With its 50 trillion cells and thousands of organs, the human body may seem to be a structure of forbidding complexity. Fortunately for our health, longevity, and self-understanding, biologists of the past were not discouraged by this complexity, but discovered patterns that made it moreunderstandable. One such pattern is the fact that these trillions of cells belong to only 200 different types or so, and these cells are organized into tissues that fall into just four broad categories: epithelial, connective, nervous, and muscular tissue.
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