Did you know that there are tiny vesicles in your pee?
A 2004 PNAS article by Pisitkun et al demonstrates that this is the case; the vesicles are secreted by tubular epithelial cells at the apical surface and contain numerous membrane proteins, such as (for example) aquaporin. A significant percentage of the proteinuria detected in dipstick analysis is found in these exosomes.
Urinary exosomes are of interest for several reasons, with one potential application being the ability to generate an enriched population of exosomes from human urine, which is being used for the identification of new biomarkers for acute kidney injury.