MIT Department of Biology 7.013: Introductory Biology - Spring 2004 Instructors: Professor Hazel Sive, Professor Tyler Jacks, Dr. Claudette Gardel 7.013 Problem Set 5 FRIDAY April 2nd, 2004 Problem sets will NOT be accepted late. Problem 1 A great medical revolution has occurred with the ability to assist the large number of couples that previously would be incapable of having children. Generally, about half of infertility cases are due to paternal issues. Congratulations, after inspiration from 7.013 you decided to forsake a lucrative career in computer science and enter the world of medicine. After an additional four years of medical school, and numerous years of training you are fiscally broke, but are now a medical professional treating infertility. A young and otherwise healthy couple approaches you after experiencing difficulty in conceiving despite over one year of attempts. The woman has had a child from a previous husband. You suspect that the man has infertility issues and begin a work-up to determine the problem. A. A sample of freshly collected semen is analyzed microscopically for sperm number and activity. Unlike the videos shown in class of vigorous sperm ascending towards the egg, the sperm sample indicates a normal number of sperm swimming rather lazily in a dish. a. How many sperm are typically present in a human ejaculate? Why are there so many? There are approximately 5x107sperm in a normal ejaculate. That many sperm are necessary because very few sperm actually reach the egg, and the more sperm are included the higher the chance for fertilization. b. What (other than an actual defect) might explain the languorous behavior of thefreshly ejaculated sperm in vitro? Sperm require capacitation by the vaginal environment in order to achieve full activity. c. Human ejaculate is a complex mixture of many substances. One of the primary ingredients is fructose (a simple disaccharide). What two properties of sperm may explain the presence of fructose in ejaculate? Sperm are very small and highly motile. They have little storage for energy sources but have high energy demands. The presence of fructose can supply the sperm with fuel for their journey.B. After determining that the number, morphology, and activity of the sperm are normal, you next seek to test whether the sperm are capable of fertilizing an intact egg. However, with human eggs hard to come by you are forced to use hamster eggs. Hamster eggs are easily obtained by hormonal treatment of female hamsters inducing super-ovulation. A large number of eggs can be recovered by removal and gentle tapping of the uterine horn with eggs dropping out into a dish. a) What hormone was likely used to induce superovulation of the hamster? What is the normal fate of these extra eggs (without the introduction of hormone)? FSH stimulates the development of more immature follicles to produce mature ovum. Normally, in humans, only a single egg is released and the rest of the follicles recede and the immature ova die. b) Have the collected eggs completed meiosis? If not, at what meiotic stage are the eggs frozen? What prompts the completion of meiosis? No, at the time of ovulation eggs are frozen at Metaphase II. Only fertilization of the egg is able to induce the completion of meiosis. c) Would you expect a human sperm to fertilize an intact hamster egg? What component of the hamster egg prevents fertilization from occurring? No, human sperm would not be able to bind to the outer coating of the egg called the zona pelucida. C. A simple enzymatic treatment of the hamster eggs can effectively remove thecomponent that you named above. After removal of this component, human sperm can now fertilize the treated hamster egg to form a zygote. Worry not; the resulting zygote cannot actually develop into a human/hamster hybrid. a) Assuming that the man’s sperm can fertilize the treated hamster egg, at what stage of fertilization are the man’s sperm possibly failing. Because the sperm are active, and capable of fertilizing an egg lacking a zona, it is likely that the sperm are either failing to bind to the zona or there is a defect in the acrosomal reaction. b) Are the couple’s plans for children doomed? What mode of ART would you suggest to overcome the man’s difficulties? No, the sperm can be injected directly into mature eggs harvested from his wife. This is called Intracytoplasmic Sperm Injection (ICSI)Problem 2 Teratocarcinoma is a rare form of cancer that typically forms on the testes or ovary. As opposed to most cancers that are composed of accumulations of cells that are similar in appearance, teratocarcinomas are quite heterogenous. Both gross and histologic examination of a teratocarcinoma reveal a variety of differentiated tissues present within the cancerous mass including: hair, teeth, intestinal epithelium, skin, etc.. a) Was the cell that gave rise to the teratocarcinoma differentiated? No, the presence of the many different differentiated cell types within the tumor suggests that the primary tumor was made up of non-determined non-differentiated cells. b) Assuming that the teratocarcinoma cells can give rise to all the tissues present in an adult how would you describe their potency? The teratocarcinoma cells are totipotent. Teratocarcinoma cells have proven quite useful in vitro(grown in culture) to define both inducers and determinants of cell differentiation. Exposure of undifferentiated teratocarcinoma cells to a given extracellular signal results in their specific differentiation into a given cell fate. c). Despite their similar potency as embryonic stem cells, teratocarcinoma cells are not used in vivo and have no therapeutic potential. Why not? As opposed to embryonic stem cells, teratocarcinoma cells are cancerous. In general one does not want to inject cancer as a therapy. Below is an illustration showing the cross-section of a blastocyst… Inner Cell Mass d) Label the section of the blastocyst that gives rise to embryonic stem cells.Grown in culture, embryonic stem cells form colonies of cells. You would like to use the cells to identify novel determinants of differentiation. You expose a plate of ES cells to retinoic acid (Vitamin A). Immediately after exposure the ES cells begin to express Gene N. One week after exposure you observe that the cells in the dish have the appearance of mature neurons. e) Please match the items below with all of the terms that apply. i. ES cell 1.
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