These neurons differ in the rat strain from which they were isolated: Sprague Dawley vs. Fisher 344. These neurons don't expand anymore. They just grow neurites. The manual (https://assets.thermofisher.com/TFS-Assets/LSG/manuals/MAN0001574-PrimaryRatCortex-HippocampusNeurons-UG.pdf) has instructions for culturing the cells. You can use Neurobasal Plus/B-27 Plus/GlutaMAX to culture them, or just Neurobasal/B-27/GlutaMAX. However, the former combination provides much superior performance than the latter and can maintain neurons longer.