Red blood cells lose their nuclei at a certain stage of development in order to become more efficient at carrying oxygen. Nuclei take up space and require energy to maintain, so getting rid of them allows red blood cells to carry more oxygen and move through blood vessels more easily. However, this also means that red blood cells cannot repair themselves or reproduce, so they eventually die and are replaced by new ones.

Brain cells, on the other hand, keep their nuclei throughout their lives. This allows them to maintain their complex structure and function, and it also gives them the ability to repair themselves and generate new cells. As a result, brain cells can live for much longer than red blood cells, potentially up to a hundred years or more.