Biology 441, Spring 2014


Review questions for second hour examination (Feb 21, 2014)

NOTE: Because of the two snow days, the topic scheduled for Monday Feb 17 ("Fertilization and mechanisms that minimize polyspermy") will be delayed until after the second exam.

The following will be the primary topics of the second hour exam:

Endoderm and the organs that develop from it (including 2 of the 4 extraembryonic membranes)

Also including the stomodeum, teeth, palatal shelves, palate, nasal cavity, proctodeum, cloaca, bladder rectum, allantois, yolk sac, amnion, chorion, amniotic cavity, bag of waters, extraembryonic membrane

Mesodermal subdivisions, and the organs that develop from them, especially kidneys and heart.

Pronephros, pronephric duct (also called the Wolffian Duct), mesonephros, metanephros (the duct of which is the "ureter")

Regarding mesodermal organs: You ought to be able to draw the notochord, somites, subdivisions of the somites, intermediate mesoderm, lateral plate mesoderm, coelomic cavity, location of the heart.

The path of blood flow through the heart of mammals (including human), and how this path changes at the time of birth.
(Including the functional reasons for this change, the names and descriptions of the two openings through which blood flows before birth, descriptions of the processes of closure of these two openings.)

Extra-embryonic membranes:
The four extra-embryonic membranes in reptiles, birds and mammals;
the two extra-embryonic membranes in teleost fish
What are the names, locations, functions, and contents of each of them?
Sketch their geometry.

What would you reasonably expect should be the result of damage or under-development of each of these six extraembryonic membranes?

Know something about the advantages and disadvantages of more permeable placentas; for example between placentas of primates as compared with pigs, horses or cows

Identical twinning (= monozygotic twinning) If human twins are contained within the same amnion, then at what stage did the twins separate?
(And please make a sketch, that clarifies your reasoning.)

What about if a pair of human twins are enclosed within the same chorion, but each has its own separate amnion? (and please make a sketch, that clarifies your reasoning.)

What if each of a pair of twins is surrounded by its own, separate chorion: does that in itself tell you whether these twins are monozygotic (identical) or dizygotic ("fraternal") twins? Why, or why not?

Conjoined twins are enclosed within which of the four extraembryonic membranes? Why are you sure of this conclusion?

Suppose you were to dissect two early chicken embryos out of their eggs, into tissue culture dishes, and lay one embryo right on top of the other: then make a time-lapse video of somite segmentation in both embryos. You might need to focus up and down frequently, so as to see both the upper and the lower embryo. Alternatively some very long working distance microscopy might be possible - or confocal microsopy? The goal is to find out whether the embryos somehow influence each other's times of somite separation. For example, these times might tend to synchronize! Would you interpret synchronization as evidence supporting the clock and wave-front hypothesis? Or would synchronization be evidence in favor of control by a reaction-diffusion system? Or would synchronization be predicted by both those theories?

Would it matter whether both embryos were lined up, with heads in the same direction? What if you laid them down at right angles, one perpendicular to the other? What if you laid them down in opposite directions, with the head end of one directly above the tail end of the other embryo? What if you laid them down with the head of one over the neck of the other; or with the head over the middle of the other?

In all cases, we will assume that A and B morphogen chemicals can diffuse the short distances between parts of the embryos directly above and below each other, and will also assume that whatever "clocks" and "wave-fronts" exist will also extent their effects from one embryo to the one above or below it. Assuming that these hypothetical signals will exert effects from one embryo to the closest parts of the other embryo, then what do you expect to happen? What evidence would confirm that the signals from each embryo will sometimes change the time and locations of somite segmentation in the embryo? What do the different theories predict about these induced changes? What differences should there be in these effects, if the axes of the embryos are perpendicular? If the axes are pointed in opposite directions? If the axes are at some other angle? If one embryo is slightly older than the other?

What if you had different mutant strains, one of which formed more somites than the other? Or one of which formed somites faster than the other? ...and you plop one isolated embryo directly on top of the other. The goal is to create a situation in which the different alternative theories make sharply different predictions.

What is chemotaxis? What are at least three different responses of cell locomotion to differences in chemical concentrations that can cause motile cells to accumulate at locations of highest concentration of that chemical.

Will a chemotactic attractant necessarily stimulate the speed of cell locomotion to increase where and when the concentration of the attractant chemical is a maximum. Suppose cells stopped locomotion where a chemical is higher than some threshold amount: would that produce negative chemotaxis. Suppose cells respond to a chemical gradient by weakening their adhesions; would that be chemotaxis.

Computer simulations of chemotaxis: part one and part two
Smaller (.mp4) versions of part one and part two

Tissue culture cells preferentially move up gradients of adhesiveness (take a look at these videos).

cells on a grid of differential adhesiveness

cells on an adhesion gradient

How would the "Differential Adhesion Hypothesis" explain this movement up adhesion gradients? (hint: that cells are physically pulled by the process of formation or enlargement of adhesions)

The darker the substratum, the greater the adhesiveness at that area. From looking at the video, are there any particular phenomena that might support or contradict the DAH explanation (such as how long cells become in areas of greater versus weaker adhesiveness; how frequently cell protrusions are retracted, whether cells accumulate where adhesion is strongest, or along the boundaries between greater and less adhesiveness.

Be prepared to answer the same sorts of questions about the video that shows cells cultured on plastic coated with little square islands where the adhesiveness is greatest. Are cells pulled by the process of formation of adhesions? Or do the cells pull themselves, using adhesions to exert contractile forces? How can you tell?
By what criteria would you decide? Do cells maximize their adhesions? Do cells ever spread off of these little square islands of greater adhesiveness.

Please invent some variations on these experimental substrata (for example, other geometric patterns of adhesive islands or adhesive gradients? Bigger, smaller, rectangular, in rows, different distances apart, steeper gradients of adhesiveness: little adhesive island super-imposed on a diffusion gradient, or vice-versa; gradients of widths of adhesive islands, gradients of adhesiveness of adhesive islands, comparisons of behaviors of different sized cells on these adhesion spots and gradients.

If you had a gradient of widths of adhesive islands, and cultured cells of different sizes on them, what would it mean (suggest possible explanation(s) if cells tended to accumulate of whichever adhesive islands approximate their own size, but spread off islands that are smaller than they are? What if they tended to move off of adhesive islands that are bigger than they are? What if they tended to line up along the edges of large adhesive islands.

What if converting cells to cancerous behavior increased their ability to spread from adhesive areas onto less adhesive areas? (Which happens to be true) How could you interpret that? How, in terms of differences between cancer cells and equivalent normal cells? How, also, in terms of reasons for the greater invasiveness of cancer cells.

Suggest possible explanations why macrophages respond to adhesive islands and gradients opposite to all other cell types, by which I mean that they preferentially move onto the less adhesive areas (off the islands, and down the adhesive gradients).

Suppose that bone destroying osteoclasts responded to adhesive islands in the opposite way than bone-forming osteocytes; the osteocytes moving preferentially onto more adhesive islands and osteoclasts behaving like macrophages.

These questions are meant to stimulate experimental design, interpretation of experiments, and recognition of medical relevance of experimental differences.

Therefore, please invent other sets of experiments, interpretations and medical relevance, for other phenomena that you have learned about in this course, such as chemotaxis. As a starter, suppose that you could attach chemotactic attractants to plastic surfaces in any geometric pattern you can imagine: what could you learn about mechanisms of chemotaxis (in one cell type versus another).

Could endothelial cells use chemotaxis to vascularize tissues in an optimal pattern? (get enough oxygen everywhere with the minimal total length of capillary?) Suggest some experiments, or other criteria to test your ideas.

Could sensory and motor nerves use chemotaxis to guide them toward connection to the proper locations in the skin, or on muscles? What difficulties can you think of? Remember, blood is flowing through capillaries.


More questions, added February 18th

You are responsible for understanding all the photographs and diagrams on the four sets of lecture notes (Feb. 3, Feb. 7, Feb. 12, Feb 17).

The next exam might include one of the photographs, and ask you to explain what it is and what significance in has for embryology, ask you to label its parts, or explain how it was related to other illustrations and text from these four sets of web pages, or web pages from earlier sets of lecture notes. Individual pictures from time lapse sequences count as "photographs", in the sense that one or more might be included as part of the next hour exam.

Another possibility is that the next exam might include a diagram or drawing with its labels removed, and you could be asked to re-label the diagram, and/or to explain its significance, or you could be asked to change a diagram, so as to make it illustrate some principle. For example, an all too common birth defect is for an opening to exist between the two ventricles of a baby's heart: "Please label the diagram and add arrows showing the paths and relative amounts of blood flow that would occur in such a baby, with one diagram showing the situation before birth and another diagram showing the situation after birth." Another possible question is that I might draw a slightly different diagram that shows connections that can (and do) occur in other kinds of birth defects; you would be asked to figure out what went wrong in development to produce the abnormality, and why it would harm the baby's health.

Yet another possibility is that you might be asked to draw a diagram of how a two-headed fish is formed, or to draw a graph of how the probability of turning of a crawling or swimming cell would need to change as a function of some particular variable in order to cause cells to accumulate at a certain location, or to avoid a certain location. A related possibility would be for the exam to include a graph of cell speed, turning direction, probability of turning, or some other variable, as a function of some other variable, and you could be asked to predict the net effect if cells behaved according to the rules.

Or a question might be the opposite: a behavior or result might be drawn, diagrammed or described, and the question could be asked what would be the net effect over time of cells obeying obeying this behavior shown on the graph.

Please suggest other questions for the next exam, and that includes suggesting kinds or categories of question that would test students' understanding of the subject matter, or how this knowledge could be applied to new situations.


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