Tales of Regenerative Medicine

Cells - the building blocks of our body

Have you ever seen a "cell"? Perhaps you have observed the cells of plants like onions in science experiments at school. You may also have hazy memories of pictures of "cell division", the moment when a single cell splits into two. But, if you think of it, the question "Have you ever seen a cell?" is really nonsense. I say this because our bodies themselves are just a mass of cells. The skin, the mucous membrane inside the mouth, and the blood are all made of cells. So we could say that if you have seen the human body, then you have seen cells, too.

Roughly speaking, our bodies are made up of 60 trillion cells. Sixty trillion is about 10,000 times the present population of the world (which is about 6.5 billion people). It is hard to imagine, isn't it? It doesn't seem possible that the number of cells that make up a single human body is almost the same as 10,000 times the population of the Earth. The only thing we can gather from it is that our bodies are made up of a great many cells.

Our bodies, which are collections of large numbers of cells, began as single cells called "fertilized eggs" in our mothers' wombs. A fertilized egg increases exponentially to become a baby, and the cells go on increasing after birth, until the baby grows into an adult, or a mass of 60 trillion cells. Well then, how many times does a single cell need to divide in order to yield 60 trillion cells? If we calculate this, we find that 46 divisions will yield more than 70 trillion cells. Just as one cell dividing yields 2 cells and 2 cells dividing yield 4, it only takes 46 divisions for the resulting number to exceed 70 trillion. If we look at it this way, it seems that a surprisingly small number of divisions yields an enormous number of cells. Even our own bodies are a universe of unimaginably large numbers.

Every single one of the 60 trillion cells in our bodies plays an important role. Cells can be classified into about 270 different types, depending on their role. For example, skin cells play the role of barriers separating the inside of the body from the world outside. They protect the contents from various attacks from outside. Muscle cells cooperate with other types of cells around them to create a mechanism whereby the body can be moved. Nerve cells have very long "arms" that stretch out to communicate senses like pain to the brain and send signals from the brain to the extremities of the body. Liver cells produce various substances that are needed in the body and also break down unnecessary substances and toxins that have entered the body. The liver is just like a chemical factory. The white blood cells in our bloodstream attack bacteria and other invaders that have gotten into our bodies. They are like a police force. If you look at it this way, you will be surprised not only at the sheer number of cells in our bodies but also the great number of roles that they play.

Next, let's think about how the cells that play these many roles are made.