In 1827, German physician Karl von Baer discovered that mammals grow from eggs that come from the mother's ovary. Fertilization of an egg by a sperm cell yields a zygote, a visually unimpressive cell 200 ^m in diameter. Every human being begins as a zygote, which houses all the necessary instructions for building the human body containing about 100 trillion (1014) cells, an amazing feat. Development begins with the fertilized egg cell dividing into two, four, then eight cells, forming the very early embryo (Figure 1-7). Continued cell proliferation and then differentiation into distinct cell types gives rise to every tissue in the body. One initial cell, the fertilized egg (zygote), generates hundreds of different kinds of cells that differ in contents, shape, size, color, mobility, and surface composition. We will see how genes and signals control cell diversification in Chapters 15 and 22.
Making different kinds of cells—muscle, skin, bone, neuron, blood cells—is not enough to produce the human body. The cells must be properly arranged and organized into tis sues, organs, and appendages. Our two hands have the same kinds of cells, yet their different arrangements—in a mirror image—are critical for function. In addition, many cells exhibit distinct functional and/or structural asymmetries, a property often called polarity. From such polarized cells arise
few cell divisions of a V
fertilized egg set the <
stage for all subsequent e development. A developing Ô
mouse embryo is shown at a
(a) the two-cell, (b) four-cell, y and (c) eight-cell stages. m
The embryo is surrounded b by supporting membranes. y
The corresponding steps n in human development ô '
occur during the first few e days after fertilization. e [Claude Edelmann/Photo
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