Definition
Apoptosis
Apoptosis is programmed cell death — a controlled, orderly process by which a cell self-destructs without damaging surrounding tissue. It is a normal part of development and tissue maintenance, distinct from necrosis (uncontrolled cell death due to injury).
Most neurons are post-mitotic — they do not divide — and are highly resistant to apoptosis after development. This is why neuronal loss from injury or degeneration is typically permanent: the brain cannot replace dead neurons through cell division.