From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms: Cleavage patterns followed by holoblastic and meroblastic eggs Holoblastic In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first. Because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk (i.e. Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta or milk, such as might be secreted from a marsupium. 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 fertilisation, or outside in the case of external fertilisation. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.įertilization (also known as 'conception', 'fecundation' and 'syngamy') is the fusion of gametes to produce a new organism. 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 and mesoderm) and a " vegetal pole" (future endoderm).
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