Small intestine. Coloured transmission electron micrograph (TEM) of a section through the wall of the small intestine. The surface consists of many small hair-like absorptive structures (red) called microvilli. These absorb nutrients from food as it passes through the lumen (internal space, blue, across top). The microvilli are the upper layer of the surface (epithelial) cells. These are columnar in shape (and so are called columnar epithelial cells). These cells (three seen here) extend down to lower centre, and the nuclei (yellow) are seen. Magnification: x600 when printed 10cm high.