Introduction

Life began with the appearance of the first cell. The word cell comes from the Latin cella, meaning “small room.” The cell was discovered by Robert Hooke in 1665. He first applied the name cells to the holes that he observed in the cork. He used the word cell because the walls of these holes resembled the walls of a prison cell. In 1835 Jan Evangelista Purkyně observed small “granules” while looking at the plant tissue through a microscope. The cell theory , first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann , states that all organisms are composed of one or more cells, that all cells come from pre-existing cells, that vital functions of an organism occur within cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. There are about 10 trillion (1013) cells in human body.