Regarding whether or not carbocyanine dyes would label myelin, Martin
Wessendorf suggests that dipping a myelinated nerve bundle into a dilute
solution of DiI, etc. should label the myelin.  I had previously detailed the
absence of myelin staining.  Let me clarify.
        In my own research, I have used these dyes for long-distance tract
tracing by in vitro application of solid crystals of the dye.  These crystals
were kept in place for over 1.5 years in some cases so I have observed long
term as well as short term diffusion.  I also studied a pathway (corpus
callosum) that does not begin to myelinate in rodent and cat until the second
and third postnatal weeks, respectively.  The appearance of labeling was
identical in premyelinated as well as myelinated callosum, therefore I conclude
that the myelin is not picking up any label.  However, I should slightly modify
that conclusion.  If one were to dip a myelinated nerve bundle into a dilute
solution of DiI I would expect the DiI to label the myelin sheath right around
the point of contact with the solution.  But since myelin is formed by
oligodendroglial cells that wrap around the nerve concentrically, and then the
adjacent piece of nerve is wrapped by another glial cell, there is no reason to
believe that the adjacent region of myelin will pick up any label.  My own work
shows that properly prepared tissue does not permit leakage of DiI from the
nerve membrane to the myelin, so only the "dip point" will show labeling of the
myelin.  Let me know if you want further discussion on this.
        Regards,  ANDREA ELBERGER
                  Dept. of Anatomy and Neurobiology
                  The University of Tennessee, Memphis
                  [log in to unmask]"
                                                                lepie d f l(osal
f ebnatffdt