In its modern form, the theory that all cells come from previously existing cells and that they are the fundamental functional units of all living organisms. Considered to be one of the most important theories in biology, it was originally proposed by Matthias Schleiden (1804-1881) in 1838 and again the following year by Theodor Schwann (1810-1882), it became known as the Schleiden-Schwann cell theory. Their theory proved to be untenable, largely because they claimed that cell generation started with a process of ‘crystallisation’ within the intracellular substance followed by progressive enlargement, a claim refuted by others such as Rudolf Virchow (1821-1905) who pointed out that every cell comes from a pre-existing cell. Subsequently, there were problems in extending cell theory to nerve cells because at the time they were difficult to reconstruct. However, this was eventually achieved with the appearance of the neuron doctrine.
See Axon, Cell, Neuron, Neuron doctrine, Progenitor cells, Proliferative cells, Stem cells