Definition: The programmed death of a cell. Scientists believe that this process is governed by chemical signals a given cell receives from its neighbors.

# It is thought some forms of cancer may result when this process of cell death is somehow interrupted, allowing cells to grow unchecked, with the result being a cancerous tumor.

Explanation: In biology, apoptosis (from the Greek words apo = from and ptosis = falling, pronounced ap-a-tow'-sis[1] (http://www.nih.gov/sigs/aig/Aboutapo.html)) is one of the main types of programmed cell death (PCD). As such, it is a process of deliberate suicide by an unwanted cell in a multicellular organism. In contrast to necrosis, which is a form of cell death that results from acute tissue injury, apoptosis is carried out in an ordered process that generally confers advantages during an organism's life cycle. For example, the differentiation of human fingers in a developing embryo requires the cells between the fingers to initiate apoptosis so that the fingers can separate. As will be explained further on, the way the apoptotic process is executed facilitates the safe disposal of cell corpses and fragments.

Since the beginning of the 1990s, research on apoptosis has grown spectacularly. In addition to its importance as a biological phenomenon, defective apoptotic processes have been implicated in a very wide variety of diseases. Too much apoptosis causes cell-loss disorders, while too little results in uncontrolled cell proliferation, namely cancerous tumors.

Not all forms of PCD share the characteristic shapes (the morphology) and sequences of apoptosis, but all types of PCD are highly regulated processes.

Source: http://www.answers.com/apoptosis