Scientists observe that cells in the human body change and eventually die as part of the aging process.  Studies that involve cells within a human body are referred to as in vivo.  While human cells do not exist in isolation it is very difficult for scientists to study individual cells in the body because of the many variables affecting the individual cell’s processes.  These variables include both genetics and lifestyle.

As an alternative scientists often choose to study individual cells outside the human body.  These studies take place in laboratories where scientists can control all lifestyle variables and manage genetic variables and are referred to as in vitro.  In order to study cells in a culture, first they must be harvested from a human body, then they may be frozen in liquid nitrogen, before being thawed, and used in a study.

While it is much easier for scientists to study cells outside of the body, it is important for scientist to consider if in vitro studies produce valid results.  Scientists have had to answer the question: do cells in cultures behave similarly to cells in the human body?  In order to better understand how environment affects cells scientists utilized information they already understood regarding how cells behave in the body.  Each cell type will only divide for a known quantity of time.  Scientists found that these same cell types will only divide for the same quantity in a culture. This supports the conclusion that in vitro studies provide valid and valuable information regarding cells.  Ongoing studies continue to support this conclusion.