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Please invent ten new kinds of contraception, using information you have learned about normal fertilization. Do sperm of all species swim? What is at least one specific exception? What is Kartagener's syndrome (or triad), and how are the three symptoms related to each other? What function might be served by selective adhesion and chemotaxis in making sure that eggs of a given species are fertilized (as much as possible) only by sperm of the same species? *In terms of evolution, will sperm tend to fertilize any egg they can, or will mutations that favor that tend to be eliminated? In fertilization, are two sperm better than one? Are two better than none? Why not? What word describes the situation when an egg cell is fertilized by more than one sperm? (Hint: it starts with a p, and ends with a y) What different kinds of mechanisms serve to prevent this from happening? One of these mechanisms is similar (in what ways) to the mechanism of nerve impulse conduction? Are calcium ions involved in fertilization? The membranes of the cortical granules fuse with what other membrane? And for what purposes? (Hint: increased cytoplasmic concentrations of calcium ions favor this fusion) What is a ball-park estimate of the number of cortical granules in each oocyte? In what 3 ways (at least) are cortical granules analogous to acrosomes? What do acrosomes contain? And what are the functions of these enzymes? The membrane surrounding the acrosome fuses with what other membrane. What third fusion between (which?) membranes is a necessary part of fertilization? In what sequence do these their kinds of membrane fusions normally occur? Which first, which last, which second? If the goal were to prevent fertilization, could that be done by changing which of these membrane fusions occurred first? Could fertilization be prevented by blocking one of these 3 examples of fusion between two membranes? For each of the 3, how could its prevention tend to prevent successful fertilization? Contrast the times when meiosis occurs, in spermatogenesis as compared with oogenesis? In which do the meiotic divisions occur before fertilization, and in which do they occur after or during differentiation? Functionally, why is there such a difference? How many sets of chromosomes are there in a human egg cell just before it is fertilized? What about just after fertilization? In what sense are we triploid for the first hour or so of development? Are any kinds of animals effectively pentaploid then? Are there any species in which neither meiotic division has occurred in recently fertilized oocytes? Who first saw sperm cells through a microscope? What did he think they were? Did anyone else accept that interpretation? (Like, for 200 years!) What is meant by capacitation? So sea urchin sperm undergo capacitation? (hint: no). How and why is the fertilization membrane formed? What is the source of the protein that the fertilization membrane is made out of? By what mechanism, and at what time, do mammal egg cells become much less adhesive to sperm? (And what important purpose does this accomplish?) Are oocytes about twice as big as average body cells of the same species, or what? In ordinary fast-growing cells of the body (and in tissue culture cells), about how long (in minutes, hours, days, weeks?) does it take to complete one cell cycle (from one mitosis to the next mitosis)? In comparison, about how much time passes from one cleavage to the next in early embryonic sea urchins, frogs, flies, or mammals? Which of these has the fastest cell cycle times? And which the slowest? What are the 4 subdivisions of the ordinary normal cell cycle? Can any parts of the cell cycle ever be skipped? When does this occur, in what stages of development? Does that ever occur in mammal embryos? (hint: no) At what stage do the normal cell cycles resume? What does it mean to say that certain embryonic cell cleavages are synchronous? Which kinds of animals have synchronous cleavage, and at what stages? Is this correlated with whether they are skipping any stages of the cell cycle? ** What is the logical connection between these correlated phenomena? (And if you can figure out this connection, please explain it to me, because I don't get it, but the textbook seems to imply that it makes sense.) When the cells of a blastula rearrange, what is that called? How is this rearrangement related to the formation of the anterior-posterior body axis? What controls the specific locations where this rearrangement will occur? Note that this control seems to be very different in flies, frogs, birds and mammals! These differences are in time and also the kind of signal or trigger that is responsible; and you should remember the differences. What is the "mid-blastula transition"? What is the difference between holoblastic cleavage and meroblastic cleavage? What are some examples of groups of animals in which cleavage is holoblastic, and some examples in which cleavage is meroblastic? *Are these differences in cleavage related to patterns of cell internalization in gastrulation? Compare radial versus spiral cleavage? What are examples of kinds of animals that have each of these patterns of cleavage? What is so strange about cleavage in embryos of flies and (most) other arthropods? Are there any examples in which the future differentiation of embryonic cells is controlled by special cytoplasm being concentrated in certain parts of the oocyte? What kinds of animals have polar lobes? What kind have yellow crescents? What kind have gray crescents? In sea squirt larvae, what controls which cells will differentiate into muscle? What does it mean to say that the embryonic development of a certain kind of animal is very regulative? What is meant by mosaic development? What are some kinds of animals whose development is very mosaic? What are some whose development is very regulative? Which of these have consistent cell lineages? Are there any kinds of animals in which every individual has exactly the same number of cells? Are there any in which cell fate has still not been decided as late as the early blastula stage? On the basis of what experiments can I say that echinoderm embryos can "regulate" over at least an eight-fold size range? What do embryologists mean by a chimera? *Should it be easier to create chimeras using embryos of species with mosaic development, or regulative development? Could you argue pro or con whether the difference between mosaic versus regulative development is only a matter of time when cell differentiation is decided, versus being a result of different fundamental mechanisms for deciding cell fate? (If it's a matter of time, which decides early? and if it's a matter of control mechanism, then how might that relate to induction versus ooplasmic segregation?) Contrast early fly development with early human development? **Do either of them resemble your previous general conception of embryos, prior to taking this course? If you were a mad scientist, inventing life, how would you design embryos to control cell differentiation? Describe how the bicoid protein is distributed in early fly embryos, and what function this has been thought to serve. What controls which end of a fly embryo becomes the head, and which becomes the tail? What controls this difference in frogs? What controls it in nematodes? What controls it in teleost fish embryos? **What controls this in mammal embryos? **By what experiments might you hope to find out? What are the five families of genes that control embryonic development in flies, and compare the structural abnormalities that result from mutations of each of these 5 classes of genes? Do any of these genes code for proteins that are transcription factors? Is there any of the 5 for which all the genes don't all code for transcription factors? Describe the phenotype of homeotic mutations. Which of these families of genes do each of the following genes belong to: bicoid, krupple, even-skipped, armadillo, antennapedia, and bithorax? Could you sketch a graph of the relative amounts of the proteins coded for by each of these genes in different parts (along the anterior-posterior axis) of an embryonic fly? What are imaginal discs? What kinds of animals have imaginal discs, and what purpose do they serve, and how are they related to homeotic mutations? What is meant by the phenomenon of colinearity? What is meant by hox genes? Explain why not all genes that contain a homeobox are hox genes? How long is a homeobox? How long is a homeodomain? The homeodomain part of proteins binds specifically to what? How many clusters of hox genes do people have? What about mice? What about frogs? What about flies? What are parologous groups? *How many different parologous groups are there?(Hint; it is an unlucky number). When a certain hox gene is deleted, or radically mutated, then what sort of phenotype is expected to result from this? What effect does retinoic acid have on the transcription of many hox genes? Why does this mean that retinoic acid is a very serious teratogen? *Can you invent a hypothesis to explain the spatial patterns of hox gene's expression in vertebrate embryos? What are some of the advantages of using amphibian (frog or salamander) embryos to do experiments on the mechanisms of development? What is their major disadvantage for this purpose? What is the distinction between nurse cells versus follicle cells? What kinds of animals have nurse cells? Which have follicle cells? Do any have both? What are ovarian follicles? How many planes of mirror-image symmetry can a mammal oocyte have? *Contrast this with the amount of symmetry in the oocyte of a fly! * What is meant by "symmetry breaking", and what are some specific examples from embryonic development? Do embryos become more and more symmetrical as they develop: or do they become less symmetrical? *Do these changes in symmetry occur suddenly, or in jumps? *Are external signals used to reduce, or to increase, the symmetry of developing embryos? **Can you think of some examples from physics or chemistry, or everyday life, in which natural objects have certain kinds of symmetry. *How is the symmetry of the earth related to the forces acting on it and within it? *What about the symmetry of a soap bubble? For what major discovery was Hans Spemann awarded the Nobel Prize in the 1930s? What did Spemann mean by primary induction, in contrast to secondary inductions? What inductive effect do the future notochord and somite cells have on the formation of the central nervous system, and on the differentiation of ectoderm cells? What is exogastrulation? Why was it thought that no nervous system develops in embryos that have undergone exogastrulation? What results did Pieter Nieuwkoop get when he put different parts of blastula stage amphibian embryos into tissue culture? *What parts of these results were not surprising? * What parts were surprising, and why? Would you have expected any nerve cells to differentiate from cells isolated at the blastula stage? Why or why not? If geneticists have named a gene "Headless Horseman", then what normal function does the gene probably serve? And what is the effect of mutating or deleting this gene? What is the gray crescent? Where does it form? * In what sense is experimental embryology a form of practical joking? How is the location of the gray crescent related to what will happen if you separate the first two cells of an amphibian embryo? (Or separate the 4-cell stage into two groups of 2 cells each?) Would you expect a fly egg to form a gray crescent? * Compare the gray crescent with the yellow crescent (in terms of what kinds of organisms have them; what effects they have in development; and where they are located). Explain what better controls should have been done in each of the following experiments, and what false conclusions seemed to have been proven:
B) The experiment in which a vital stain (chemical dye) was used to "prove" that fish enveloping cells undergo invagination or ingression during gastrulation? C) The experiment which used actinomycin D to "prove" that salt water fish embryos synthesize all the messenger RNA needed for all their cell types to differentiate in the first hour after development, before cleavage? Describe gastrulation in fish eggs. What are some of the main advantages of Zebra fish as a model organism for embryological research? What are some of its DISadvantages for this purpose? What is the "yolk syncytial layer"? How does it form? Describe its subsequent movements. Does it form any part of the anatomy of the fish? Describe the "enveloping layer"? What is its function in fish development? What is the eventual fate of the enveloping layer? What is meant by the "deep cells" in teleost embryos? What do they form? Where does the head form, in a fish embryo? Where does the tail form? How is the formation of the fish's body axis related to the epiboly of the enveloping layer? What is different about the formation of the neural tube in teleost fish embryos, in contrast to the formation of the neural tube in mammal or amphibian embryos? *Can you suggest other embryological examples in which a given organ or germ layer develops by what seem to be completely different mechanisms in kinds of animals that are relatively closely related? *(What about the geometry of gastrulation in amphibians versus birds? What about the formation of the epiblast and hypoblast in mammals versus birds? |