Re: Origin of programmed cell death.

Hello Matthew,

I have a very interesting answer (at least I hope) to a very interesting question.

First I would like to define a little bit what exactly I understand as cell death. There are two types of cell death which occurs in every pluricellular organisms. The first one, and the best understood, is necrosis which I call cell death. The second one is called apoptosis or programmed cell death. There is a world of difference between them and the official definition for each is irrelevant here. Let's just say that necrosis is a pretty nasty death that could be viewed as a massacre while apoptosis is a clean and noble death that could be viewed more as a sacrifice on the part of the cell for the good of the whole organism.

Apoptosis is crucial to every pluricellular organism since it controls the proliferation (i.e. growth) of unwanted cells. For example, when you were just a foetus, your hand had no defined fingers at first, just like a fish's fin. In order to "create" fingers, cells between each fingers were "told" to die and this massive cell sacrifice molded your fingers. Apoptosis occurs at every moment of your life in your body since its the only way for your body to ensure that new cells replace old ones without creating a "mess". Necrotic cells spills their content in their surroundings and cause inflammation (like when you cut your finger) while apoptotic cells keep their content inside and try to make as little mess as possible, thus keeping the immune system silent.

Smaller and more simpler organisms also have apoptosis mechanisms in their cells. In fact, the nematode Caenorhabditis elegans that is composed of 959 cells (no more, no less) has been one of the greatest player in understanding how apoptosis occurs and what proteins are involved in this event. Three proteins have been discovered that regulate apoptosis in C. elegans: CED-3, CED-4 and CED-9. CED-3 promotes apoptosis while CED-9 inhibits it. If the levels of CED-3 in the cell increase or the level of CED-9 decrease, apoptotosis follows. These proteins have led scientists to discover similar proteins in higher lifeforms and it seems that events and proteins that controls apoptosis are nearly identical (okay, a little bit more complicated) in higher pluricellular organisms.

This means that apoptosis has appeared very soon in the evolution and that it conferred a survival advantage for the organism. This advantage comes from the control that the organism as a whole has on its constituents. Pluricellular organisms involves many unique and specific cells (neurons, muscle cells, etc.) that depend on many other in order to survive. Apoptosis ensures that every cells in the body has its place and that when a cell is out of place or is not needed anymore, it dies for the good of the organism.

Here is a picture of the nematode Caenorhabditis elegans:

In short, if apoptosis had not appeared in evolution, pluricellular organisms would not have appeared either (or they wouldn't have last very long!)

I hope this answers your question.

Ciao!

Mike