Apoptosis is the process of programmed cell death (PCD), whereas Necrosis is the premature death of cells and living tissue. Processes of disposal of cellular debris whose results do not damage the organism differentiate apoptosis from necrosis.
Apoptosis (pronounced /ˌæp.ə.ˈtoʊ.sɪs/ ăpˈə-tō'sĭs ˌæpəpˈtoʊsɨs, ăpˈəp-tō'sĭs) is the process of programmed cell death (PCD) that may occur in multicellular organisms. Programmed cell death involves a series of biochemical events that lead to a variety of morphological changes, including blebbing, changes to the cell membrane such as loss of membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation.
Necrosis (from the Greek νεκρός, "dead") is the premature death of cells and living tissue. Cells which die due to necrosis do not usually send the same chemical signals to the immune system that cells undergoing apoptosis do. This prevents nearby phagocytes from locating and engulfing the dead cells, leading to a build up of dead tissue and cell debris at or near the site of the cell death. For this reason, it is often necessary to remove necrotic tissue surgically.
edit Causes and Symptoms
Necrosis is caused by external factors, such as infection, toxins or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death. While apoptosis often provides beneficial effects to the organism, necrosis is almost always detrimental, and can be fatal.
For example, the differentiation of fingers and toes in a developing human embryo occurs because cells between the fingers apoptose; the result is that the digits are separate.
Excessive apoptosis causes hypotrophy, such as in ischemic damage, whereas an insufficient amount results in uncontrolled cell proliferation, such as cancer.
Between 50 and 70 billion cells die each day due to apoptosis in the average human adult. For an average child between the ages of 8 and 14, approximately 20 billion to 30 billion cells die a day. In a year, this amounts to the proliferation and subsequent destruction of a mass of cells equal to an individual's body weight.