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One of the most interesting puzzles in nature is the transformation of a single cell, the fertilized egg, into a smiling, crying, cuddly baby. How does this happen? What is the role of genes in making it happen?

Although we have learned much about the process of development, deep mysteries remain. The purpose of today's exercise is to let you investigate for yourself some of the mechanisms of development by making simple observations of zebrafish embryos by testing hypotheses of your own design. The exercise has 5 parts:

• to think a bit about gene action
• to think about the main processes that must happen to make a baby
• to discover first hand some of the mechanisms of the earliest stages of development
• to discover similarities and differences between development in zebrafish and humans
• to discuss some social issues impacted by this study of development.

1. What genes do

Observe the adult zebrafish in the tank. One reason zebrafish are useful to biologists is that it is relatively easy to make mutations in them. Biologists use mutants to investigate the function of genes. For example, albino people lack pigment in their skin. Thus, in these folks, a mutation has disrupted a gene necessary to make skin pigment. So, the normal job of the normal alternative of the albino gene is to make dark skin pigment.

Several mutant zebrafish have been brought in. Pick one mutant and compare it to the wild type with respect to at least 3 traits.

  Give your mutant a name _______________________________________________________________

• What do you think could be the function of the normal alternative of the gene that is mutated in your mutant fish? Suggest a hypothesis or two.
• What experiment could you do, given appropriate equipment, to test your hypothesis?

2. Fundamental processes in development

An egg becomes a baby. In non-technical language, what are some of the main processes that must occur for this to happen?

3. The mechanisms of early cell division

An egg cell divides into many cells. How does it happen? What is the pattern of division of the egg? Suggest 3 possible patterns whereby the egg can divide into two cells.

• Now observe the zebrafish embryos.
• Which of your hypotheses are ruled out by observation?
• What can you conclude?

List 3 possibilities for the orientation of the second cleavage.

Which were ruled out by observation?

Now, make a prediction for the 3rd and 4th cleavages which give an 8- and a 16-cell embryo.

What actually happened?

What can you conclude?

• What is the mechanism of cleavage?
• How long is it, on the average, between each cleavage?

Hypothesis 1. The force comes from outside, like a drawstring on a purse.

Hypothesis 2. The force comes from inside, like construction of walls in a house.

What predication does each hypothesis make?

Make observations to test your hypotheses.

Based on which general model is supported by data from your observations of the embryo, devise a hypothesis for a sub-cellular mechanism to account for cleavage in zebrafish embryos.

How would you test your model?

4. Comparative embryology

How do the stages of development in zebrafish embryos compare to those in human embryos?

I have collected zebrafish embryos of different ages and labeled them with letters, but accidentally mixed them up while bringing them from the fishery. I need your help to put these embryos of various ages in proper order.

Place two embryos from each of the labeled beakers into labeled petri dishes. By observing the embryos, try to place them in order.

youngest

1

2

3

4

5

6

7

8

oldest

Sketch each stage:

• What principles guided your choice of embryo orders?
• What main changes occurred between each step?

Ooops! I just checked my pockets again and found my notes on the ages of the embryos.  How close did you come to the actual order?

• How long does it take for the eyes to form?
• When does the heart start to beat?
• When is movement first apparent?
• When does hatching occur?
• Now, contrast zebrafish embryos of 2, 4, 8, and 16 cells with human embryos of the same stage pictured in the handout.

Contrast the patterns of cleavage divisions.

• How are they similar?
• How are they different?

Suggest a hypothesis that could account for the difference.

• How could you test your hypothesis?

Compare later developmental stages with human embryos. Match equivalent stages and compare actual times.

• What generalization might you make about the orders of different developmental processes and the relative time scales of the two animals?
• At what time in the development of each animal have all of the fundamental processes been established?

First, I guess we have to define "fundamental processes"

• How would you define it?
• When are they established in zebrafish? _____________hours
• When are they established in humans? _______________days
• How long after a woman has missed her period have these fundamental processes been established? ______________days
• How big is the embryo at this time? Indicate actual length by a line drawn here:

If you wanted to learn the fundamental shared mechanisms of development, which would be a better embryo to study -- the human, the mouse, or the zebrafish?

If it is a question of morality, why is it considered immoral to experiment on human embryos but not immoral to experiment on zebrafish embryos?

5. Science applied to social issues

• When does the human embryo become a "person"?
• Why is that an emotionally charged question?
• What is the role of science in the abortion issue?
• Think about the domain of science. What types of problems are its methods spectacularly useful to investigate, and what sorts of problems are its methods inadequate to probe?
• How does the question, "Should abortion be legal?" differ from the question, "How does an embryo's eye develop?"
• Why don't we do experiments on and utilize aborted human fetuses?

A critical part of teaching is to constantly evaluate what worked and what didn't work about a lesson plan. Please help me to improve this exercise.

1. What was the most useful part of the exercise for you?

2. What was the least useful part of the exercise for you?

3. How did the lack of a prior lecture or reading assignment on embryology make you feel in performing the exercise?

4. Did this feeling you had (regarding the lack of previous fact-based learning on zebrafish embryos) improve or inhibit your take-home messages from the exercise?

5. What is the single most important thing that could be done to increase the value of this exercise?

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