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The theory of embryonic development we follow today was proposed 2000 years ago by Aristotle This idea called epigenesis the form of an animal arises gradually from a generally formless egg The organisms development is largely determined by the genome of the zygote and the organization of the cytoplasm of the egg cell The first part of the development of a new animal is fertilization The gametes the sperm and the egg are both highly specialized haploid cells that will combine chromosomes to produce the diploid zygote The union of the sperm with the egg activates the egg triggering the onset of embryonic development The mammalian egg is cloaked by follicle cells that were released with the egg during ovulation The sperm must migrate through this before it reaches the zona pellucida the extracellular matrix of the egg To get there the acrosome of the sperm tip releases hydrolytic enzymes to the zona pellucida enabling sperm to enter egg The zona pellucida is made of 3 different glycoproteins The protein ZP3 functions as a sperm receptor In non mammals the acrosomal reaction releases hydrolytic enzymes that enable an elongating acrosomal process to penetrate the jelly coat of the egg and to bind to the vitelline layer When the sperm meets with an egg the sperm undergoes a acrosomal reaction the reaction releases hydrolytic enzymes that enable the sperm or sperm nucleus to enter the egg The tip of the acrosomal process is coated with a protein that binds to a specific receptor molecules just outside the plasma membrane of the egg This lock and key enables only gametes of the same species to fertilize The acrosomal reaction leads to the fusion of the sperm and the egg The fusion of the egg and the sperm causes ion channels to open allowing sodium to enter the cell changing the membrane potential This blocks other sperm cells from entering the egg and is called the fast
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