The human body is composed of over 37 trillion cells, each with a limited lifespan. These cells are continuously replaced to maintain organ and system function. Yet over time, or as a result of damage, the number of functioning cells can decrease to a level that causes symptoms or even organ failure.
Regeneration of organs and systems is a scientific holy grail that relies on stem cells, but due to their limited number and slow division rate, this isn’t a practical route to organ regeneration. It would take many years to repopulate all the cell types needed.
However, some people see organs “reappear”, like Katy Golden who had her tonsils removed for a second time as an adult after they grew back over 40 years.
