Review Questions for Third Examination.Embryology Biology 441 Spring 2008 Albert HarrisReview Questions for Third Exam April 2, 2008What is neurulation? Into what three parts does the ectoderm become separated by the process of neurulation? What is the epidermis? What role do tight aggregations of dermal mesenchymal cells serve in the formation of hair, feathers, an the scales of reptiles, birds and some mammals? Enamel is to dentine as what is to hair? List at least eight cell types that develop from neural crest cells. What important experiment was done by Koller and Fisher? What did their results suggest about the genetic changes that cause modern birds not to form either the dentine or the enamel of teeth? Hint: did they lose the genes for making enamel? Or did they lose the genes for detecting by which epidermal cells detect inductive stimuli from neural crest cells equivalent to those that would have formed dentine in reptile teeth? What genes do seem either to have been lost or to have been inactive? * Suppose that some of the mouse odontoblasts that they used in their experiment had either been contaminated with a few mouse ameloblasts, or had somehow switched differentiated cell types, from being odontoblasts to becoming ameloblasts: how would that affect the interpretation of their experimental results? * Invent some possible experimental criteria by which they might have been able to exclude both those possibilities. Hint: Should they maybe better have used stomodeal epithelial cells from Japanese quail, instead of from chickens? Make a rough drawing of the cellular structure of a single hair, including its mesodermal papilla. Sketch the early development of feathers. Is there any relation between the mechanical tension in an embryo's tissues and the normal spacing of developing body parts. Nerve "outgrowth" is really an example of what? Secretion of axons? Assembly of short pieces? Or What? 2) How was this true cause of nerve fiber extension proved? Approximately when? 1907 But it wasn't done by a UNC faculty member! (But by one at Johns Hopkins, who then moved to Yale!) 3) What very important method was invented for the specific purpose of proving how nerve fibers extend? 4) Why was this a key breakthrough in the development of vaccines against polio & some other diseases? Because tissue cultures of human (first) and later monkey and chimpanzee cells allowed polio viruses to be cultured in Petri dishes, much like the culture of bacterial germs on agar, so that vaccines could be made for viral diseases using variations on the methods that had already been developed for making vaccines for diseases that are caused by bacteria. 5) Do nerve axons actually get attracted toward certain places? No; but the active crawling of nerve growth cones toward directions of increased adhesiveness, or toward increasing concentrations of chemotactic attractants, have approximately the same net effect as if the nerves were being pulled up these gradients. Metaphorically, one might say that the smell of good food "pulls" people's noses toward the dinner table; whereas the forces that move people toward dinner are exerted by their feet. Sometimes the textbook is not clear that it is using "pull" and "push" metaphorically, and in a few specific cases the textbook author himself seems to forget whether he means such terms literally or metaphorically. An example was the earlier section on "thermodynamic" explanations for cells sorting out, which could only be valid if cells were passively pulled (pulled in a literal sense) by gradients of cell-cell adhesions, as opposed to their active locomotion being biased in directions of increased adhesion. 6) When our textbook talks about nerve axons being "pushed" away from certain places during embryonic development, does Scott Gilbert literally mean there is a pushing force, or what? I believe he means this metaphorically, because the effect of ephrin gradients is to induce retraction of crawling nerve fibers. Ephrin binds to ephrin receptors on the nerve surface, and this stimulates detachment of adhesions, so that the axons own contractility pulls it back away from places where ephrin is concentrated. 7) What is meant by a "neural projection"? Can you sketch one, using series of letters and numbers to indicate patterns of connections of nerves at in one area, connecting to some other area of the body? 8) Are there "projections" of motor nerves, as well as projections of sensory nerves? Sure there are. 9) What about projections of nerve connections between one part of the brain and another? (yes, lots of them) 10) Which neural projection has been the subject of the most experimental research? (The retino-tectal projection, of course!) From which of the three main subdivisions of the ectoderm do each of the following develop: The lens of the eye? Motor nerves? Postganglionic autonomic nerves? Sensory nerves of the skin? Hair? Feathers? Scales of reptiles? Lateral lines of fish and amphibians? Semicircular canals? The Cochlea? Sensory nerves of the nose? Schwann cells? Pigment cells of the skin (melanocytes)? The pigmented retina of the eye? The optic tectum? The neural retina? The optic nerve? The ganglion cells of the eye? The rod and cone cells of the eye? The enamel layer of the teeth? The dentine layer of the teeth? Rathke's pocket? The optic tectum? The brain? Bones of the face? From what subdivision of the mesoderm do each of the following develop? The dermis layer of the skin? (Sample answer: The cells of the dermis come from the dermatome of somites, and somites form from paraxial mesoderm.) What is the embryological origin of all skeletal muscle cells? What is the origin of cardiac muscle cells? ...of the pronephros? ...of the mesonephros? ...of the metanephros? ...of the pronephric duct? ...of the sperm duct? ...of the vas deferens? ...of the uterus? ...of the fallopian tubules? ...of the oviducts? ...of the chondrocytes of the back-bone?
What are the meanings of each of the following terms? Neuron? Neurocoel? Glia? How can a gradient of ephrin cause nerves to behave as if they were undergoing either negative chemotaxis or negative haptotaxis? *As a discussion question: Can you argue either pro or con whether such a guidance of cell locomotion should be regarded as chemotaxis, as haptotaxis, or called by some different name (perhaps newly invented for this purpose)? If axons from many different neurons each have a different concentration of ephrin receptor on their plasma membranes, then if their growth cones encounter a gradient of concentration of ephrin on the outside surface of other cells (perhaps in the brain?) then which growth cones will be able to reach the areas where the ephrin concentrations are the largest, and which will be halted where there are intermediate or low concentrations of ephrins? * Would some other form of positive chemotaxis or haptotaxis also probably need to be affecting the direction of the growth cones' locomotion. (Hint: Ask yourself why any of the growth cones, including those with the lowest concentrations of ephrin receptors, move past the area with the lowest concentration of ephrin?) Thought question: How might pre-patterns of expression of either ephrin genes or ephrin receptor genes (or both) be a possible mechanism for the separation of somites from each other? Even better thought question: Invent how a "clock and wave-front" mechanism for controlling somite segmentation be based on time oscillations of amounts of ephrin present, combined with a rearward-moving spatial gradient of concentration of ephrin receptors on the surfaces of paraxial mesoderm cells. In which department of what university did Julie Nowicki and Anne Burke do their best research on the control of body segment differentiation by Hox genes in chickens? (As cited in the textbook). Hint: Dr. Nowicki was one of the two teaching assistants in this course for two years. So was Dr. Stopak, who is about to be mentioned below. Describe the physical structure and mechanical properties of notochords. How are the structure and the properties related to each other? What function does the notochord serve in tadpoles and young fish? Roughly describe the theory proposed by David Stopak as a mechanical explanation for the formation of tendons and skeletal muscles? What evidence supports this theory? What evidence is there that a similar mechanism also causes collagen fibers to become wrapped tightly around arteries? In what department of what university was that work done? (hint: the artery part was done in the Biology Department of Stanford University.) Explain how and why the anterior and posterior parts of sclerotomes become parts of different vertebrae in the back-bone of vertebrates. Sketch the geometric division of an individual somite into to form a dermatome, a myotome and an anterior and a posterior sclerotome. The locations where somites form controls the later locations of which different segmental structures, including several whose cells were never parts of somites? Why do hearts and kidneys become functional so much earlier than the brain or other parts of the nervous system? Contrast the two very different solutions to this need for early function: in the case of the kidneys in contrast to the case of the heart? Hint: do embryos first develop a very simple heart, and then develop a more complex heart from the tissues behind the first heart, then pump blood with this second heart during most of development, while building a third, separate adult heart to be used after birth? No, they don't; but what embryonic structures do develop somewhat like that? Explain how the human heart manages to avoid pumping much blood to the lungs before birth; but then begins pumping lots of blood to the lungs immediately after birth? What is the foramen ovale? What is the ductus arteriosus? How are the functions of these last two structures related to each other? What would happen if one or both failed to form in the embryo? What would happen if one or both failed to close at birth? If a hole remains open between the right and left ventricles during embryonic development of a human, why does this not cause much if any problem until after birth? Thought question: It is a fact that the muscular walls of right and left ventricles have the same thickness before birth, but the left ventricular muscles become much thicker after birth, in proportion to the much higher blood pressure in the left ventricle: What does this suggest about the mechanism that controls enlargement and strength of cardiac muscle cells? What if the walls of both right and left ventricles thicken equally babies with holes connecting their two ventricles ("septal defects"), until these ventricles are surgically closed, after which the wall of the left ventricle becomes thicker? What is misleading about an article (that I just read) which says that "septal defects are holes in the walls of the ventricles of the heart?
The following is a question about the relation between curvatures, pressure differences, and tensions: If you look at a horizontal cross section of an adult human heart, then the left ventricle will appear nearly circular, whereas the right ventricle will be shaped like a c or a parenthesis beside a circle, somewhat like this: o) This completes the list of study questions. There will not be any questions on plant development on this exam. However, there will be questions on this topic on the final exam in May.
|