Hi,
    As I understand it, the crosslinking of the tissue elements
continues as long as there are reactive areas in the tissue and free
aldehydes to form the methylene bridges. This makes it much more
difficult for the desired elements to be accessed by your staining
chemicals. Also for immunohistochemistry to work you need to break some
of these methylene bridges to expose the antigen to the antibody. If the
tissue is overfixed then the epitope retrieval described above will not
be as effective.
    I have not yet encountered problems with archived, processed blocks.
They last a long tome. This is the route I would suggest.
Amos Brooks

MICHELLE LOWE wrote:

> I was told by a co-worker: the longer tissue is kept
> in formalin, the more it losses its staining
> properties.  We were discussing keeping wet tissue vs.
> blocks for back up special stain/immunohistochemistry
> controls.
>   Does it really matter how long the tissue is in
> formalin?
>   What are the effects of tissue kept too long in
> formalin?
>   It is ideal to have 5-10 year old blocks to use as
>     controls?
> I know that this question seems a little generic, but
> I'm still a rookie!! :)
>
> Michelle Lowe
> Sup. Anatomic Pathology/Histology
> Valley Children's Hospital
> Madera, CA
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