Cells are their microscopic building blocks of the body. Among the smallest human cells are some nerve cells, while the largest cell, the ovum, has a diameter of 5 1,000th of an inch, the size of a pinpoint. Each cell is a self-contained living unit, which becomes modified and specialized as it develops and is integrated into the body's systems. An actual functioning cells in tissues and organs are packed together in close proximity, they take on a variety of shapes. Muscle cells, for example, are elongated into fibers. Liver cells are hexagonal, and the cells which line the intestine are column-shaped. The cutaway view of a generalized cell, as presented here and magnified to 6,000 times the average size, reveals the working parts concerned with systems of cell support, maintenance, repair and multiplication. The cell nucleus, a spherical structure, contains the basic genetic material, dioxyribonucleic acid, or DNA, in the form of chromosomes. The nucleus controls the overall activity of the cell. The nuclear envelope is a two-layered membrane surrounding the nucleus. The envelope acts as a sieve, controlling the continuous flow of chemicals to and from the nucleus. The nucleolus, housed within the nucleus, has under a microscope a granular appearance. This is due to its concentration of ribonucleic acid or RNA. Cytoplasm is the watery medium of the cell in which all the structures are suspended. Most of the work of the cell is carried on in the cytoplasm. The cell membrane is a soft, pliable skin, which acts as a control gate for the entry and exit of substances involved in cell function. The endoplasmic reticulum is a network of flattened sacks and tubes, which provides a communication channel for materials passing between the nucleus and the cell's environment. The lysosomes are simple sacks which store enzymes, the powerful chemicals involved in digestion. The mitochondria are the main sites of energy production within the cell. Cytoplasm is the surface of the cell, captured fluid which contains large molecules such as proteins and carry it toward the interior. The ribosomes are the sites where proteins are made. They contain RNA similar to that in the nucleus. The centrioles are involved in cell division, during which they move apart to opposite poles of the cell, leaving a delicate tracery within the cytoplasm. And Golgi's complex acts as a depot for the storage and secretion of cell products. Working around the clock, the human cell is comparable to a small factory. Like a factory, the cell takes in a continuous flow of raw materials for processing and packaging of its own chemical waste ready for disposal.