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Unformatted text preview: Biology 131 Outline of lectures on animal development and Problem Set 5 In the study of development, we wish to understand how the complexity of the adult multicellular animal emerges from the relative simplicity of the egg. It has long been established that that the degree of preformation of the embryo in the egg is slight; the pattern of the embryo (and the adult) develops gradually by the process of epigenesis. I. Fertilization Development is a cyclical process. Adult organisms, when reproducing sexually, make haploid gametes (egg and sperm) that unite to form a fertilized egg, the zygote. The zygote then undergoes a series of mitotic cell divisions to form a blastula, which then gastrulates to form the three layered embryo. Through the process of epigenesis , the form of the embryo gradually emerges, until an adult finally appears. In many cases, the adult form is preceded by a free- living larval form, such as the tadpoles of amphibians. In other cases, such as mammals, the adult develops directly from the embryo. We first considered how the gametes arise. Early in embryonic development, a special group of cells, the germ line cells, are set aside. These cells are already fated to become the gametes of the future adult. The processes of spermatogenesis and oogenesis, each involving meiosis, lead to the formation of sperm and eggs, respectively in the gonads. The point in the meiotic cycle that an oocyte becomes a fertilizable egg varies considerably from species to species. In the case of the frog, the oocyte matures to second meiotic metaphase, where it arrests and becomes a fertilizable egg. The arrest is relieved by fertilization, and the egg then finishes meiosis before the eggs nucleus fuses with the sperm nucleus. In the case of the sea urchin, the egg is not fertilizable until meiosis is completed. Strictly speaking, the female gamete is called an oocyte before it is fertilizable and an egg after it becomes fertilizable, regardless of the meiotic state. The fertilization process in sea urchins begins when the eggs and sperm are released into the sea water and the sperm sense a chemical signal that attracts them to the eggs. When a sperm encounters the egg jelly, it triggers the acrosome reaction , which involves the fusion of the acrosomal vesicle with the sperms plasma membrane and the release of its contents to the extracellular space, the polymerization of the pool of G-actin into filamentous f-actin, and the extension of the acrosomal process. This process occurs in many sperm cells that are attracted to a single egg as they all vie to become the one that fertilizes the egg. Even if thousands of sperm contact the outer coats of an egg, only one actually fuses with the egg and fertilizes it. If more than one sperm fertilizes the egg, that condition is called polyspermy and is lethal in sea urchins, as well as in many other (but not all) species....
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- Spring '08