Villi are finger-like projections that extend into the lumen of the small intestine. Usually attributed with the role of increasing surface area in the gut for increased absorption of nutrients, they are complex structures, with defensive as well as digestive functions.

Villi are covered with a simple, columnar epithelium that features a brush border of microvilli, goblet cells that secrete mucus into the lumen, and intra-epithelial lymphocytes. The connective tissue beneath the epithelium is reticular, with few ground substances, to minimize retention of fluids (or conversely, to maximize flow of fluids into a central lymph lacteal, then out into regional lymph nodes). Within the connective tissue at the core of each villus, there are many defensive cells. These are typically arranged in a somewhat predictable order or layering.

At the tips of the villi, there are many macrophages and t-cells. Further down the villus, mast cells, t-cells and other leukocytes may be observed between the smooth muscle cells, fibroblasts, and small blood and lymph vessels. At the base of each villus there is typically a large concentration of plasma cells, which are differentiated b-cells that are producing antibodies.

Slide 146 is from an older human. The macrophages contain a golden-brown pigment called lipofuschin, or old age pigment. This is a remnant of non-digested, secondary lysosomes. These accumulate with time, hence the name ‘old age pigment'. Moving from the tips to the central parts of villi, you should be able to observe pink-staining cells with granules. These are mast cells which regulate permeability of the venules, thus controlling lymph formation. There are also lymphocytes (mainly t-cells) and other leukocytes. At the base of each villus, there are concentrations of uniform-looking cells with bluish-purple cytoplasm and spotted or striped nuclei. These are plasma cells. A basal location is ideal for a cell that depends on macrophages and t-cells for stimulation and control.

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