Hello Muhammed Zidan Khan

There is a general trend among researchers to store tissues indefinitely in 10% NBF without harm. If you stain such section under routine histological staining such as Hematoxylin and Eosin stain, it will look fine. But the problem will arise when such material is used for immuno-staining, since the cross-linking action continues and may irreversibly change the conformation (i.e., reduce the antigenicity) of antibody targets. So, you need to be careful not to 'over-fix' the tissue.

For tissue samples, the optimum fixation in 10% NBF is achieved in 24 hours with a fixation window of 8 hours to 48 hours. For routine fixation in 10% NBF, the tissue is dissected as rapidly as possible and immersed in a large volume of fixative (i.e., at least 20X volume of fixative to tissue). After fixation, wash the tissue well in several changes of PBS. The tissue may be stored in cold PBS for short periods of time (2 or 3 days) but should be placed in 70% ethanol for extended storage.

The tissue will be safe in 70% ethanol since there is no danger of bacterial degradation. Use two changes of 70% ethanol to wash off the residual formalin and stop formalin fixation. Please note that the samples may be stored in 70% ethanol for up to one month. Extended storage in 70% ethanol beyond one month will result in alcohol fixation artifacts in the tissue.

Best.

Dear Mohammad you can keep the tissue in formaline 10% to routine histochemical analysis for along time but for immunohistochemical you must use it within 24 hour.with my best wishes.