each cell has a defined role
The cells in our body each perform their work according to their various roles. Cells that play an amazing number of functions are produced from the single cell that started out in a mother's womb. You all must have heard the words "cell division" at some time. As the words suggest, this means a single cell divides into two to increase the number of cells. Well then, what about the word cell "differentiation"? This may not be as familiar to you as "division". I would like to now discuss cell "differentiation" a little.
This may seem like a sudden change of subject, but actually cell "differentiation" means that the various types of cells each have some type of role. For example, in order for cells that are going to be liver cells to fulfill their role as the liver, they must have a form and functions that are suited to that role. Cells that are going to be muscle cells must also acquire suitable abilities. When individual cells have adopted such roles, we say "The cells have differentiated into liver cells" or "The cells have differentiated into muscle cells", etc.
Cells with their respective roles increase in number by repeatedly undergoing "division". By "differentiating" further, they then acquire the functions that correspond to their roles. By differentiating into skin cells, differentiating into nerve cells, differentiating into bone cells, or differentiating into blood cells, individual cells determine how they will live. But some cells still have not decided upon specific roles. We call these "undifferentiated" cells. "Undifferentiated" cells undergo a steady process of preparation for differentiation. They are just like children going through years of study while deciding what they want to be in the future.
A single cell (fertilized egg) in the womb of a mother goes through round after round of division to multiply into a large number of cells. As the number of cells increases, some of the cells will start to differentiate. Gradually, there will be a rough sort of classification as cells that will become skin separate from those that want to become muscle like the heart, and those that want to become the brain.
Some of the cells that were starting to differentiate as skin may decide that what they would really like is to become the mucous membrane inside the mouth. The skin and the oral mucosa have a mutual relationship like close relatives. The mucous membranes of the stomach and intestine are also like close relatives of these. Some of the cells that were headed toward becoming muscle might decide they want to be bone instead. The bones and muscles and cartilage are all close relatives, and if you trace back a little, you will find they originated as the same type of cells.
Thus, through a series of small choices that is just like evolution, they form increasingly specialized communities of cells. The body is a group of 60 trillion specialized cells. It is truly a clever arrangement.