NOVA scienceNOW: Stem Cells Breakthrough
Student teams research how stem cells become specialized cells. They summarize their findings with flowcharts and clay models.
Students will be able to:
define the term "stem cell."
demonstrate that specialized cells arise from stem cells.
explain how embryonic stem cells form during early development.
show how embryonic stem cells differentiate into distinct tissue layers.
One class period
While current controversy centers on human embryonic stem cells, your students may be surprised to learn there are stem cells in their bodies right now. From birth, all humans—infants, children, and adults—house populations of undifferentiated stem cells in the tissues of their bodies. These so-called "adult" stem cells (the name is something of a misnomer, as these cells are also present in infants and children) give rise to new specialized cells, such as neurons, blood cells, and muscle cells, as the body grows and repairs itself over time. Blood-forming stem cells in the bone marrow, for example, give rise to red blood cells, white blood cells, and platelets.
Adult stem cell research is a rapidly expanding field filled with many important, as yet unanswered, questions. What, for example, is the source of the stem cells found in adults? Are they leftover embryonic stem cells that never differentiated? Where in the body are they manufactured? How plastic are they—that is, how many different kinds of structures can they differentiate into? As you discuss stem cell science with your students, remind them of the compelling questions remaining in this broad and emerging field.
To help students understand stem cells, it is useful to review the basics of fertilization and differentiation. When a sperm cell fertilizes an egg cell, the resulting fertilized egg, or zygote, has the potential to divide and differentiate into every single type of cell in the body.
The fertilized egg, then, is the ultimate stem cell, an unspecialized cell that divides repeatedly and, when the right set of signals is received, differentiates into the specialized cells that make up the body's heart, bones, skin, muscles, and other tissues and organs. Scientists sometimes call the fertilized egg totipotent, meaning it has the potential to differentiate into any cell type in the body. The term comes from the Latin roots totus, or "entire," and potens, "able."
Soon after fertilization, the egg cell begins to divide, and after about 4-5 days, the dividing cells take the shape of a hollow ball called a blastocyst. In animal development, the blastocyst is one of the first stages in the formation of an embryo. By the time the blastocyst forms, initial stages of differentiation are already taking place—some cells form the outer wall of the sphere, while a cluster of cells called the inner cell mass begins to assemble on one end inside the sphere. This cluster of inner cells eventually becomes an embryo and then, as development proceeds, a fetus; the outer cells ultimately differentiate into the placenta.
Inner mass cells, also called embryonic stem cells, have the ability to differentiate into almost any cell in the human body. Embryonic stem cells retain this potential early in development, but approximately two weeks after fertilization in humans, the cells begin to move, arranging themselves into three layers. The outermost layer, the ectoderm, eventually becomes the skin and components of the nervous system. The middle layer, or mesoderm, becomes the muscles, blood, bones, heart, and circulatory system, and the inner layer, or endoderm, becomes the lungs, digestive tract, bladder, and glands such as the pancreas and liver.
This differentiation process, called gastrulation, is an important step in development. Cells become successively bound to a particular destiny as development proceeds; when the cells of the inner cell mass migrate to the ectoderm, for example, they become committed to a pathway that will lead them to differentiate into skin or nerve cells, but not blood cells. Later steps in differentiation will lead some ectoderm cells to become skin cells, but not nerve cells, and so on. Differential gene expression—turning different genes "on" or "off"—drives this process. As students will learn in the NOVA scienceNOW segment Stem Cell Breakthrough, some scientists are trying to cause cells to revert to an earlier stage in differentiation, or to coax them to follow different pathways, by manipulating their genes. In this way, researchers hope to develop novel medical treatments, such as making new nerve cells to replace damaged ones.
Scientists investigating human embryonic stem cells obtain these cells from the inner cell mass of blastocysts at fertility clinics. The cells derived from these sources are obtained only with donor consent and the understanding that the fertilized egg will be used strictly for research. Still, research on human embryonic stem cells remains a hotly debated topic in the United States today. As of this writing, federal funding for research on human embryonic stem cells is limited to the cell lines produced prior to August, 2001.
In this activity, your students will make a flowchart to describe the early phases of embryonic development, from fertilization through gastrulation. Then they will model some of the specialized cells in the body and predict how undifferentiated cells begin to change into these specialized structures.
Before the Lesson
- As a class, watch the NOVA scienceNOW segment Stem Cell Breakthrough.
- Ask students what questions they have about the program, and explain that they will model some of the cell types and processes discussed in the segment.
- Give each student a copy of the Understanding Stem Cells handout and divide the class into pairs. Have each pair view the Human Development and Stem Cells and Mapping Cell Fates animations.
- Have students develop flowcharts to summarize the processes that take place as a fertilized egg cell becomes a multicellular organism. They should use both animations as guides. You may have student pairs develop flowcharts together, or have students develop them individually. A sample flowchart with answers is provided in the assessment section.
- Next, have students use clay to model the kinds of cells that arise from adult stem cells in their own bodies. Assign student pairs to model one of the following types of specialized cells: skin, bone, nerve, skeletal muscle, and red blood cell. Students may use the animations What is a stem cell? and What are some different types of stem cells? as references.
- Have teams label their models and bring them to a central table where everyone can look at them. Ask students what structures these cells all have in common (With the exception of red blood cells, they all have a nucleus, cell membrane, and organelles such as mitochondria and ribosomes). What makes these cells different? (They are specialized to perform different functions, such as carrying nerve impulses or transporting oxygen). You also may ask students to identify from which germ layer the cell they modeled originated.
- Lead a short discussion that relates to the original segment students viewed, Stem Cell Breakthrough. Return to the point made in the video about "turning back the clock" on skin cells. Recall and consider questions such as, "What can scientists learn by taking an adult cell back to a form that mimics a pluripotent embryonic cell?" and "What questions might this process help scientists answer?"
As an extension, lead a brainstorm session about the four genes that can make an adult cell pluripotent, as referred to in the segment. Talking points to consider could include the question, What might these genes do? One of them is known to be an oncogene, or a gene that causes the unchecked cell division that brings on cancer. Why might an oncogene be active in an undifferentiated stem cell?
As an additional extension, you can try having students use clay to model the formation of a fertilized egg, the blastocyst, and the division into germ layers during gastrulation. To help them remember which types of cells originate in each germ layer, have them watch the Video Extra at Stem Cells Breakthrough.
Sample flowchart answer:
(Student charts may vary. At some point, discuss with the class how each step, beginning with the formation of the blastocyst, is a "determining" step that commits cells to a particular pathway. Also be prepared to discuss differences in student charts; you may have one pair draw their chart on the board and ask if other students' charts looked similar or different).
Student Handout Questions
- What is a stem cell? Where do stem cells come from?
A stem cell is an undifferentiated cell. It can become many different types of specialized cells. Stem cells come from a variety of sources. A fertilized egg is a stem cell; embryonic stem cells come from early-stage embryos a few days after fertilization. Stem cells also exist in our tissues as we age—they too can give rise to new cells.
- How do stem cells differentiate?
The stem cell receives signals that tell its genes which proteins to make. Signals also tell some genes to turn off and not make proteins.
- What is the role in the body of the cell you modeled out of clay?
Bone cells: building scaffolding and renewing bone material; comes from mesoderm
Nerve cells: transmitting signals throughout the body; comes from ectoderm
Red blood cells: transporting oxygen to the body's cells; comes from mesoderm
Skeletal muscle cells: moving the body; comes from mesoderm
Skin cells: providing outer layer of protection for the body; comes from ectoderm
Where in the body would you expect to find the stem cells that produce these specialized cells?
Answers may vary; some answers are found in the What is a Stem Cell animation. Look for evidence that students have thought creatively about the tissues in the body that might serve as reservoirs for these specialized cells. From the animation, they learned that:
Skin cells originate deep beneath the skin's surface.
Red blood cells originate in the bone marrow
Nerve cells are made mostly before birth and so would originate in the embryonic tissue layers.
The animation doesn't provide information about the origin of each cell type, however. In fact, scientists stll don't know much about the origin of adult stem cells in the body's tissues. Some structure, such as blood, skin, and the brain, are known to house adult stem cells, but scientists are still learning about where these stem cells come from.
- Can the process you described in your flowchart run backwards to produce stem cells from specialized cells such as the cells that make up your bones? Why or why not? What are some steps that would need to take place for this process to run backwards?
Answers will vary. Look for evidence that students understand that different genes control the switches that regulate differentiation, and that by manipulating these, scientists may be able to "turn back the clock" on adult specialized cells.
Use the following rubric to assess each team's work.
|Completing flowcharts and building cell models
- Students use Web resources effectively to
answer questions, develop flowcharts, and build models.
- Students show ability to independently integrate information from multiple resources.
- Students raise questions showing critical thinking skills and creative thinking.
- Students need assistance while using resources.
- Students have difficulty integrating information from more than one resource.
- Students have difficulty building 3-D models based on Web resource materials.
- Students raise only minimal questions.
- Students have difficulty connecting the Web resources to the model-building activity or make little effort to build models and answer follow-up questions.
- Students cannot integrate information from multiple sources.
- Students raise no questions and appear otherwise unengaged with the activity.
The Stem Cell Breakthrough activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).
Science Standard C
Science Standard E
Science and Technology
Understanding about science and technology
Classroom Activity Author
Jennifer Cutraro and WGBH Educational Outreach Staff
Jennifer Cutraro has 12 years of experience in science writing and education. She has written text and ancillaries for Houghton Mifflin, K12, and Delta Education and has taught science and environmental education at science centers across the country. She also contributes news and feature stories about science and health to media outlets including The Los Angeles Times, The Boston Globe, Science News for Kids and Scholastic Science World.