may ultimately solve the ethical problems that have made stem cell research controversial.
• #12: Remind students that they should have been using their vocabulary charts to record new terms from the presentation. Have students work in pairs to compare lists of terms, adding any they forgot, and to compare definitions. Students can add additional terms on the back of the sheet as necessary. After a few minutes, go through the terms and their definitions with the class. Ask volunteers to share their ideas for how they will remember the meaning of certain terms. Worksheet Answers: Be sure students have defined at a minimum the following terms: differentiation, totipotent, pluripotent, multipotent, stem cell, blastocyst, embryonic stem cell, and induced pluripotent stem cell. Individual ideas for the right column will vary widely.