How does a single fertilized egg become a human embryo?
Quick Answer
A fertilized egg divides repeatedly by mitosis and begins forming distinct cell layers within days. By about the end of the second week, it has implanted in the uterus and started organizing into early tissues.
The Full Story
After sperm and egg combine, the resulting zygote starts rapid cell divisions called cleavage. Over the next several days, it becomes a blastocyst, a hollow cluster of cells with an inner cell mass that will form the embryo. Around 6 to 10 days after fertilization, the blastocyst implants in the uterine lining. From there, cells begin differentiating into the three germ layers that will produce the body's organs and tissues.
Key Facts
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Suggested video title for this topic:
"From one cell to a body: the first days of human life"
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