Cell Theory

Cell Theory is one of the unifying principles of biology, and like most great scientific ideas it was not the work of a single person but was built up gradually through the observations of several scientists across roughly two decades in the nineteenth century. The first major step came in 1838, when the German botanist Matthias Schleiden, after studying a wide variety of plant tissues under the microscope, concluded that all plants, without exception, are composed of cells. A year later, in 1839, the German zoologist Theodor Schwann extended this same conclusion to animal tissues, and went further -- he proposed that the cell wall was not essential to a cell's identity (since he had studied animal cells, which lack one) and put forward the idea that cells were the common structural unit for both the plant and animal kingdoms. It was Schwann who first coined the term "Cell Theory" for this combined proposal: that all living organisms, whether plant or animal, are composed of cells and of the products cells secrete.

However, this early version of Cell Theory had a significant gap: it did not correctly explain where new cells come from. Schwann himself believed cells arose out of a formless nutrient fluid inside or around existing cells -- an idea similar in spirit to the old theory of spontaneous generation, and one we now know to be incorrect. It was the German pathologist Rudolf Virchow who, in 1855, supplied the missing and crucial third postulate. Virchow proposed that all cells arise only from pre-existing cells, through the process of cell division, a principle he summarised in the now-famous Latin phrase "Omnis cellula e cellula" -- "every cell from a cell." This was a direct and decisive rejection of spontaneous generation as an explanation for the origin of new cells, and it completed the theory in essentially the form it is taught today.

Combining these contributions, the modern statement of Cell Theory rests on three central postulates. First, all living organisms are composed of cells and of materials that cells produce (cell products). Second, the cell is the basic structural, functional and organisational unit of all living things -- it is the smallest unit that can, on its own, display all the properties associated with life. Third, all cells arise from pre-existing cells by division, so that every cell alive today traces an unbroken lineage of cell division back through its ancestors. …