Effect of long-term fixation on samples - (Jun/21/2011 )

At the time I put some of my samples in formalin (I believe 4%) for fixation I asked someone in the lab how long I could keep them in the fridge. The answer I got was something like "it doesnt matter". Because of this, I assumed I could store them for a couple of weeks. Recently I mentioned to my supervisor I still had the samples in formalin in the fridge, after which he replied that it should not be stored for such a long time (almost 4 weeks now).

I was wondering what the effect would be on my samples? What kind of staining methods can I still perform? I would like to do H&E staining and maybe some immunostaining. I doubt immunostaining will still work, but I would really appreciate some insight into this.

Thanks in advance!

I think it depends on which tissue and what staining that you perform,
but most likely no effect on your HE staining,

Immunostaining is depends on what epitope you will stain. Some IFC wont work or become very weak, if you fix too long.
for that reason, I normally fix only few hours.

probably by now you've already processed your samples, it'd be about 8 weeks if my calculations are right.
i've left samples after fixation up to 5 weeks and they've been fine for embedding and sectioning. i still have some samples in the fridge that i didn't process and i believe they would still be ok to process in case it was necessary. according to some protocols, after the vaccuum infiltration you can leave your samples for months in the fixative, or in 100% ethanol.