How Do Scientists View Planarian Stem Cells?

What can I learn from worms? June 2012

Lesson 5: How can we see the worms’ cells during regeneration?

How do scientists view planarian stem cells?

Think about the planarians that you cut in Lesson 3 and their regeneration.

1. How are the planarians regenerating at the cellular level? What is happening each day as the planarians grow new tissue?

2. Based on the BrdU reading, “Investigating Planarian Cell Regeneration using BrdU” how would the number of cells, and their location within the planarian’s body, change each day the planarian regenerates?

3. How would you draw and describe your cut planarian’s number and location of stem cells the first day it was cut (Day 0) compared to Day 7? Remember that your claim should have evidence that supports your idea and scientific reasoning that links the claim and evidence.

Initial illustrations and explanation:

Where are the stem cells in a planarian?

You just read about a process that allows scientists to localize a specific molecule in a planarian, but what does this look like in a real planarian?

In Figure 2, scientists have labeled three different strains of planarians with a fluorescently labeled antibody that allowed them to see where BrdU was in the worms. Since BrdU is incorporated into the DNA during cell division, the scientists were able to label the stem cells and all the “daughter” cells in the planarians.

4. In the figure above, three different strains of planarians were labeled for BrdU. What do you observe about the patterns of BrdU labeling in the different strains?

5. Why do you think there are differences in the BrdU labeling pattern in these three strains?

Figure 3. Cell migration was visualized by BrdU labeling. Planarians were fixed at (A) 8 h, (B) 48 h, and (C) ~6 days postfeeding with BrdU. Scale bars equal 50 µm. (taken from Newmark and Sánchez, 2000 Developmental Biology)

6. As you’ve learned in Lesson 3, the planarians have no stem cells in the tissue that is anterior (above) to the eyespots. While the images in Figure 3 were taken from three different planarians (of the same strain), they essentially show what happens over time after the planarians were fed BrdU. How does the staining pattern change from Fig 3A to 3B to 3C? Why do you think that you see these changes?

Now what happens if one injures the worm?

To document cell migration in a regenerating planarian, scientists cut off the head of a planarian and took pictures once a day (see Figure 4). To determine the role that stem cells play in regeneration, the scientists cut the planarians after they were exposed to BrdU and let the worms regrow before fixing them and looking to see locations of BrdU-labeled cells. The results of this experiment are shown in Figure 4 to the left.

7. What do you notice about the regenerating cells when you look at the time-course?

8. Based on what you have learned in this lesson and previous lessons, what do you think is happening to the planarian’s tissue?

Figure 4. Time-course of the planarian, Schmidtea mediterranea, undergoing cephalic (head) regeneration. The blastema is unpigmented. Numbers refer to days after decapitation. (Taken from Newmark, Sánchez, Nature review Genetics, 2002)

9. Look at Figure 5A and 5B below and then look back at Figure 4 on the previous page. Draw the images shown in Figure 4, Days 3 and 7 in the space below and indicate where you think the BrdU cells would be (based on the images in figure 5). Explain why you think that they would be there.

Day 3: Day 7:

Figure 5. BrdU-labeled neoblasts contribute to the formation of the regeneration blastema and differentiate into blastemal epithelial cells. (A) Five-day anterior (head) regeneration blastema resulting from amputation 17 h after initial BrdU labeling. Regenerated pigment cups of the photoreceptors are seen within the blastema. (B) A 3.5-day posterior blastema (indicated by dashed line) resulting from amputation 18 h after BrdU labeling. Note the accumulation of labeled cells within both blastemas and the BrdU-labeled epithelial cells at their edges (arrowheads). (Taken from Newmark and Sánchez, 2000)

10. Does the BrdU-staining pattern look different in the planarian that regenerated a head (Figure 5A) versus the one that regenerated a tail (Figure 5B)? Describe any similarities/differences you observe between the two images.

11. Would you expect the BrdU-staining pattern to look different in the regenerated head vs. regenerated tail? Why or why not?

12. Now, go back to your initial illustrations and explanation on page 1 and revise your illustration and/or add in additional information to make a stronger explanation for what your cut planarian might look like on Day 0 compared to Day 7. Make sure your revised explanation includes not only a claim, evidence, and scientific reasoning, but also a rebuttal, which states why another claim, would not be possible based on evidence and reasoning.

Revised explanation:

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