In biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells. Cells are the basic unit of structure in all organisms and also the basic unit of reproduction. With continual improvements made to microscopes over time, magnification technology advanced enough to discover cells in the 17th century. This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientists. Most of these debates involved the nature of cellular regeneration, and the idea of cells as a fundamental unit of life. Cell theory was eventually formulated in 1839. This is usually credited to Matthias Schleiden and Theodor Schwann. However, many other scientists like Rudolf Virchowcontributed to the theory.The three tenets to the cell theory are as described below:All living organisms are composed of one or more cells. (However, this is considered a controversy because non-cellular life such as viruses are disputed as a life form.See Non-cellular life.)The cell is the basic unit of structure and organization in organisms.Cells arise from pre-existing cells.

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