Why is cell differentiation needed?
Every tissue and organ is made up of living cells. Some cells provide protection; some give structural support or assist in locomotion; others offer a means of transporting nutrients. All cells develop and function as part of the organized system — the organism — they make up. Yet each of us originated as a single, simple-looking cell — a fertilized egg, or zygote — so tiny that it can barely be seen without a microscope. (A human egg cell is about 1/100th of a centimeter in diameter, or a bit smaller than the width of a human hair.) Shortly after fertilization, the zygote begins dividing, replicating itself again and again. Before long, a growing mass, or blastula, of dozens, then hundreds, then thousands of cells called stem cells forms; each stem cell is only one-fourth to one-tenth the diameter of the original zygote, but otherwise nearly identical to it. The majority of organisms, however, consist of many more than one type of cell. Indeed, about 200 different types of cells –
