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"Microvilli in duodenum. Coloured transmission electron micrograph (TEM) of a section through the human duodenum, showing microvilli on the surface epithelium (lining). The duodenum is the first part of the small intestine. The microvilli (pink) appear as tiny projections from the surface of the epithelial cells (at lower frame). Microvilli are present on two specialised cell types that comprise the duodenal epithelium. One type, goblet cells, secrete mucus; a second type, secretory cells, secrete digestive enzymes and an alkaline fluid. The existence of microvilli serves to maximise the duodenum's surface area and hence its capacity to secrete. Magnification: unknown." (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)