When a cell divides, it actually multiplies. Huh? This may not be possible mathematically but it possible biologically. Yes, that is how we get to have many cells, by division.
We all started as one fertilized egg cell. The cell then divided and divided until there are hundreds of millions of cells (refer to Nov 10 post, “cells touch”). See, by division a single cell has multiplied into millions of cells. Imagine that! This cell division is called mitosis.
Through mitosis, new cells are formed to replace dying, dead, or old worn out cells. Through mitosis, new cells are also formed to repair wounds or to even grow new parts. Unfortunately, as a cell becomes more specialized, it loses its ability to divide. Thus, nerve cells which are highly specialized cannot be replaced once they die because there no new source of cells. Other cells however are rather “enterprising”. When they divide, only some of the cells become specialized, the others remain unspecialized and they can keep on dividing and dividing so there is a constant source of new cells. These unspecialized cells are called stem cells.
Skin cells have their own stem cells, so do our intestinal cells and blood cells. Thus we have new skin every month and new intestinal cells every week. Sperm cells also have their own stem cells but egg cells do not have any (alas!).