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Cleavage and Blastulation
The first two processes of embryological development are cleavage and blastulation. Following ovulation, the secondary oocyte (egg cell) makes its way into the fallopian tube. If sperm has been deposited into the female reproductive tract, then fertilization will take place within the thickest portion of the fallopian tube. Once the sperm fuses with the egg cell, an influx of calcium ions into the cytoplasm leads to a series of metabolic process, such as the cortical reaction (blocks other sperm cells from entering) and protein synthesis. Shortly after fertilization (about 24 hours), the first mitotic division takes place in which the unicellular zygote forms a two-celled embryo. These two cells continue to divide and eventually form a 32-cell stage called the morula. Each one of the cells in the morula are called blastomeres and they are identical in size, shape and carry the same genetic information. Collectively, these quick mitotic cell divisions following fertilization are known as cleavage. During cleavage, the cells do not actually grow in size but rather become even smaller so that the overall bundle of cells is the same size as the original zygote. Cleavage simply partitions the zygote into many identical cells that can eventually be used as the building blocks for the developing embryo. The blastomeres (i.e. cells) of the morula will continue dividing via mitosis and eventually will form a spherical structure called a blastula (blastocyst in humans and other mammals). This blastocyst contains a hollow cavity that is filled with a nutritious fluid. The outer cells of the this structure make up the trophoblast, which will eventually form the chorion and placenta. The inner cell mass consists of cells that eventually form the entire organism itself. Cleavage takes place as the zygote travels along the fallopian tube while blastulation takes place when the growing embryo makes its way into the uterine cavity (before implantation).
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