Chapter 47 Animal Development 1 Chapter 47 Animal Development Introduction Embryonic development involves common stages that occur in a set order The first is fertilization the fusion of sperm and egg which forms a zygote Development proceeds with the cleavage stage during which a series of cell divisions divide or cleave the zygote into a many celled embryo Gastrulation occurs when the blastula folds in on itself rearranging into a three layered embryo Organogenesis is the last major stage of embryonic development when local changes in cell shape and large scale changes in cell location generate the rudimentary organs from which adult structures grow The cleavage division converts the embryo into a hollow ball of cells called a blastula Concept 47 1 Fertilization and cleavage initiate embryonic development Fertilization Fertilization is the formation of a diploid zygote from a haploid egg and sperm The sperm first dissolves or penetrates any protective layer surrounding the egg to reach the plasma membrane Molecules on the sperm surface then bind to receptors on the egg surface helping to ensure that a sperm of the same species fertilizes the egg Changes at the surface of the egg prevent polyspermy the entry of multiple sperm nuclei into the egg If polyspermy were to occur the resulting abnormal number of chromosomes in the embryo would be lethal The Acrosomal Reaction The Cortical Reaction When sea urchins release their gametes into the water the jelly coat that surrounds the egg exudes soluble molecules that attract the sperm which swim toward the eggs The acrosomal reaction beings as soon as the head of the sperm contacts the coat of the egg This reaction begins with the discharge of hydrolytic enzymes from the acrosome a specialized vesicle at the tip of the sperm The enzymes digest part of the jelly coat and the acrosomal process elongates the sperm and allows it to penetrate the coat Protein molecules on the tip of the extended acrosomal process binds to specific receptor proteins that extend out from the egg plasma membrane The lock and key recognition is important for external fertilization because the surrounding water may contain gametes of other species The sperm nucleus then enters the egg cytoplasm as ion channels open in the egg s plasma membrane Sodium ions diffuse into the egg and cause depolarization a decrease in the membrane potential By preventing additional sperm from fusing with the egg s plasma membrane this depolarization acts as a fast block to polyspermy A longer lasting block to polyspermy is established by cortical granules vesicles that lie beneath the egg plasma membrane in the rim of cytoplasm known as the cortex The cortical granules fuse with the egg plasma membrane and are released into the space between the plasma membrane and the surrounding vitelline layer a structure formed by the extracellular matrix of the egg Enzymes trigger a cortical reaction which lifts the vitelline layer away from the egg and hardens the layer into a protective fertilization envelope Together the fertilization envelope and other changes in the egg s surface impede the entry of additional sperm nuclei and thus acts as a longer term slow block to polyspermy Formation of the fertilization envelope requires a high concentration of calcium ions in the egg They found that calcium ions spread across the egg in a wave the correlated with the appearance of the fertilization envelope The study demonstrated that the release of calcium ions into the cytosol from the endoplasmic reticulum is controlled by a signal transduction pathway activated by sperm binding The acrosomal and cortical reactions during sea urchin fertilization Contact the sperm contacts the egg s jelly coat triggering exocytosis of the sperm s acrosome Acrosomal reaction hydrolytic enzymes released from the acrosome make a hole in the jelly coat Growing actin filaments form the acrosomal process which protrudes from the sperm head and penetrates the jelly coat Proteins on the surface of the acrosomal process bind to receptors in the egg plasma membrane Contact and fusion of sperm and egg membranes fusion triggers depolarization of the membrane which acts as a fast block to polyspermy Cortical reaction cortical granules in the egg fuse with the plasma membrane The secreted contents clip off sperm binding receptors and cause the fertilization envelope to form This acts as a slow block to polyspermy Entry of sperm nucleus Egg Activation Cleavage Fertilization in Mammals Chapter 47 Animal Development 2 A major function of fertilization is the combining of haploid sets of chromosomes from two individuals into a single diploid cell the zygote The events of fertilization also initiate metabolic reactions that trigger the onset of embryonic development thus activating the egg Studies show that injecting calcium ions into an unfertilized egg activates egg metabolism in many species despite the absence of sperm Egg activation requires only the proteins and mRNAs already present in the egg cytoplasm The rise in calcium ion concentration that causes the cortical reaction also causes egg activation Mammal fertilization occurs internally Secretion in the female reproductive tract provides a moist environment for the sperm motility and structure Changes in the sperm motility and structure allow the sperm to fertilize the egg in a process of capacitation The sperm must travel through a layer of follicle cells that surround the egg before it reaches the zona pellucida the extracellular matrix of the egg Binding of a sperm to this receptor induces an acrosomal reaction facilitating sperm passage through the zona pellucida to the egg This binding also exposes a protein on the sperm that binds with the egg plasma membrane Sperm binding triggers changes within the mammalian egg that leads to a cortical reaction the release of enzymes from cortical granules to the outside of the cell These enzymes catalyze changes in the zona pellucida which then functions as the slow block to polyspermy After the egg and sperm membranes fuse the whole sperm is taken into the egg The chromosomes are organized into a single mitotic spindle and only after the first division is there a true diploid nucleus with a nuclear membrane Cleavage divides the cytoplasm the large fertilized eggs into many smaller cells called blastomeres Once fertilization is complete many animal species undergo a succession of rapid cell divisions that characterize the