PREPARATION OF BIOLOGICAL SPECIMENS Fixation of tissues is the
most crucial step in the preparation of tissue for observation
in the transmission electron microscope. Fixation consists of
two steps: cessation of normal life functions in the tissue
(killing) and stabilization of the structure of the tissue
(preservation). The goal of fixation is to preserve structure as
faithfully as possible compared to the living state. The three
most important parameters to remember about fixation are: (1)
keep the time between killing and fixation to a minimum. (2)
keep the size of the tissue as small as possible without losing
information or destroying the tissue. If a large specimen must
be fixed, keep one dimension less than 1 mm, or else nick areas
of the specimen that can be discarded so that the fixative can
penetrate. Effective penetration of fixative is about 0.5 mm for
osmium. (3) keep gross tissue deformation to a minimum by using
sharp implements and keeping manipulation of the specimen to the
minimum necessary.