Chapt 47 - CHAPTER 47 - ANIMAL DEVELOPMENT A. The Stages of...

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A. The Stages of Early Embryonic Development 1. From egg to organism, an animal’s form develops gradually: the concept of epigenesis Preformation : the egg or sperm contains an embryo that is a preformed miniature adult. Epigenesis : the form of an animal emerges from a relatively formless egg. An organism’s development is primarily determined by the genome of the zygote and the organization of the egg cytoplasm. 2. Fertilization activates the egg and brings together the nuclei of sperm and egg Sea urchins are models for the study of the early development of deuterostomes. Sea urchin eggs are fertilized externally. Sea urchin eggs are surrounded by a jelly coat. The Acrosomal Reaction. Acrosomal reaction : when exposed to the jelly coat the sperm’s acrosome discharges its contents by exocytosis. Hydrolytic enzymes enable the acrosomal process to penetrate the egg’s jelly coat. The tip of the acrosomal process adheres to the vitelline layer just external to the egg’s plasma membrane. The sperm and egg plasma membranes fuse and a single sperm nucleus enters the egg’s cytoplasm. Na + channels in the egg’s plasma membrane open. Na + flows into the egg and the membrane depolarizes: fast block to polyspermy. The Cortical Reaction. Fusion of egg and sperm plasma membranes triggers a signal-transduction pathway. Ca 2 + from the egg’s ER is released into the cytosol and propagates as a wave across the fertilized egg—IP 3 and DAG are produced. IP 3 opens ligand-gated channels in the ER and the released Ca 2 + stimulates the opening of other channels. High concentrations of Ca 2 + cause cortical granules to fuse with the plasma membrane and release their contents into the perivitelline space. The vitelline layer separates from the plasma membrane. An osmotic gradient draws water into the perivitelline space, swelling it and pushing it away from the plasma membrane. The vitelline layer hardens into the fertilization envelope : a component of the slow block to polyspermy. The plasma membrane returns to normal and the fast block to polyspermy no longer functions. Activation of the Egg. High concentrations of Ca 2 + in the egg stimulate an increase in the rates of cellular respiration and protein synthesis. In sea urchins, DAG activates a protein that transports H+ out of the egg. The reduced pH may be indirectly responsible for the egg’s metabolic responses to fertilization. In the meantime, back at the sperm nucleus. .. The sperm nucleus swells and merges with the egg nucleus—>diploid nucleus of the zygote. DNA synthesis begins and the first cell division occurs.
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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Chapt 47 - CHAPTER 47 - ANIMAL DEVELOPMENT A. The Stages of...

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