Biology 155 Study Guides

		Study Guide 17 Animal Development can be divided into a series of stages Fertilization - sperm finding and penetrating egg, followed by fusion of sperm and egg nuclei Cell cleavage - the single celled zygote becomes many undifferentiated cells by mitotic divisions Blastulation - becoming a hollow ball of undifferentiated cells Gastrulation - invagination of cells produces the three primary tissue layers : endoderm, mesoderm, and ectoderm Neurulation - folding of ectoderm produces the nervous system Cell Migration - some cells move to specific locations in the embryo and give rise to specific tissues Organogenesis and Growth - tissues associate to become organs and the embryo increases greatly in size Both sperm and egg are active participants in fertilization sperm cells are specialized for delivery of a nucleus and penetrating an egg the acrosome of the sperm has digestive enzymes that are specific for the jelly coat many sperm cooperate to digest the "jelly coat" an egg sperm find eggs by swimming toward the source of chemicals released from the egg - Ca ions are one important substance released from the egg when one sperm cell reaches the egg membrane the egg becomes activated an activated egg envelops the head of the sperm and then takes action to prevent fertilization by additional sperm - polyspermy massive amounts of Ca are released cortical granules are released - their contents form a layer that is impenetrable - this is the fertilization membrane Cleavage involves a series of mitotic divisions of the original zygote - early cleavage always follows a specific pattern the embryo has a polarity - the animal pole has less yolk than the vegetal pole - cells at the animal pole are more active in forming the body of the embryo holoblastic cleavage is found in embryos that contain little yolk - all cell divisions are complete meroblastic cleavage is found in embryos that contain large amounts of yolk - early cell divisions are incomplete - the early embryo is a disk of cells sitting on top of a ball of yolk All embryos go through a blastula stage - a hollow space develops inside the embryo Gastrulation involves a movement of cells that creates an embryo with three primary tissue layers: endoderm, mesoderm, and ectoderm endodermal tissues form the lining of the gut and associated organs ectodermal tissues form the skin and associated glands and the entire nervous system mesodermal cells form all skeletal structures, muscle, the circulatory system, and most internal organs Neurulation is the formation of the neural tube - the notochord forms from mesodermal tissue lying the region that will become the back of the embryo - the ectoderm above the notochord folds into the neural tube. Cells in the early embryo receive messages from their neighbors that tell them what type of tissue to become - cells are induced by their neighbors the notochord induces the formation of the neural tube the optic stalk induces the formation of the lens a cell that adopted a fate has been determined determination always preceeds differentiation - taking on the characteristics of a specific cell type Some cells in the embryo serve as organizers for the rest of the body of the embryo Organizers secrete substances called morphogens - concentrations of morphogens give information about the position of cells with the embryo The early blastomeres of a vertebrate embryo are totipotent - capable of giving rise to a complete embryo cells loose totipotency when they become determined Under special conditions determined and differentiated cells can be dedifferentiated and become totipotent again - this is the basis for cloning Vertebrates and their relatives have regulative development - the early blastomeres are totipotent - removal of blastomeres does not impact development - the fate of a cell is determined by interactions with its neighbors Many invertebrates have mosaic development - the early blastomeres are not totipotent - removal of blastomeres results in embryo lacking body parts - the fate of cells is determined by chemicals that are depositied in specific patterns within the egg as it formed In spite of major differences in developmental plans, all animals appear to share a common set of genes that control some aspects of development - the homeotic genes

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