Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial). The different cells derived from cleavage, up to the blastula stage, are called blastomeres. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula. Cleavage and MorulaĬell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called cleavage. Slow block begins the first few seconds after fertilization and is when the release of calcium causes the cortical reaction, various enzymes releasing from cortical granules in the eggs plasma membrane, to expand and harden the outside membrane, preventing more sperm from entering. Fast block, the membrane potential rapidly depolarizing and then returning to normal, happens immediately after an egg is fertilized by a single sperm. To prevent more than one sperm fertilizing the egg (polyspermy), fast block and slow block to polyspermy are used. The fertilized egg cell is known as the zygote. Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. įertilization is the fusion of gametes to produce a new organism. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane. The first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals). It is covered with protective envelopes, with different layers. The egg cell is generally asymmetric, having an animal pole (future ectoderm).
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