Introduction
Parasitology studies parasites and their interactions with their natural
hosts. Parasites are diverse and range from unicellular to multicellular
organisms. The size of parasites can vary between micrometres for
unicellular parasites up to several millimetres or even centimetres for
multicellular organisms. A recent review shows how different imaging
modalities (light, X-ray, electron and ion microscopies) can be used to
study parasites1. The highest resolution structural
studies of parasites are usually performed using electron microscopy.
For transmission electron microscopy, parasites are thick samples and
cannot be studied as is, they must be thinned down. This is classically
performed by conventional EM on dehydrated resin embedded samples (Fig.
1, top-left) which consists of different sample preparation steps:
- fixation with paraformaldehyde and glutaraldehyde
- staining with osmium tetroxide (also acts as a fixative agent) and
heavy metals (e.g. uranyl acetate)
- dehydration in ethanol (and acetone)
- embedding the sample in a resin which can be solidified using heat or
UV-light
This eventually creates a solid block of sample that must be cut into
thin sections using a diamond knife in an ultramicrotome (Fig. 1,
bottom-left). Then, the sections can be observed at the electron
microscope using TEM. Conventional TEM is an extremely useful method to
study the ultrastructure of cells and organisms as it allows tracking
membrane and organelle contours permitting to identify cell
compartments. Resin blocks can be stored for several years and sections
deposited on EM grids can be re-imaged several times if required. For
decades, the structure of parasites has been explored using conventional
TEM2. These studies allowed describing the internal
organisation of the cells and helped with the phylogenetic
classification of parasite species.