Apoptosis

Medical Meanderings 5 August 2009

All Good Things… ©

To everything there is a season, a time for every purpose under the sun. A time to be born, and a time to die…
- Ecclesiastes 3:1-2

Each of us starts out in life as a ball of cells called a blastocyst. You may notice, however, when you look at (most of) your friends and neighbors, that they are not spherical. How do we grow from a ball into two-legged, two-armed, one-headed adults? Of course, the first thing needed is growth—through cell division, over and over, tissue is added. Second, cells have to become differentiated. That is, groups of cells start to specialize into nerve or bone or muscle cells. Each cell switches certain genes on or off, like a handyman selecting what tools he’ll need for a job and putting others away. Once transformed into a certain type of cell, the identity is permanent.

The third process that must occur for us not to become giant human spheres is called apoptosis (ay-poh-TOE-sis), or programmed cell death. Apoptosis is derived from the Greek “apo-,” meaning “away from” and “ptosis,” meaning “to fall.” That is, cells undergoing apoptosis are falling away, like autumn leaves. These cells essentially commit suicide in a very organized fashion, right on time. This allows certain tissues to shrink as others grow, shaping our organs and limbs, even our brains.

Apoptosis can by triggered from outside the cell, when chemical messengers attach to “death receptors” on the cell surface, or from inside the cell, due to damage from chemicals and radiation. In either case, the control center of the cell, the nucleus, starts the self-destruction as DNA clots together and breaks apart. Then, a series of enzymes is activated, leading to a cascade in which the cell’s own proteins are digested. Through this process, the membrane of the cell, its “skin,” is kept intact. This keeps the cell contents from leaking out into the surrounding tissue, thereby avoiding a huge mess to clean up. Finally, the cell corpse (scientists actually call it that) is ingested and digested by surrounding cells.

Not only is apoptosis vital for normal growth and development, it is important in the proper function of our immune system’s defenses. When you get an infection, the white blood cells replicate and swell their numbers in order to fight the battle against invading bacteria or viruses. However, once the battle is over, the extra troops can’t be sent home (to where?), so they self-destruct. After all, we don’t want millions of extra white blood cells floating around with nothing to do. Apoptosis plays a role in autoimmune diseases such as lupus and rheumatoid arthritis in an opposite way—white blood cells active in inflammation fail to self-destruct as they should, instead hanging around causing mischief in healthy organs.

All good things must come to an end, including living cells. The wisdom of our bodies’ tissues shows us that not only is it important to know when to prosper and grow, but that there comes a time, even for individual cells, when it’s time to bid good-bye.