Embryology Biology 441 Spring 2007 Albert Harris and Andrius Masedunskas


Review Questions for Second Exam

What are the major subdivisions of the mesoderm? Describe the notochord? Describe its cellular and molecular structure, and its physical properties? Draw a sketch of a notochord. Where is the notochord located? (below what? between what? The cells of the primitive node become the notochord! Can you visualize why? Hint: it's because they are the last mesodermal cells to move into the interior in animals like birds, reptiles and mammals, which have the primitive streak pattern of gastrulation.

What are somites? Where do somites form? Describe the process of separation of somites from each other? What are the dermatome, myotome and scerotome? Draw their shapes and geometric arrangements relative to each other. What parts of the adult body develop from each of these subdivisions of each somite? Draw the geometrical shape of a somite, as it forms from strips of paraxial mesoderm, and then the process of subdivision into parts. Draw the subdivision of the anterior and posterior sclerotomes of a somite. After these have separated from each other, then what do these two halves of each scleroderma become attached to. How is this fusion related to the formation of the vertebra (the individual bones of your back-bone?

What is the dermis? How is it related to the epidermis? What (and where) is the intermediate mesoderm? What parts of the body does the intermediate mesoderm develop into? What is the pronephros? Where does it form? How many of them does each vertebrate embryo form, during its development? (Hint: 2, one on each side, behind the head.) Do adult animals still have a pronephros? What happened to them? What "grew" rearward from each pronephros, and does still exist in some of us?

What is the Wolffian duct? Do only Wolves have them? What about Virginia Woolf; did she still have a Wolffian duct. Why not? What is the mesonephros? What does it develop from? What function does it serve? What duct are the mesonephric tubules connected to? In males, what other tubules do the mesonephric tubules become connected to? By what sequence of tubules and ducts do sperm reach the outside of the body. In amphibians, why do the sperm have to share this pathway with urine? In mammals, birds and reptiles, why does the urine no longer flow out of the body through the same tubules as the sperm? (Hint: remember that our adult kidney is the metanephros, and it is connected to the bladder by a different tube called the ureter. Remember that most mammals have a bladder, separate from their rectum, but in contrast amphibians, reptiles and birds have one space for both urine and feces. This one space is called the cloaca, and we had one during earlier stages of embryonic development, before it subdivided into a (more anterior) bladder and a (more posterior) rectum. The large intestine empties into the rectum and the two ureters empty into the bladder.

What is the vas deferens? What function does it serve? Is it present in members of both sexes? What happens to it in the other sex? Note that the oviduct develops from yet another tube, called the Mullerian duct, which is considered to form from lateral plate mesoderm. In males, this duct forms, but then is induced to degenerate by a certain small protein secreted by cells in the tested. In female mammals, including humans, the upper parts of the oviducts remain narrow, and are called the Fallopian tubes (named after an Italian anatomist). (Muller and Wolff were Germans)

What and where is the lateral plate mesoderm? What does lateral plate mesoderm develop into. Where does the coelom form? From what tissue does the heart form? What and where is the foramen ovale? What function does it serve? It allows blood to flow from where to where? Why and when is that desirable? If the foramen ovale closed prematurely, then blood would be forced to flow where. Do you still have a foramen ovale? Explain why or why not? What and where is the ductus arteriosus? What function does it serve before birth? When should it constrict, and prevent flow of blood between where and where?

Is it unusual for arteries to have walls that are made mostly of smooth muscle cells? Do smooth muscle cells ever block arteries, except by means of their own contraction? Can you draw a diagram of the path of blood flow before birth? What major changes in the paths of blood flow occur at birth? Can you explain why many kinds of major birth defects in heart structure produce no bad effects until birth, but then become major, life-threatening problems at the time of birth, and afterward (until surgically fixed)? What is meant by "blue babies? What is meant by septal defects?

The circulatory system and the excretory system have to begin functioning very early in embryonic development, in contrast to the nervous system for example, which does not need to function until late in development. Imagine a life form is which a succession of three different hearts form during development, one after the other, with more and more complicated structures, and with the two earlier hearts degenerating once the third heart becomes functional. Compare this imaginary sequence of heart development with what actually happens in the development of our kidneys. Also compare it with the actual sequence of development in our heart.

What are some examples in which type I collagen becomes arranged into many layers, with the fibers of each layer oriented perpendicular to the layers directly above, and directly below, it? What are endothelial cells? What are "blood islands"? Compare the structure of capillaries, arteries and veins? In terms of what you know about curvature, tension and pressure difference, please explain why capillaries can withstand almost as much blood pressure as arteries, without bursting. Embryonic arteries also have comparatively thin walls, for the same reason.

THOUGHT QUESTION: It is an anatomical fact that the muscular walls of the left ventricle are much thicker that the walls of the right ventricle, and it is a physiological fact that the blood pressure is about 4 or 5 times larger in the left ventricle than in the right ventricle. That pressure difference is why the heart seems to be on the left side of your chest, instead of being right in the middle. Before birth, however, the thicknesses of the muscles are the same for the left ventricle as for the right ventricle; but after birth the left ventricle becomes much stronger, and its walls thicker. Is this analogous to arm muscles getting stronger in weight-lifters?

What is meant by "embryonic regulation". What is meant by saying that mammals have very regulative development, in contrast to nematodes and flies, whose development is highly mosaic? Who originally discovered embryonic regulation, using the embryos of what kind of animals? What are some other examples of kinds of animals whose embryonic development is regulative? What are some other examples of embryos with mosaic development? At a causal level, what can you suggest about possible reasons for development in a given kind of animal being particularly regulative, or particularly mosaic?

What experiment is Roux famous for? If he had known about regulative development, why would he probably have been surprised by what he observed? And are you surprised! What other result might you have expected? Suppose that Driesch had chosen to study embryos whose development was more mosaic; how would his results have differed? Would he have hypothesized that embryos have "entelechies"? (Hint: probably not; but why?)

What is some evidence that mammal development is extremely regulative? How is regulative development related to the current political debates about "embryonic stem cells"? How does the "Theory of Positional Information" interpret embryonic regulation as evidence for control of cell differentiation and shape formation by linear diffusion gradients of chemicals called "morphogens"? Are there other ways to interpret embryonic regulation? What is different about the embryonic development of the neural tube of teleost fish?

What do physiologists mean by homeostasis? In what sense is embryonic regulation analogous to homeostasis? Why is it misguided to think that any phenomenon that "homes-in" toward a particular stable state must be minimizing either potential energy or minimizing thermodynamic free energy. [Don't panic or worry too much about the preceding two questions; it's OK if you can explain either what you don't understand about these issues, or why you disagree with what was in the lecture and on the web page. Good counter-arguments will earn as good or better grades as regurgitation.

What are some examples of self-organizing structures and patterns in non-living matter? What are some examples in which people make things by taking advantage of spontaneous shape-generating or pattern-generating properties of raw materials? Explain the relevance of those drawings of "punching dummies" on the web page, and in the lectures? How are they meant to relate to spontaneous embryonic events, and also to embryonic regulation? [Do you prefer punching dummies to entelechies, or do you think that embryos drive scientists somewhat crazy!]

What are several different kinds of symmetry? One example of a "symmetry operation" is reflection in a plane: what are some other examples of symmetry operations. Do embryos become more symmetrical, or less symmetrical, in the process of embryonic development. What is an example of reduced displacement symmetry. What kind of animals and plants have more than one plane of reflection symmetry? What is Curie's Principle? What implications does it have for embryology? What is an example of the application of this principle to crystallography?

Turing's "reaction-diffusion" mechanisms are an example of "breaking" what kind of symmetry? The word "morphogen" was invented by Alan Turing to mean something somewhat different, but also somewhat related", to what is meant by a morphogen in the theory of positional information: what are the similarities and differences? What will happen if there are two chemicals A, and B; and if increases in the concentrations of both at each location are somehow caused in proportion to the current local concentration of substance A, while substance B somehow causes destruction of both A and B; both of which substances can diffuse from place to place, but B diffuses faster than A. Was Turing's original hypothesis more complicated than this? (Hint: yes: but the rules just listed will produce patterns just as well, and are logically equivalent.)

What are "Liesegang rings"? How can you cause them to form? *Can you suggest some possible mechanisms capable of explaining the formation of Liesegang rings? * What is the key issue?

What are some examples of quantitative variables that are scalars? What is special about vectors, and variables that are vectors. What is the definition of curvature? (for example, the curvature of a line). Compare the curvature of a small circle, a large circle, and a straight line. [answer: small circles have large curvatures, large circles have smaller curvatures, and straight lines have zero curvature.] Compare the curvatures of a small sphere, a large sphere, a flat plane, a hen's egg, and a saddle.

What is the distinction between stress and strain? Compare tensile stress with compressive stress. What is Hooke's Law? Do materials often obey Hooke's Law? Do they often disobey it? Why is does a single isolated soap bubble become spherical? Hint: because its shape is a stable balance between what two opposed forces, and both these forces have what symmetry. In an inflated cylinder, what is the ratio of stresses in the circumferential direction, as compared with the longitudinal direction. What happens when Hydra reproduce asexually?


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