Insights into intermediate phases of human intestinal fluids visualized by atomic force microscopy and cryo-transmission electron microscopy ex vivo

The current work aimed to study at the ultrastructural level the morphological development of colloidal intermediate phases of human intestinal fluids (HIFs) produced during lipid digestion. HIFs were aspirated from the ligament of Treitz at 30 and 60 min after administration of a heterogeneous liquid meal into the antrum. The colloidal structures of individual total aspirates and supernatants of aspirate after ultracentrifugation (micellar phase) were characterised by means of atomic force microscopy (AFM) and cryogenic transmission electron microscopy (Cryo-TEM). AFM revealed domain-like structures in HIFs aspirated at 30 min and both vesicles and large aggregates in HIFs aspirated at 60 min. Rough surfaces and domains varying in size were frequently present in the micellar phase of HIFs aspirated at 30 or 60 min. Cryo-TEM revealed an abundance of spherical micelles and occasionally present worm-like micelles coexisting with facetted and less defined vesicles in HIFs aspirated 30 or 60 min after meal administration,. At 60 min oil droplets were visualised with bilayers closely located to their surface suggesting a lipolytic product phases accumulated on the surface of the oil droplet. In the isolated micellar phase of HIFs aspirated 30 min after meal administration Cryo-TEM revealed the presence of spherical micelles, small vesicles, membrane fragments, oil droplets and plate-like structures. At 60 min the only difference was the absence of oil droplets. Visualisation studies previously performed to biorelevant media revealed structural features with many similarities as presented in the current investigation. The impression of the complexity and diversion of these phases has been reinforced with the excessive variation of structural features visualised ex vivo in the current study. The current study visualises for the first time the colloidal structures in the human intestinal fluids and provides insights at the ultrastuctural level of intermediate phases which impact drug solubilisation.