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Implantation bleeding occurs between 7 and 14 days after fertilization, [57] and is a small amount of light vaginal bleeding or spotting that can occur in early pregnancy due to the blastocyst penetrating the lining of the uterus during implantation.
Early development of the human embryo from ovulation through implantation. The zygote undergoes several rounds of mitosis. After the 3rd cleavage division, the embryo begins the process of compaction, which, in human, is only completed when the embryo consists of 8-16 cells, [7] [8] then becoming known as the morula.
At the 16–cell stage the compacted embryo is called a morula. [14] [15] Once the embryo has divided into 16 cells, it begins to resemble a mulberry, hence the name morula (Latin, morus: mulberry). [16] Concomitantly, they develop an inside-out polarity that provides distinct characteristics and functions to their cell-cell and cell-medium ...
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After the seventh cleavage has produced 128 cells, the morula becomes a blastula. [8] The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity the blastocoel. [citation needed]
During these cellular divisions, the zygote remains the same size, but the number of cells increase. The morula enters the uterus after three or four days—during which a cavity, called the blastocoel, is formed to produce the blastocyst. [4] Once the blastocyst is formed, it undergoes implantation into the endometrium. [4]
After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. [8] Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. [9]
The division of blastomeres from the zygote allows a single fertile cell to continue to cleave and differentiate until a blastocyst forms. The differentiation of the blastomere allows for the development of two distinct cell populations: the inner cell mass, which becomes the precursor to the embryo, and the trophectoderm, which becomes the precursor to the placenta.