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Animal Development

Animal Development - A nimal Development After...

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Animal Development After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis o Fertilization The union of egg and sperm, combines haploid sets of chromosomes of these specialized cells and activates the egg by initiating metabolic reactions that trigger embryonic development. In the external fertilization of sea urchins, the egg releases molecules that attract the sperm. When a sperm comes in contact with the jelly coat of an egg, the acrosome at the tip of the sperm ddischarges hydrolytic enzymes. This acrosomal reaction allows the acrosomal process to elongate through the jelly coat. Fertilization within the same species is assured when proteins on the surface of the acrosomal process attach to specific receptor proteins extending from the plasma membrane through the vitelline layer of the egg. In response to the fusion of sperm and egg plasma membranes, ion channels open and sodium ions flow into the egg. The resulting depolarization of the membrane prevents other sperm from fusing with the egg, providing a fast block to polyspermy. The sperm nucleus enters the egg cytoplasm. Membrane fusion also initiates the cortical reaction, which involves release of calcium ions from the endoplasmic reticulum. In response to the Ca2+ increase in the cytoplasm, cortical granules located In the outer cortex release their contents to the perivitelline space. The released enzymes and other macromolecules cause the vitelline layer to elevate and harden to form the fertilization envelope, which functions as a slow block to polyspermy.
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The rise in Ca2+ concentration increases cellular respiration and protein syntehsis, known as egg activation. Parthenogenetic development can be initiated by injecting calcium into an egg. After the sperm nucleus fuses with the egg nucleus, DNA replication begins in preparation for the cell division that begins the development of the embryo. Following the internal fertilization in mammals, secretions of the female reproductive tract enhance motility of the sperm. A sperm migrates through the follicle cells released with the egg to the zona pellucida, the extracellular matrix of the egg. Complementary binding of a molecule on the sperm head to a receptor in the zona pellucida induces an acrosomal reaction. The hydrolytic enzymes released from the acrosome enable the sperm cell to penetrate the zona pellucida. Binding of a sperm membrane protein with the egg membrane leads to the release of enzymes from cortical granules. This cortical rxn causes alterations of the zona pellucida and a slow block to polyspermy. The entire sperm enters the egg. The centriole of the basal body of the sperm flagellum generates the mitotic spindle. Nuclear envelopes disperse and first mitotic division
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Animal Development - A nimal Development After...

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