A new set of carbohydrate-positive vesicles in encysting Giardia lamblia.

Giardia lamblia is a protozoan parasite that presents both trophozoite and cyst forms. In this study, the distribution of the different sugar residues and the origin of the carbohydrate components of the cyst wall were studied using transmission electron microscopy, ultrastructural cytochemistry for carbohydrate detection and immunocytochemistry. Immunofluorescence microscopy using anti-cyst wall protein 1 (CWP1) and gold- and fluorescent-conjugated lectins, such as WGA and DBA, were also used. Interestingly, a population of carbohydrate-containing vesicles, distinct from the encystation-specific vesicles (ESVs) was found in the encysting cells and was named encystation carbohydrate-positive vesicles (ECVs). The differences between the ECVs and the ESVs were: (1) they are electron-translucent, whereas ESVs are electron dense; (2) they do not react with antibodies against cyst wall proteins; (3) the contents are positive for carbohydrates, whereas ESVs display a negative reaction; and (4) they exhibit a positive labeling for DBA indicating the presence of N-acetyl-galactosamine, whereas ESVs are negative. To evaluate if ECVs could be vesicles involved in the endocytic pathway, endocytic markers were used. No co-localization of these markers with ECVs was observed. We suggest that the ECVs may represent a new structure involved in cyst wall formation.