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Duodenum secretory cells. Coloured transmission electron micrograph (TEM) of a section through the human duodenum, showing secretory cells of the surface epithelium (lining). The duodenum is the first part of the small intestine. A row of columnar-shaped cells are seen, each with a rounded nucleus (brown) and mitochondria (purple) in the cytoplasm. Microvilli appear as tiny projections from the surface of the cells (at top). Secretory cells secrete digestive enzymes, and an alkaline fluid into the pancreas which neutralises stomach acids. Microvilli serve to maximise the duodenum's surface area and hence its capacity to secrete. Magnification: unknown. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE GSCHMEISSNER)