OBJECTIVES: Studies on the characterization of ascending colonic fluids are scarce, limited to physicochemical characterization of their composition, and little is known for the morphology of the produced colloidal phases. In an attempt to gain insights for their structure at the ultrastructural level, samples from the lumen of ascending colon were collected from patients with ulcerative colitis in remission.

METHODS: After ultracentrifugation, the supernatants of two samples (one with high and one with low cholesterol level) were visualized by means of cryogenic transmission electron microscopy.

KEY FINDINGS: In the supernatants with high cholesterol content, micellar-like structures, bilayer fragments, open vesicles, and uni-, bi- and trilamellar vesicles were abundant. In addition, crystals of cholesterol were frequently observed. In contrast, in the sample with low cholesterol content, oily solids, plates of cholesterol monohydrate and elongated structures were present. Few unilamellar vesicles were occasionally visualized.

CONCLUSIONS: The current study gives direct evidence, for the first time, of the existence of 'remnants' of lipolytic products in the fasted ascending colon. The impact of these structures to colonic absorption of drugs is an open question.