Biology 441 Spring 2009

Embryology Biology 441 Spring 2009 Albert Harris

REVISED Review Questions for Third Embryology Exam, April 8
You need to be able to identify and explain all of the photographs and drawings on the following web pages which have been posted for this course:

# Ectoderm Development
# Retinotectal Projection
# Notochord and Somite Development
# Heart and Kidney Development

Sample questions: (* stars like this mean that a question is unusually difficult; and I won't ask too many such unusually difficult questions; but I probably will ask some)
What are the three major subdivisions of the ectoderm (in vertebrates)? What decides which one of these subdivisions a given embryonic cell will become part of? List the organs, tissues and differentiated cell types that develop from each of these 3 subdivisions.
Draw a neurulating embryo in a series of cross sections. What is different about neurulation in the embryos of teleosts? What does Scott Gilbert mean when he distinguishes between "primary neurulation" and "secondary neurulation"? In addition to teleosts, are there any other examples of secondary neurulation? * Johannes Holtfreter did the following experiment (among many others): He dissected the neural plate out of developing amphibian embryos, and dissociated (separated) the tissue into its individual cells, and then mixed these cells back together, in a random mixture, kept this random mixture alive for a day or so, and then fixed, stained and sectioned it (using the same methods as for ordinary histological sections). *List some different alternative results that he might (in principle) have observed. *Suppose that the randomly mixed neural plate cells had cavitated and formed a (somewhat) normal-looking neural tube: How would you interpret that result, in terms of what it tells you about the normal mechanism of neurulation? How do you think Holtfreter interpreted that result? How would Trinkaus interpret the result? *Suppose the same experiment had been done using teleost neural tube cells, and the result had been the same: *What do you think that might imply about the normal mechanisms of neurulation?
What is the epidermis? Compare the embryological origin of the lens of the eye, the inner ear, the lateral line system, and the nostrils. What are at least three specific examples in which a sharp bend in an epithelial cell sheet then becomes the boundary between cells that differentiate into very different cell types? *In terms of curvature, define or describe a sharp bend in a sheet of material? What is the olfactory placode? What are "neuromast cells"? Where are they located? How does your body detect the direction of gravity (in the sense of which way is down)? How do we detect rotation? How do we detect sound? How are these 3 detection systems related to the otic placode? What do sensory nerves develop from? What about post-ganglionic autonomic nerve cells? What are Schwann cells? What do they develop from? Draw the stages of formation of the nose.
Draw the stages of formation of the eye. Draw the stages of formation of the inner ear. Draw the stages of formation of the kidneys and the heart. What is "spina bifida"? What are some other birth defects that have a similar cause? Invent a new (or hypothetical) kind of birth defect, based on the partial failure of some normal embryological mechanism. Draw and describe the embryonic development of vertebrate teeth. Draw a melanocyte. Draw the lens of a vertebrate eye, including the shapes and relative sizes of the cells of the lens.
What is the stomodeum? YOU SHOULD BE PREPARED TO ANSWER ANY OF THE QUESTIONS THAT ARE LISTED AT THE END OF THE ECTODERM WEB PAGES (Those dated March 6 and March 16).
What is a "retino-tectal projection". Which nerve cells of which organ extend nerve axons along what pathway, and connect in what geometric pattern to what part of what organ? How are ephrin proteins and receptors for ephrin proteins believed to be a major part of the cause of the creation of these (and maybe other?) neural projections? What is the optic chiasm? Invent one or more experiments that could be capable of DISproving what scientists now believe about the function of ephrins in creating neural projections. (Such experiments could include surgery, isolation of proteins, isolation of fragments of proteins, making mono-clonal antibodies against ephrins or other proteins, or isolation of mutant organisms with abnormal neural projections.)
What are the four main subdivisions of the mesoderm? Describe the structure and function of the notochord. Draw the correct relative positions of the notochord, the neural tube, the somites and the skin in a cross section of a vertebrate embryos. About how many pairs of somites are there in a human embryo? Where do somites form? Do they form all at once? Or two at a time? Or what? * Why does it make sense that the notochord is as big or bigger than the neural tube in the embryos of amphibians and fish, but in the embryos of birds and mammals, the neural tube is much bigger than the notochord? During development, what eventually happens to the notochord? What happens to each somite? What do the sclerotomes develop into? What does the inner ("dermis") layer of the skin develop from? What are myotomes? What cell types differentiate from each subdivision of the somites? If the bones of the arms and legs develop from lateral plate mesoderm, then what does that tell us about what the skeletal muscles of the arms and legs develop from?
*A mutation that resulted in complete failure of formation of the notochord could be expected to produce what different abnormalities of the animal as a whole (don't forget inductive effects)? Draw a picture of a notochord in side view. Draw cross sections of 3 somites (including their subdivisions), in all three dimensions. (Longitudinal cross section, etc.)
What sort of material is collagen? Is it mostly inside cells, or outside cells? About how much of the total amount of protein in the body is collagen? Are there different kinds of collagen? And what are they called? What two rather different commercial products have collagen as their major component? What important discoveries about collagen rearrangement was made by a UNC graduate student who was a teaching-assistant in this course? Some of these experiments involved fluorescent labeling; but many of them didn't; please include both in your answer.
* Please invent your own theory about the mechanical and/or chemical cause of the formation of somites, and describe it, along with any evidence that supports it or future experiments that could either support or disprove your experiment. What is meant by "somitomeres"? *Suppose that you had a microscope that could somehow "see" tensions in pressures inside living tissue (for example, that made pressure look bright red and made stretching look bright blue), then suggest what you might see during the separation of somites from each other, and during the subdivision of somites into sub-parts.
Approximately what is meant by the "clock and wave-front" theory. *Invent experimental criteria by which this theory could possibly be DISPROVED (the experiments could involve grafting tissues, fusing whole embryos, implanting barriers inside embryos, or any combination of these or analogous experiment methods. Suppose that you found a mutant mouse or zebra-fish, or something, that formed a different number of somites than the normal members of that species: They how could you use such a mutant strain of animals to do experiments to test theories of the mechanism of somite formation? By dissecting an adult animal, how could you tell how many somites it had formed as an embryo?
*How do elevators work? What is the Wolffian Duct? Explain and draw the sequence of embryological events by which the human Wolffian Duct becomes the sperm duct.
From what part(s) of the mesoderm does the heart develop? By what experimental disturbance can you cause an embryo to form two hearts? *Would you expect these two hearts to be mirror images of each other, and if so (or if not) explain your reasoning. Remembering Hans Driesch and his experiments and interpretation, why might he had concluded that each lateral plate mesoderm contains its own entelechy?
You should be able to draw a diagram of the paths of blood flow through the 4 chambers of the heart before and after birth (and before and after hatching). What is the foramen ovale? What is the ductus arteriosus? What flows through each of them? When does this flow stop? Why is it important that they were both open at the same time, and that they both close at the same time? Could either serve any function without the other? What would you expect the medical consequences to be if one or the other failed to form? What if both failed to form? If the wall of tissue between the two ventricles failed to form, then when would the serious symptoms begin? *In fact, most of the embryo's blood flow to the placenta is through arteries of the body circulation (associated with the allantois), but try to imagine another line of evolution in which extensions of the lungs developed into an extraembryonic membrane of their own, then explain why these creatures would or would not need the ductus arteriosus and the foramen ovale. The thickness and strength of the ventricular heart muscles are the same as each other before birth, but then later in life the left ventricle muscle becomes 3 or 4 times thicker and stronger: What do you conclude from this? [Please think seriously about this one; it is hard, but I didn't give it a star, and may well put it on the exam]
Explain the functions and embryological origins of the pronephros, the mesonephros and the metanephros. The kidneys that you have in your body develop from which of these? What is the reason for developing three sets of kidneys, instead of just one set? *Imagine a kind of animal that had evolved from ancestors that had all had placentas, going very far back in time, with no egg-laying ancestors: Explain why this kind of animal might be less likely to develop either a pronephros or a metanephros. * Imagine an animal that develops 3 different hearts, one after the other one in development (analogous to the 3 pairs of kidneys), and suggest the differences in structure and artery connections of these three successive hearts.
THE QUESTIONS BELOW THIS LINE WILL NOT BE ON THE THIRD HOUR EXAM
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YOU WILL NOT BE RESPONSIBLE FOR THEM UNTIL THE FINAL EXAM
REPEAT: The third hour exam will NOT include any of the questions below.
What is the technical difference or distinction between "stress" as opposed to "strain"? If you draw a graph of the amount of stress in an object as a function of the amount of strain causing this stress, then the slope of the graph will be approximately what shape curve? The slope of this graph is named _____'s M_______. Hooke's Law predicts that this graph will be what shape? Is Hooke's Law always true? Is it a law of nature, or what? *What are some other examples in which proportionality constants are sometimes more or less constant, but sometimes not constant? [Have you ever heard of Ohm? Have you ever heard of transistors?] What is the definition of the curvature of a surface? Is surface curvature larger in a basketball or a baseball? If the diameter of a sphere is two feet, then what is its radius of curvature? *In that case what is the curvature of its surface in feet per radian? In the case of a cylinder one millimeter in diameter, what is the curvature of its surface? [trick question!]!
Does a cylinder have the same curvature in all directions? What are the only two shapes that do have the same curvature in all directions? [Hint: the other one is a plane.] If you form a soap bubble across an irregularly-bent loop of metal, then what can you predict about the amounts of curvature of the soap film in different directions and at different points? In terms of surface curvatures in different directions at different places, describe the surface curvatures: Of a lemon? Of an apple? Of an orange? Of a football? *Of a branch in an artery? Of the neural plate before it folds? Of the neural tube and surrounding future epidermis during neurulation? Of the eyeball, including its slightly protruding cornea? Of the lens of a human eye? Of two or three other parts of an embryo, or of the body?
What is the key distinction between "Differential Geometry" as opposed to "Analytic Geometry" (=coordinate geometry, or Cartesian Geometry)? *What would I mean if I were to claim that the popularity and plausibility of "Positional Information" theories results not so much from data or logic, but from the historical fact that every biologist has taken courses in analytic geometry, but hardly any biologists have even heard of differential geometry? Could you argue pro or con this opinion? (either opinion would be equally valued and respected in grading; and a good counter-argument should earn more points than simple agreement).
What are at least four examples of tensor variables? (Three second-order tensor variables, and one fourth order tensor variable) Another word for a first-order tensor is what? What are some example of scalar variables? What does it mean to say that a string stretched around a curved surface exerts a force against the surface equal to stress times curvature? If two soap bubbles are fused together, sharing one common side, then what equation describes (predicts) the different amounts of curvature in the surfaces of the bubbles, and the curvature of the side that they share.