Embryology   Biology 441   Vertebrate Embryology, Spring 2016   Albert Harris


Review questions for the last part of the course - part two

Sketch a mammal or bird limb bud, at 3 or 4 stages of embryonic development.

Draw a sketch of the Apical Ectodermal Ridge in a bird embryo in cross-section. Contrast the AER structure in embryos of birds, mammals, frogs, salamanders, and fish.

*Can you deduce what might be special about the tension in the surface membrane at the AER, relative to other parts of the skin? (There is more than one possibility.) Can you invent experiments capable of proving and/or disproving which possibility is true?

What happens to limb development if you surgically remove the apical ectodermal ridge at different stages of development?

What happens if you graft a second apical ectodermal ridge to a developing limb bud?

What is unusual about the apical ectodermal ridges in salamander limb buds? ?
Hint: They don't exist.

What else is unique about salamander legs? ?
Hint: They can regenerate.

When a salamander limb regenerates, the muscles near the cut surface "dedifferentiate" (become undistinguishable from cells that had been skeletal cells), and then grow and divide until the cell mass is nearly as big as the amount of tissue removed. Then muscle cells re-differentiate (only) into muscle cells, and skeletal cells redifferentiate (mostly) into skeletal cells. ?
What can you conclude about the mechanism of pattern formation in limb buds? ?
Is it by rearrangement of cells according to cell type? ?
Or is it by re-differentiation of cells according to position?

Compare these alternatives to what H. V. Wilson hypothesized about the reformation of functional anatomy by dissociated sponge cells. ?
Hint: Wilson preferred to believe that sponges re-formed by differentiation of the equivalent of stem cells, or at least by switching from one cell type to another; He hated the idea that being differentiation causes cells to move actively to their correct relative locations, although rearrangement is really what happens, and what Wilson is credited with discovering.

How can you cause vertebrates to form 3 legs along each side, instead of two?

How can you cause legs to branch into three hands, wings or feet? How can you cause them to branch into four distal ends? ?
Hint: by grafting what? to what location?
Who discovered both these phenomena? ?
John Saunders

Why branch into three when a limb bud is grafted backwards?
Hint: response of anterior tissue to being grafted very close to posterior tissue

How could you produce a 6 legged salamander (or chicken)? (Perhaps as extras in a John Carter of Mars movie?) ?
Hint: implant sources of fibroblast growth factor.

Can you figure out any logical reason why the same category of protein (Fibroblast Growth Factor) induces third limbs and also causes the medio-Lateral axis of limb buds? (I am not sure myself why this should be true.) ?
Hint: Substituting for the AER in maintaining limb proximo-distal development is kind of like inducing an additional limb to form.

What about the paracrine protein Wnt? What axis does it control in developing limb buds? ?
Hint: dorso-ventral

What properties of the neural retina does this axis correspond to? ?
Hint: Geometry of innervation of the brain by axons from retinal ganglion cells.

Argue pro or con: If the muscles of a regenerated leg consist entirely of cells that were muscles in the stump, and if all the skeletal cells in the stump become skeletal cells in the regenerated leg, that means that regeneration results from rearrangement of differentiated cells, instead of what most people assume (spatial control of undifferentiated stem cells).

Argue pro or con: Will stem cells be able to regenerate complicated anatomical structures? (instead of just particular cell types, that either have a very simple geometry, like skin and intestine, or have no geometry, like blood)

What would be some medical uses of a method that could cause cells of one differentiated cell type to convert to cells of a different cell type?

Describe the sequence of events that occur when a newt or other salamander regenerates one of its legs.

What is a blastema?

When salamanders regenerate the skeleton and musculature of one of their legs, do any of the previous chondrocytes redifferentiate as muscle cells? What is some of the experimental evidence for or against this fact?

Likewise, do any of the previous muscle cells redifferentiate as skeletal cells? And what evidence would be needed to prove or disprove this?

*Discuss why, or why not, you would or would not have expected these results, drawing on several particular facts and principles that you have learned in other parts of the course.

*What are some facts that would have led you to expect these results?

*Conversely, what facts would have led you to expect that dedifferentiated blastema cells would re-differentiate into whichever differentiated cell type is needed at each particular location?

Important: Compare the limb bud development with development of the entire animal of sea urchins, frogs, and mammals. Also compare with heart, uterus, oviducts.
Hint: What happens when limb buds, and embryos are split in two, or fused side by side? Also, compare or contrast the development of anterior-posterior axes in limb buds as compared with whole embryos.

Imagine an experiment in which somebody takes an early snake embryo, soaks a plastic bead in fibroblast growth factor, and then surgically implants this bead into the somatic layer of the lateral plate mesoderm, and the snake then develops a leg, with fingers and claws, at the location where the bead was implanted! What might this result tell you about the molecular changes in evolution by which snakes stopped developing legs? (Specifically, as opposed to what other molecular changes that could have produced this same result). ?
Hint: Remember Kollar and Fisher's experiment involving bird teeth?

What controls whether turtle and alligator embryos will develop into males or females?

What is unusual about sex determination in fish?

What tissues are the testis and the ovary and the sex ducts derived from? (in addition to primordial germ cells)

What is the Müllerian duct? What is the Wolffian duct? What happens to them in embryonic development?

Where do the testes originate in development of mammals, and what happens to their locations during development?

Contrast the internal histology of the testes and the ovaries.


Suppose that at the one cell stage of an embryo, two chromosomes happen to break at locations between promoter regions of certain genes and the parts of those genes that code for the protein, and then each chromosome fragment rejoins incorrectly with the DNA of the other chromosome. If the genes are normally transcribed only in differentiated cells (and NOT in the same cell type), then what will happen as a result of this genetic translocation?

What is metamorphosis?

What are at least three different examples of metamorphosis (e.g. caterpillars change into what? Plutei change into what? Tadpoles change into what?)

What are at least 7 different changes that occur when a tadpole metamorphoses into a frog?

What is thyroxine? What functions does it serve? (You don't need to know the molecular structure)

What is ecdysone?

What is meant by molting, in relation to the growth of insects and other arthropods?

What is Juvenile Hormone? (again, what is its function, not its chemical structure)

Does Juvenile Hormone participate in the control of molting and metamorphosis of insects? What does it cause? What does it prevent?


A common misconception is that people today live much longer than they did a few hundred years ago. Explain why they are wrong.
Hint: What is the meaning of "life expectancy"?


Antibodies are synthesized by which particular kind of differentiated cells?

Suggest why it's necessary that the great majority of lymphocytes undergo apoptosis.
(Hint: Remember the randomness of the VDJ recombination of lymphocyte DNA! This is important!)

Which of the following is closest to the reason that people don't normally make antibodies against their bodies' own antigens?

    a) Because every molecule in the body is labeled with special molecules that are special for them, and which lymphocytes recognize as "self", in the sense of avoiding attacks or binding to any molecule with that "self" label ?

    b) Genetic linkage prevents anybody who inherits the gene for the A antigen from also inheriting the gene for antibodies that bind to the A antigen, and likewise for every combination of antigens and antibodies?

    c) Transcription of RNA is selectively inhibited for any antibody whose binding site would fit any of a person's own antigens?

    d) Because some mechanism weeds out those V, D and J regions of DNA that would code for antibodies that would fit any of that person's own antigens?

    e) Some mechanism selectively kills, inactivates or sequesters all B and T lymphocytes whose combinations of V, D and J regions of DNA actually do code for antibodies (or T-cell receptors) that actually do fit any of that person's own antigens?

    f) When antibodies bind to antigens that are present in large enough amounts, then those antibodies are quickly filtered out of the blood.)?

    g) Some other mechanism? Put it in your own words.

You should be able to write a few sentences about each of the following:
    autoimmune disease
    immune tolerance

Explain the errors in the following misleading statements:

    Autoimmune diseases result from failure of lymphocytes to distinguish between "self" and "non-self" histocompatibility antigens. ?

    Autoimmune diseases are caused by excess strength of the immune system.?

Which of the following are already known? (And briefly explain the known mechanism). Also tell which of the following are not known.

    a) How does the immune system produce genes for antibodies that fit (="recognize"="selectively bind to") molecules of your own body. Hint: Explain VDJ recombination. ?

    b) Does your immune system ever contain genes for antibodies that selectively bind ("recognize") molecules that neither you nor any of your ancestors have ever encountered? (same hint) ?

    c) How does the immune system (or any system) "recognize" something that it hasn't encountered before. ? (same hint)?

If a B lymphocyte's antibody binding sites fit a certain molecule, then based on what criterion does the lymphocyte "recognize" whether the bound molecule is "self" or "non-self"??

    a) If the molecule had been self, then that lymphocyte would have been weeded out = induced to die) during embryonic development. Therefore if a lymphocyte still exists, then whatever antigen it binds to must be non-self?

    b) All your cells have the equivalent of identification badges, and if a lymphocyte's badge matches the badge of another cells, then the latter must be "self"?

If immune tolerance really did work by means of matching badges, then by what means would you treat, or try to treat, autoimmune diseases? People who have type A blood somehow avoid making antibodies whose binding sites fit the type A blood group antigen. If they did make such antibodies, that would cause a very severe autoimmune disease. How do their lymphocytes avoid making antibodies with that specificity?

Tens of thousands of people become autoimmune to the special kind of myelin made by oligodendrocyte cells inside the central nervous system. (Many more than become autoimmune to any other differentiated cell type).
Absolutely nobody develops autoimmunity to Schwann cell myelin! ?
Can you invent some possible reasons for these facts? Why do you think each occurs?

Explain why the following would be a possibility:
Perhaps oligodendrocytes differentiate later during embryonic development than Schwann cells. (After the process of weeding out lymphocytes whose randomly-generated binding sites fit "anti-self" binding sites?)
Suggest and support a hypothetical explanation.

All cells of the body contain all the enzymes and signalling proteins needed to cause the cell to digest itself from the inside out (apoptosis). Why should cancer researchers be trying to cause apoptosis in all cells that have any cancerous properties, as opposed to trying to inhibit DNA synthesis and mitosis in fast-growing cells

How could you use apoptosis as part of a cure for autoimmune diseases?


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