All of us start as a single cell. When a sperm gets together with an egg, the process of forming an embryo, then a fetus, and on to a baby, gets started. But a single cell can’t do all the things required for a fully-grown person. At some point the cells must change and become different – ultimately becoming neurons, muscles, bones, liver cells, kidney cells, and every other type of cell in the body. This process is called differentiation.
When the newly formed embryo is only a few cells in size (called a zygote), all of those cells are the same. However, because every cell contains a complete set of genetic information, those early cells are capable of becoming just about any specialized cell in after future divisions.
The process of development of an embryo is called embryology. This is a hard and complex topic, often taking entire semesters of study and filling thick textbooks. For this reason, it is impossible to describe the detailed process of cell differentiation in this space. I can, however, provide an overview and summary.
Once the zygote implants and begins to grow, a series of complex chemical reactions begins to guide the further development of the soon-to-be embryo (and eventually, baby). At first, these changes are subtle. Two layers of tissues form, then a third. It’s not for a few weeks that the first early “organs” begin to take shape. But with each cell division, the cells become more and more different from each other. Most of these changes are due to changes in which genes are expressed, and which ones are suppressed. Different genes produce different proteins, which makes for cells that are different. Sort of makes sense, doesn’t it?
Interestingly, as cells become more and more specialized, most of them loose the ability to change to some other sort of cell. For example, a muscle cell cannot change in to a neuron. The muscle cell is “stuck” – and any daughter cells produced by its splitting (via a process known as mitosis) is also going to be a muscle cell.
There are some cells in the body which retain the ability to become any other type of cell. These cells are referred to as pluripotent stem cells. Technically a cell that become any other type is know as totipotent, and a pluripotent cell is somewhat limited, but these distinctions are often lost on anyone who is not a cellular biologist.
The differentiation of a cell has more implications than just the development of a new zygote. It’s also important in cancer. Cancer is the abnormal and out-of-control growth of a set of cells. As those cells grow like mad, they can change. The more a cell changes, the more differentiated it becomes from a normal cell. This can have a wide range of consequences when a doctor is trying to determine how advanced a cancer is and when treatments are being considered. The degree to which a group of cancer cells has differentiated is known as the “grade” of the cancer.
So there you are, the most basic of summaries of cell differentiation. As I mentioned above, this is a gigantic topic – one which often takes months to years of study to learn all the details.