We do not know at what age postnatal neurons require FGF and a higher osmolality medium, or how this could vary based on the species from which the neural tissue is isolated. Without this knowledge, we generally recommend switching to the Neurobasal-A Medium for isolates from animals above 1 week of age. You will need to determine if the Neurobasal-A Medium is optimal for neurons you have from one day old rats.
Further, the addition of bFGF can increase the viability of neurons of any age. Therefore, caution needs to be made in impure neuronal cell culture as bFGF can increase the proliferation of non-neuronal cells. We recommend a pilot experiment to test the impure culture in the presence and absence of bFGF.
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