Barry Burbach asks if there is a definitive work on DiI.  There have been many
publications, and I'm sure that each lab feels that their work is the
definitive study.  In my case I have used DiI in several species (mouse, rat,
cat, bushbaby) and for longer periods of diffusion than most.  The reports of
transcellular diffusion vary greatly according to species, age, tissue
preparation and tissue studied.  We use DiI to study central nervous system
connections during development, and with good tissue preparation there is
rarely any transcellular transport.  When it does occur, it is easlily
recognized because it is fainter than the primary labeled cell, is immediately
adjacent to it, and is glial in nature.  But this does not occur in every
section, nor in every brain.  Other labs have reported the same occurrence, yet
some labs claim widespread transcellular diffusion.  So either their techniques
are poor, or their system and/or age just lends itself to this annoying
problem.  As for the mounting medium, we use 65% glycerine in PBS and when
tissue is prepared well, can re-analyze tissue over 12 months or more.  We use
this mixture, then a glass coverslip, and then go with dry or oil immersion
objectives as needed.
        ANDREA ELBERGER
        Dept. of Anatomy and Neurobiology
        The University of Tennessee, Memphis
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