chapter47 - CHAPTER 47 ANIMAL DEVELOPMENT Animals develop...

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CHAPTER 47 ANIMAL DEVELOPMENT Animals develop throughout their lifetime. Development begins with the changes that form a complete animal from the zygote and continue as progressive changes in form and function. I. The Stages of Early Embryonic Development A. From egg to organism, an animal's form develops gradually: the concept of epigenesis Epigenesis proposes that the form of an embryo gradually emerged from a formless egg. - Originally proposed by Aristotle Modern biology has found that an organism's development is mostly determined by the zygote's genome and the organization of the egg cell's cytoplasm. - The heterogenous distribution of messenger RNA, proteins, and other components in the unfertilized egg greatly impacts the development of the embryo in most animals. - After fertilization, cell division partitions the heterogenous cytoplasm in such a way that nuclei of different embryonic cells are exposed to different cytoplasmic environments. - These different cytoplasmic environments result in the expression of different genes in different cells. - This leads to an emergence of inherited traits that is ordered in space and time by mechanisms controlling gene expression. B. Fertilization activates the egg and brings together the nuclei of sperm and egg Fertilization is important because: - It forms a diploid zygote from the haploid sets of chromosomes from two individuals. - It triggers onset of embryonic development. 1. The acrosomal reaction The acrosomal reaction is the discharge of hydrolytic enzymes from a vesicle in the acrosome of a sperm cell (based on studies with sea urchins): - Upon contacting the egg's jelly coat, the acrosomal vesicle in the head of the sperm releases hydrolytic enzymes via exocytosis. - These enzymes enable an acrosomal process to elongate and penetrate the jelly coat. - A protein coating the tip of the process attaches to specific receptors on the egg's vitelline layer (just external to the plasma membrane). - This provides species specificity for fertilization. - Enzymes of the acrosomal process probably digest vitelline layer materials allowing the tip of the process to contact the egg's plasma membrane. - The sperm and egg's plasma membranes fuse, allowing the sperm nucleus to enter the egg and causing a depolarization of the plasma membrane that prevents other sperm cells from also uniting with the egg. 2. The cortical reaction The fusion of the egg and sperm membranes stimulates a series of changes in the egg's cortex known as a cortical reaction . The cortical reaction results in the vitelline layer of the egg to become harden to the fertilization membrane. - The fertilization membrane prevents entry of additional sperm. 3. Activation of the egg
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This note was uploaded on 04/05/2008 for the course BIO G 006 taught by Professor Macneill,a. during the Spring '08 term at Cornell.

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chapter47 - CHAPTER 47 ANIMAL DEVELOPMENT Animals develop...

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