Michelle
this is a very good question and one that will probably generate a lot
of debate

The stable chemical bonds that formaldehyde forms continue for several
years. This results in decreased ability to carry out staining or IHC.
However, even tissues that have been fixed for several years can be
treated with antigen retrieval etc to unmask various antigens. To a
large extent this depends on the antigen you are trying to demonstrate.
As far as blocks  go, tissue that has been processed will remain
relatively stable although there are indications that demonstration of
some antigens  is decreased on storing. Again this is not a hard and
fast rule and may vary considerably with the antigen.
My gut response would be to go with the processed block.
Another option is to fix in formalin and then store tissues in 70%
ethanol. You have to ensure that excess formalin has been washed out and
prevent tissuses from drying (5-10% glycerin in with the alcohol).
Barry

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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