Due to the presence of mechanisms that regenerate portions of tissue after disease or damage, it is possible that excessive or inappropriate growth may occur. In humans, when a portion of tissue grows excessively, it is often described as a cancer; however, there are many forms of excessive growth that are limited and not life-threatening. At the heart of a cancerous growth is the inappropriate response of the cell to its microenvironment. Sensory modules that determine mechanical aspects of the microenvironment contribute to cancerous growth. In particular, the depletion of rigidity-sensing modules enables cells to grow on soft surfaces, much in the way that loss of sensing in self-driving cars causes inappropriate movement. There are common features of inappropriate growth that are considered in the rubric of cancer biology and many books are written on the details. Here we consider growth in the context of cell states and the response of the cell to its microenvironment. By definition, cells in cancers have ignored normal growth controls and have grown excessively. Because tissue growths need vascularization to grow rapidly beyond millimeter dimensions, they are normally self-limiting. Further, telomere shortening will limit the number of divisions of cells. Thus, these two factors act as safeguards to keep most tumors small in size and benign. When cells move to states that continue growth, elongate telomeres and promote vascularization, they can be life-threatening. Even then, surgeons can remove the growths and the normal cells can often heal the area and restore function. What is life-threatening is the ability of inappropriately proliferating cells to disseminate to other parts of the body and to differentiate and grow in many different environments. Then, the surgeon cannot remove all of the tumors and the multiple, differentiated cells will not be killed by a single therapy. Furthermore, the act of killing the tumor cells often elicits a general repair response in the body that will promote further growth of tumors. The lessons of Cancer Biology can tell us a lot about biology in general.

In the previous chapter, we discussed normal development and the process of differentiation. When a loss of cells occurs during normal development, neighboring cells typically divide to fill the gaps. This is important, because with the loss of cells in a tissue, both physical mass and functional performance are lost.