Hair, skin, and fingernails are replaced constantly by cell division at a rapid rate throughout our lives. But, brain and nerve cells in the central nervous system do not divide at all after we are a few months old.
Subsequently, if the brain and nerve cells are destroyed later, the loss is usually permanent, as in the case of paraplegics.
Liver cells usually do not reproduce after an individual has finished growing and are not replaced except when there is an injury.
Red blood cells are also somewhat of an exception. While they are being constantly produced in our bone marrow, the specialized cells from which they come do not have nuclei nor do the red blood cells themselves.
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