The earliest stages of human development are difficult to study for obvious (and important) ethical reasons. Therefore, much of what we know about early human development is inferred from the study of animal development. This is a microscopic image of a mouse embryo that is about 3 days old. The zona pellucida (royal blue) is the strong outer covering that originally formed around the developing egg; the sperm penetrated this covering to fertilize the egg. The zona pellucida will disappear as the embryo begins to implant in the uterus. The embryo itself is represented by the mass of bright red nuclei; at this point, all of these cells are pluripotent (i.e., able to form any cell of the body). The dark red nuclei will be extra-embryonic tissues (trophoblast) that serve to attach the embryo to the uterine wall and contribute to the formation of the placenta. Proper formation of all of these cells is critical for successful implantation and continuation of pregnancy.
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