Fixation of trypanosomatids for electron microscopy with the glutaraldehyde-tannic acid method

Epimastigotes from Trypanosoma cruzi and promastigotes from Herpetomonas samuelpessoai were fixed with glutaraldehyde-tannic acid. Different concentrations of tannic acid were tested. With this technique the cellular membranes appear in negative contrast offering the same aspect as seen in cells fixed in glutaraldehyde only without post-fixation in osmium tetraoxide. An electron-dense deposit appears on the surface which possibly represents positively charged groups. The structure of the sub-pellicular microtubules appears well defined and it was possible to distinguish the 13 protofilaments which compose the microtubule wall.