Small intestine villi. Fluorescence light micrograph of a cross-section through a normal human small intestine mucosa, showing the villi and crypts of Lieberkuhn. The villi are finger-like projections which increase the surface area of the intestine, improving the efficiency of food absorption. The crypts are tubular invaginations of the epithelium around the villi. The green stain highlights the nucleoli of the rapidly growing epithelial layer of both the villi and the crypts. All the nuclei are stained red and the epithelial cell membranes are stained blue.