Biology 441 Spring 2009

Embryology Biology 441 Spring 2009 Albert Harris

Added Review Questions for Feb 2 Embryology Exam
A star* means a difficult question; I won't ask very many such difficult questions.
Two stars** means nobody knows the answer, but that it's an interesting question that you should have some ideas about.
Approximately how many differentiated cell types do human and other mammal bodies contain? What are ten specific examples of differentiated cell types? What are some kinds of animals that have many fewer differentiated cell types?
Name the three primary germ layers? Do they have anything to do with germs in the sense of infectious diseases? Hint: no, they don't. What are several major organs that develop primarily from each of these primary germ layers?
What is gastrulation? What is neurulation? Neurulation subdivides which of the three primary germ layers into what three sub-divisions? Gastrulation subdivides early embryonic cells into what three categories? Compare and contrast the geometric pattern of gastrulation in the embryos of sea urchins, teleost fish, frogs and salamanders, birds, and mammals.
Compare the cleavage divisions of the embryos of these same different kinds of animals. Which ones have holoblastic cleavage? Which ones have meroblastic cleavage? Which ones form a primitive streak, with a Hensen's node? Which ones form a blastotpore? Which form the blastopore by invagination? Which form the blastopore by involution? *What is similar about the formation of the epiblast and hypoblast in mammal embryos, as compared with the formation of the primary and secondary mesenchyme of sea urchin embryos?
Embryos have synchronous cleavage in which kinds of animals? Which of these kinds of embryos take the longest time between early mitotic divisions. What do the brain and spinal cord develop from? Describe and draw sequential stages of the process of gastrulation as it occurs in sea urchins, frogs, fish, birds and mammals?
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? *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 is some evidence that mammal development is extremely regulative? *Can you suggest any relationship between regulative development related to the current political debates about "embryonic stem cells"? [Hint: If human development were very mosaic, then would there be stem cells? You could argue pro or con. The point is to show that you understand concepts and their medical implications.]
Describe the life cycle of Dictyostelium. What are some important reasons why so much research, including research at medical schools, has been concentrated on Dictyostelium? What is chemotaxis? What function does chemotaxis serve in the life cycle of Dictyostelium? In what sense is Dictyostelium even more regulative than sea urchin eggs? What happens if you cut a Dictyostelium "slug" into pieces? How many pieces can you cut them into, and get the same result? About how many differentiated cell types are there in Dictyostelium?
What is the role or transcription factors and promoter regions in the selective transcription of luxury genes in cell differentiation? * Can you suggest alternative molecular mechanisms for the irreversibility (or is it self-perpetuation?) of cell differentiation? ** Can you suggest molecular mechanisms to prevent cells from differentiating into a combination of two differentiated cell types at the same time? *What happens if you fuse two differentiated cells of the same cell type? What if you fuse cells of 2 or more different differentiated cell types? *An interesting exception to this rule is that if you fuse a bird red blood cell with a mammal differentiated cell of some other cell type, then the bird nucleus becomes reactivated and the fused cell makes luxury bird proteins corresponding to whatever differentiated cell type the mammal cell was! How would you interpret this result? Hint: what does it imply about whether mouse transcription factors will bind to bird promoter regions? Can you invent a cure for cancer based on selectively stimulating cancer cells to fuse with nearby differentiated cells? How about a cure for autoimmune disease based on inducing anti-self lymphocytes to fuse with, for example, epithelial cells?
* Why are lymphocytes more likely than other cell types to have chromosome translocations?
[Hint: remember that the DNA sequences that code for antibody binding sites are created by breaking and rejoining DNA.] Most cancers of lymphocytes are caused by chromosome breaks next to genes for antibodies; why does that make sense?
What is the main difference between cell locomotion in higher plant cells as compared with cells of higher animals? [Hint: the plant cells don't have locomotion.] Sketch at least five kinds of morphogenetic cell movements that occur in vertebrate embryos? Explain the key differences (or distinctions) between each of them. For each one, describe and sketch particular stages and locations in developing embryos where they occur. How is cell locomotion special in nerve cells? * Suppose that you compared the effects of inserting into developing embryos either small pieces of (impermeable) mica, or inserting thin sheets of some plastic that is permeable to small diffusing molecules: Suppose that nerve cells detoured and went around the pieces of implanted mica, but did not go around the pieces of plastic (and just tried to ram their way through) what could be the cause of this difference.
If you culture Dictyostelium amoebae on both sides of permeable sheets of plastic, then when they form aggregations on one side of the plastic, then aggregations will always form on the other side of the plastic sheet at the exact same locations: what is this evidence for? What is cyclic AMP? * What if you had sheets of several different kinds of plastics, some of which were permeable only to molecules smaller than 500 amu molecular weight, and other kinds of plastic that let bigger molecules diffuse through them, up to different maxima (like 1,000, 5,000. 20,000): how could you use these to determine the molecular weight of unknown (hypothetical) chemotactic attractants, or chemicals that induce differentiation of some cell type? * If you were trying to find out whether aggregation of some kind of cell is caused by chemotaxis, which would you do first: ? Find out the chemical nature of the diffusing chemical, or ?? Test whether the attraction signal can pass through a permeable plastic, but not through impermeable mica, or ??? Test what happens when cells aggregate with water flows slowly past them, specifically whether attraction occurs more from the downstream side than from the upstream side. * By testing the effects of the speed of water flow, could you measure the speed of diffusion of the hypothetical chemotactic attractant chemical? Or would you first need to discover what this chemical is, before you could prove conclusively whether chemotaxis is occurring? Why not? *Would you be able to identify this chemical until after you had already done these kinds of indirect experiments? ** Would sequencing the organism's genome be of any use in answering these kinds of questions?
Why were embryologists not surprised by the birth of Dolly the sheep? ** Some people (me) predicted that Dolly the sheep might die at an unusually young age: based on what reasoning might you expect that? What if Dolly the sheep had been made by injecting the nucleus from a B-lymphocyte into an enucleated oocyte? (Mice really have been made this way.) Antibodies secreted by animals produced this way make antibodies all of which have exactly the same shaped binding site: why do you think this is? Suggest how you could use nuclear transplantation to generate an animal with an autoimmune disease. ** Suggest different conceivable ways that anti-self B-lymphocytes normally get eliminated; **Suggest experimental criteria that might be able to distinguish which of these elimination methods is actually used; ** And suggest clinical treatments to treat autoimmune diseases, depending on which of the different elimination methods normally gets rid of all the anti-self B-lymphocytes.
** What is the weirdest embryological fact you have learned about so far? ** Which embryological phenomenon would you prefer to be able to control, in order to cure some diseases or improve agriculture? * Invent a "science fiction" life form whose embryological development is fundamentally different from humans, in some interesting way? In the evolution of reptiles from amphibians, suggest what intermediate structures and patterns of gastrulation might have occurred in the most primitive reptiles. What about the embryology of the intermediate stages between egg-laying mammals and higher mammals? What are several different ways in which mammal embryos can develop into identical twins (Hint: NOT separating at the two cell stage). What would happen if a mammal gastrula developed one primitive streak but two or more Hensen's nodes? Draw a normal human embryo at the stage where it has a Hensen's node. Which parts of this embryo become part of the body, and which parts become part of the placenta? Consider a chicken egg that you buy at the store: which parts become the body of the chicken? Which parts are non-living, protective coatings?