Hyperplasia is a term used to describe a particular type of growth seen in cells and tissues of your body. Normal tissues in the body have a certain size. Hyperplasia is generally considered to be a response to an injury or stimulus of some kind. This can be a physical injury, or a hormonal or chemical event. Hyperplasia rarely happens for no particular reason.
There are complex mechanisms in place to control the number of cells in a tissue and organ. Tissues undergoing changes deemed to be hyperplasia are still controlled by normal mechanisms. They are not growing out of control as is seen in cancerous tissues.
Hyperplasia is not what happens when you lift weights and get bigger muscles. Body builders don't create "more" muscle. Rather enlarge the cells that are already there. This is a process known as hypertrophy. Gaining fat works the same way. You don't get more fat cells, the ones that are already there get bigger.
Hyperplasia can be induced artificially, usually by injecting a person with human growth hormone. Of course, there wouldn't normally be a good reason to do this, but it can be done.
There are several medical conditions where hyperplasia can occur. One of these is a series of genetic disorders known as congenital adrenal hyperplasia. It most often is related to a deficiency in a particular hormone needed during fetal development. This hormone is used the development of sex characteristics in the developing infant. A lack of the hormone causes a decrease in cortisol production. The adrenal gland will then undergo hyperplasia.
Another common cases of hyperplasia is a condition known as benign prostatic hyperplasia, or BPH. This is a very common condition seen in many men as they get older. The prostate gland is located just below the bladder. For reasons that are not well understood, as a man ages, the prostate gland will undergo hyperplasia. Because the prostate gland wraps around the urethra (the tube which allows urine to drain from the bladder) as it get bigger, it can cause an obstruction to urine outflow. Men suffering from BPH will experience difficulty urinating.
The enlargement of the prostate seen in BPH is not cancer, and it must be carefully differentiated from prostate cancer.
Other organs that can undergo hyperplasia under the right conditions include the breast, endometrium (the inner lining of the uterus), and even the liver. The liver will undergo hyperplasia in response to damage. It is estimated that up to 75% of the liver can be damaged and still regrow to be fully functional.