1.2 Cells and tissues
Cells have distinctive shapes and functions which depend on how they have differentiated, the proteins that they express, and their interactions with other cell types. It is difficult to be precise, but there are probably at least 200 different types of cell within the body. Disease processes affect individual cells in many ways; they may cause them to die, to change their shape, to divide, to move or to invade other tissues. Any of these changes also affect the anatomy of the tissue. Understanding the changes that are characteristic of a disease requires a detailed knowledge of the normal appearance of cells and tissues, and the range of normality. Many tissues change considerably with age, so that something that is normal in an adult would not be normal in a child. For example, the thymus gland gradually decreases in size with age, so a large and hypercellular thymus in an old person could indicate some underlying pathology.
The term tissue is used here to describe any collection of cells; most tissues and organs consist of many different cell types with separate anatomical components, including blood vessels and lymphoid tissues, in addition to the characteristic cells of that tissue.