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Stem cells self-renew and alsohave the potential to give rise todaughter cells that can differenti-ate into many types of cells. In thisactivity, students make play doughmodels of early embryonic devel-opment. These models help visu-alize where embryonic stem cells
come from and how their potentialto develop into different types ofcells changes over time. Studenthandouts to accompany this activ-ity (which is part of a larger stemcell curriculum) can be found athttp://www.nwabr.org/education/stemcell.html.
Materials• four different colors of play
dough
• paper plates to representPetri dishes
• paper clips
• straws
ProcedureDemonstrate the steps as stu-
dents make their own models.
zygote (Figure 1)
Use a single color to make bothan egg (the size of a ping pongball) and a much smaller spermcell. Mix them together to form a zygote on a “Petri dish,”representing in vitro fertilization. The “tail” of the spermdrops off and does not enter the egg.
Early Cell Divisions
Divide the zygote in half, making two spheres. Divideeach of those two cells in half, then each of those in halfagain, until there are 16 cells.
Morula (Figure 2)
Push the 16 cells together to form a sphere (morula).Through this stage, the cells are considered totipotent –ifseparated, any one of them could become a completeindividual.
MODELING EARLY EMBRYOLOGY &
STEM CELL CONCEPTS
Quick Fix
J E A NN E T I N G CHOWN I N G JOAN G R I SWO L D JO D I E M ATHW I G D I A N N E MA S S E Y
STEM CELL CONCEPTS 77
J eanne T ing C hOWning ([email protected]) is Education Director, and J Oan grisWOld ([email protected]) is Education Outreach Coordinator,both at Northwest Association for Biomedical Research, Seattle, WA 98119. J Odie M aThWig ([email protected]) and dianne M assey
([email protected]) are science teachers, both at Kent- Meridian High School, Kent, WA 98030.
Figure 1. Zygote Figure 2. Morula
Figure 3. Blastula Figure 4. Gastrula
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Blastula (Figure 3)
The blastula stage occurs 3-14 days after fertilization. Themammalian blastula is often referred to as a blastocyst .Pick a new color, make a sphere the size of a ping pongball, then flatten the ball into a bowl. The bowl representspre-placental cells. In real life, they form a hollow ball; thebowl represents a cut-away view (see Figure 3). Use theend of a straw to make indentations that look like cells.
Make pea-sized spheres to represent the cells growing
inside the hollow ball. These are the cells of the innermass (“embryonic stem cells”). The cells have alreadygone through one “fate decision.” The cells that makethe hollow ball can only become placenta. The cells ofthe inner mass are pluripotent; they can become anytype of cell in the body except placenta.
At this point, an embryonic stem cell line could bemade by transferring cells from the inner cell mass toa culture dish and growing them in a medium that pro-
vides support and nutrients.
Gastrula (Figure 4)
Make a new early placental bowl in the same coloras the previous one. The next steps involve making athree-layered ball. Take the original embryo color andform a pea-sized ball. Flatten a marble-sized piece of anew color and wrap that around the ball. Add a differ-ent-colored layer around the outside. Use an open paperclip to cut through the center of the gastrula to show thethree different layers.
The gastrula is multipotent. The early placenta cells canstill only become placenta. The inner cell mass hasundergone another fate decision and has differentiatedinto three layers: endoderm, mesoderm, and ectoderm.Stem cells in these layers are restricted in their abilityto differentiate – they can only become cells of the type
found in that layer. This limitation is a hallmark of anadult stem cell.
Discuss the limitations of the model, including:
• The simulation shows only discrete points in timerather than continuous development.
• The different colors may give the wrong idea aboutorigins of cells—all of the colors originate from theoriginal zygote.
• Students cannot see the spherical nature of the pre-placenta.
OptionalHave students bring their blastulas in their Petri dishes
and place them in the freezer, much as blastocysts created byin vitro fertilization are frozen for storage.
AcknowledgmentsThe development of this activity was made possible by
Collaborations to Advance Understanding of Science andEthics (CAUSE, 2 R25 RR016284-04), a Science Education
Partnership Award from the National Center for ResearchResources, a component of the National Institutes ofHealth.
ReferencesGilbert, S. (2003). Developmental Biology. Sunderland, MA: Sinauer
& Associates.
National Academy of Sciences. (2007). Understanding Stem Cells. Available online at: http://delsnas.edu/bls/stemcells/
National Institutes of Health. (2006). Stem Cell Basics. Availableonline at: http://stemcells.nih.gov/info/basics/
Northwest Association for Biomedical Research. (2007). The
Science and Ethics of Stem Cell Research. Available online at:http://www.nwabr.org/education/stemcell.html
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