Bob,

There is no such thing as 'overdehydration' in 70% alcohol prior to paraffin
embedding.
In fact, 70% alcohol has been a preferred storage medium for formaldehyde
fixed tissue for many years.
The effect you are describing sounds more like alcohol fixation.
How long are you fixing in formaldehyde?
If the answer is less than 24 hours at R.T., you must consider the fact
that, for times shorter than 24 hours, formaldehyde fixation is freely
reversible.
The shorter the time in formaldehyde the more rapid the reversal.
This reversal occurs most readily in water and aqueous solutions such as
PBS.  However, it also occurs in 70% alcohol.
Upon reversal, the tissue is subsequently re-fixed by the alcohols involved
in processing. This results in the typical shrinkage and hardening obtained
following alcohol fixation.

Bryan

----- Original Message -----
From: 
To: 
Sent: Friday, February 01, 2002 5:19 PM
Subject: storing tissue after fixation


> I was wondering if phosphate buffered saline (PBS) can be used as a
temporary
> holding solution for tissue after primary fixation, and how long can the
tissue
> remain in PBS?  If not, what is a better holding solution?  Some of our
> researchers after fixation in 4% paraformaldehyde will put the tissue in
70%
> alcohol.  This is overdeyhdrating the tissue by the time we process it,
and I
> was wondering if we safely put this tissue in PBS instead.
>
> Bob Meyer, HTL
> Northwestern University
>
>