The human body is composed of many types of cells. But none of them perform their function well on their own because they were designed to function as part of something much more complexyou! An adult has about a hundred trillion cells arranged in four primary tissues that make up all the organs of the body. The four tissues are epithelium, connective tissue, muscle, and nerve. An understanding of these primary tissues greatly aids in understanding the structure and function of the organs of the body.  Although our skin and kidneys look totally different to the unaided eye, under the microscope they are seen to be made up of a unique combination of the same four primary tissues.