As a former EM tech, when performing acetylcholinesterase detection, the
DAB step was followed  with  tap  and distilled water rinses and then
the stained slides were exposed to fumes of OsO4 in a small but efficient
incubation chamber placed under the hood.  Our incubation chamber
consisted of a square Petri dish with a layer of gauze on the bottom.
The sponge inserts from coverglass boxes were cut in half lengthwise and
placed on either side of the bottom dish on top of the gauze.  These
supported both ends of the slide and elevated it above the gauze.  Two
drops of 1% OsO4 were placed on the gauze, the slides were placed
section-side up on the sponge "rack" and then the chamber was covered
with the lid of the Petri dish. Incubation time: 5 minutes.  Then the
slides were removed and rinsed in tap and distilled waters before
counterstaining with hematoxylin. The exposure to fumes only gave a very
crisp dark color to the nerve twigs but kept the background clean.
  This same type of incubation chamber is useful in hand IHC staining
also.  The gauze is wet with water to provide the moisture needed in the
procedure.
Mary North
St. Joseph Hospital
Bellingham, WA

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