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Lecture_Notes_Chapter_47 - Chapter 47 Animal Development An...

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Chapter 47 Animal Development An organism’s development is determined by the genome of the zygote and by differences that arise between early embryonic cells Cell differentiation is the specialization of cells in their structure and function Morphogenesis is the process by which an animal takes shape From egg to organism, an animal’s form develops gradually: the concept of epigenesis 1. Fertilization activates the egg and brings together the nuclei of sperm and egg 2. Cleavage partitions the zygote into many smaller cells 3. Gastrulation produces the three primary germ layers 4. Cytoplasmic determinants establish the body axes and differences among cells of the early embryo 5. Inductive signals (cell-cell signaling) drive differentiation and pattern formation 6. Morphogenesis produces body plan 1. Fertilization Important events regulating development occur during fertilization and each of the three successive stages that build the animal’s body The main function of fertilization is to bring the haploid nuclei of sperm and egg together to form a diploid zygote Contact of the sperm with the egg’s surface initiates metabolic reactions within the egg that trigger the onset of embryonic development The Acrosomal Reaction The acrosomal reaction is triggered when the sperm meets the egg Releases hydrolytic enzymes that digest material surrounding the egg Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy The Cortical Reaction Fusion of egg and sperm also initiates the cortical reaction inducing a rise in Ca 2+ that stimulates cortical granules to release their contents outside the egg These changes cause the formation of a fertilization envelope that functions as a slow block to polyspermy In mammalian fertilization, the cortical reaction modifies the zona pellucida as a slow block to polyspermy Activation of the Egg Another outcome of the sharp rise in Ca 2+ in the egg’s cytosol is a substantial increase in the rates of cellular respiration and protein synthesis by the egg cell With these rapid changes in metabolism the egg is said to be activated
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In a fertilized egg of a sea urchin, a model organism many events occur in the activated egg 2. Cleavage Fertilization is followed by cleavage - A period of rapid cell division without growth Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres The eggs and zygotes of many animals, except mammals have a definite polarity The polarity is defined by the distribution of yolk with the vegetal pole having the most yolk and the animal pole having the least The development of body axes in frogs is influenced by the polarity of the egg Cleavage planes usually follow a specific pattern that is relative to the animal and vegetal poles of the zygote Meroblastic cleavage, incomplete division of the egg occurs in species with yolk- rich eggs, such as reptiles and birds
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Lecture_Notes_Chapter_47 - Chapter 47 Animal Development An...

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