The nucleolus is the main site for synthesis and processing of
ribosomal RNA in eukaryotes. In mammals, plants, and yeast the nucleolus
has been extensively characterized by electron microscopy, but in the
majority of the unicellular eukaryotes no such studies have been
performed. Here we used ultrastructural cytochemical and
immunocytochemical techniques as well as three-dimensional reconstruction
to analyze the nucleolus of Trypanosoma cruzi, which is an early
divergent eukaryote of medical importance. In T. cruzi
epimastigotes the nucleolus is a spherical intranuclear ribonucleoprotein
organelle localized in a relatively central position within the nucleus.
Dense fibrillar and granular components but not fibrillar centers were
observed. In addition, nuclear bodies resembling Cajal bodies were
observed associated to the nucleolus in the surrounding nucleoplasm. Our
results provide additional morphological data to better understand the
synthesis and processing of the ribosomal RNA in kinetoplastids.