Cholesterol-phospholipid vesicles in human bile: an ultrastructural study.

Area of Science:

• Biochemistry
• Cell Biology
• Gastroenterology

Background:

• Cholesterol transport in human bile is essential for preventing gallstone formation.
• Phospholipid vesicles represent a newly identified, bile salt-independent mechanism for cholesterol transport.
• Previous characterization utilized light scattering and gel filtration, necessitating ultrastructural investigation.

Purpose of the Study:

• To investigate the ultrastructure of phospholipid vesicles involved in cholesterol transport in human bile.
• To determine the morphology and size of these vesicles using electron microscopy.
• To elucidate the role of these vesicles in cholesterol homeostasis within bile.

Main Methods:

• Electron microscopy techniques, specifically freeze-fracture and negative-staining, were employed.
• Analysis of hepatic and gallbladder bile samples from multiple individuals.
• Comparison of vesicle appearance in fresh bile versus chromatographic fractions.

Main Results:

• Phospholipid vesicles were consistently observed in all examined human hepatic and gallbladder bile samples.
• Electron microscopy revealed spherical, unilamellar vesicles with diameters ranging from 25-75 nm.
• Quasi-elastic light scattering measurements yielded slightly larger diameters (54-94 nm).
• Vesicle morphology remained consistent across fresh bile and isolated fractions.
• Exogenous bile salts induced the dissolution of these vesicles.

Conclusions:

• Phospholipid vesicles are a significant structural component of human bile, contributing to cholesterol transport.
• Their unilamellar and spherical nature, visualized by electron microscopy, provides key ultrastructural insights.
• These vesicles play a vital role in maintaining cholesterol solubility, particularly in supersaturated bile conditions, thus preventing precipitation.