OBJECTIVES: To investigate at the ultrastructural level, the colloidal phases formed in the lumen of the distal ileum and caecum of healthy adults.

METHODS: Cryogenic transmission electron microscopy (Cryo-TEM) was employed to image the intermediate colloidal phases of human intestinal contents collected from distal ileum and caecum of two healthy volunteers under fasted and fed state conditions.

KEY FINDINGS: In samples collected both in the fasted and fed states, Cryo-TEM study revealed the presence of large spherical unilamellar and occasionally bi-lamellar and oligolamellar vesicles with diameters ranging from 50 to 200 nm for both volunteers in distal ileum and caecum. Bilayer fragments were frequently observed in caecal samples. Plate-like structures resembling the morphology of cholesterol plates were visualised in all samples. Elongated structures were observed in the fed state in distal ileum and caecum for both volunteers, whereas no micellar structures could be detected for all samples.

CONCLUSIONS: This study provides a framework for understanding the structure of colloidal phases, and it may assist in elucidating the role of dosing conditions on drug absorption from the distal ileum and caecum.