Small intestine lining. Light micrograph of a longitudinal section through the lining (mucosa) of the ileum, part of the small intestine, showing the numerous crypts of Lieberkuhn. These invaginations of the mucosa lie between the intestinal villi (also visible here). They produce new cells by mitotic cell division, which mature and replace dead cells lost into the lumen (cavity) of the intestine. The crypts are composed of columnar enterocyte cells (purple) and mucus-secreting goblet cells.