Transmission
electron microscopy (TEM) is a time consuming and technically complicated
method, and it also requires a skilled and experienced user. However, it makes
it possible to examine the animals at very high magnifications and obtain
detailed and valuable information about their ultrastructural organization.
When an animal is prepared for TEM it is first treated with osmiumtetraoxide
and different aldehydes to fixate the muscles and cell membranes. When fixed
the animal is embedded in a polymer and cut in ultra-thin sections. The
sections are subsequently placed in the TEM where it can be investigated at
magnifications up to 300.000x. TEM is especially useful when examining
organelles in cells but
it may also be used on other structures. In L. maerski TEM has given
very important information about the ultrastructure of the jaws, and this
information has been an important key to understand the animals phylogenetic
position.
TEM-photograph
showing a section trough the pharynx of Limnognathia
maerski. The paired disc-shaped structures in the center are
transverse sections through the main jaws (click to see main jaws in SEM
and 3-D). The picture shows that the jaw elements contain lucent
"balls" with a black dot in the center. TEM-investigations of
several serial sections through the jaws of L. maerski have shown
that these "balls" are sections through long rods that seem to
be the basal brick in the formation of the jaw elements. This special
feature is only known from two other animal groups, the Rotifera and the
Gnathostomulida, and it has been interpret as an important link between
the three groups.
Transmission
electron microscopy
