Why is it that we stop growing but yet our hair and nails continue to grow?
Not only our nail and hair. If we get a cut, our skin recovers itself. If we have an accident and break a bone, it also recovers itself. Our blood cells - in fact, nearly all the cells in our body are constantly being replaced!. We are living organs, not fossils! Most people can easily tell the difference between 20, 50 and 90 years old person, simply by looking at him or her. This is because our body is in constant change throughout our lifetime. In adults, these changes are much slower than in children, but they still appear.
So, the true question, is how come we stop growing at a certain age. This happens because we are genetically programmed to do so. It is our genes, made of DNA, that determine how we grow and develop. We inherit our genes from our parents when we are conceived, and we maintain this genetic blueprint throughout life.
Our genes start working at the moment of conception when a single cell becomes a complex organism in which billions of cells work in concert. Once we are born, we continue to grow and develop until the completion of puberty. At this point our genetic program tells us to stop growing.
From an evolutionary viewpoint, once our genes have orchestrated the growth and development of the body to the point that it can reproduce, the purpose for growth is complete.
The complex interplay of genes, nutrients and hormones cause bone cells to proliferate at the growth plate of long bones. This interplay is also responsible for the cessation of linear growth and the correlating inability of these cells to further multiply and lengthen bone. The key hormones in this process are: growth hormone, thyroxin, androgens and estrogen. They are secreted by the pituitary, thyroid and reproductive glands respectively.
At the completion of puberty, the reproductive glands in both males and females increase the production of the hormone estrogen. It is the high concentration of estrogen in the blood that causes the growth plates of our bones to fuse. This fusion effectively closes the growth centers of long bones and renders them unable to respond to the hormones that initiate growth.