This is a microscopic image of a mouse intestine that was stained using fluorescent dyes. The dark area at top is the lumen of the intestine, through which the food passes; the balloon-shaped structure is called a villus. A single line of cells with greenish-tinged cytoplasm lines the outside of the villus. These cells have specialized enzymes at their apical (luminal) surfaces that act to break down the food components and help in nutrient uptake. Note that there is only one layer of these absorptive cells between the lumen and the inside of the body. The red structure at the center is a small muscle that can act to move the entire villus; the resulting pumping action helps move nutrients from the absorptive cells into the blood. The bright blue-stained cells of the villus are called goblet cells; they secrete mucous that acts to protect the cells from invasion by the trillions of bacteria that live in the gut lumen.
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