In any society, whether human or non-human, there are links and interactions between individual units. A possible method for investigating these links is to consider what happens when they are severed. For instance, what will happen to an ant which is removed from its nest and kept in isolation in a place which offers the same conditions as the nest in terms of food, temperature, humidity and other factors? Will it have a shorter or longer life than the ants which remain in the nest? If, as may be expected, its life is shortened, one would like to know how much it is shortened. Before answering this question (which we do later on in this chapter) it is important to realize its implications. This point can be illustrated through the following gedankenexperiment. Suppose you are slicing bread and the knife slips. Your finger is cut and starts to bleed. You put on a plaster and think nothing more of it. However, some skin cells have been displaced down into the muscle tissue; if they survive and divide they will produce skin cells in a location where no skin cells should exist. Fortunately, these displaced skin cells undergo self-destruction, a mechanism known as apoptosis. Other expressions such as programmed cell death (PCD) or cell suicide (Raff 1998) are also used to designate the mechanism of self-destruction.

In order to establish a connection between our previous question about ants kept in isolation and the phenomenon of apoptosis one may wonder what happens when a skin cell is removed and kept in a test tube.