The creation of a complex multicellular organism begins with two haploid cells, the sperm and the egg, merging into one diploid cell. The two genomes are first kept separated as two pronuclei (Cantone and Fisher 2013), but soon thereafter join to form the zygote. This totipotent cell will then go through a series of mitotic cleavage divisions, first creating a small mass of cells called the blastula, and then (by day 3.5 in mouse or day 5.5 in human) form the blastocyst, comprised of the outer trophoblast cells and the pluripotent inner cell mass (ICM). The latter will subsequently differentiate into the three germ layers, creating the new developing organism. During differentiation, each cell expresses a different set of genes, according to its location, function, signaling cascades, etc. Since all cells share the exact same genome, epigenetic processes will dictate the silencing/activation of desired genes, and the maintenance of cellular states.
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