Introduction

The word ‘cell’ was first used by the scientist Robert Hooke in the seventeenth century in his book Micrographia (1665), where he described observations made with his own handmade microscope (Hooke, 1665). In particular, he noticed small cavities in a piece of cork delimitated by cell walls of cellulose and suber that he called cells. However, neither Hooke nor his contemporaries realized the importance of this discovery and it was only during the nineteenth century that the cell stood out as a central dogma in biology: the cellular theory. This theory is based on the work of Matthias Schleiden and his friend Theodor Schwann, who showed that all living organisms (plants, animals, moulds) are made of microscopic building units: cells (Schwann and Schleyden, 1847). The cellular theory is based on two central ideas. First, the cell is the unit of structure, physiology and organization in all living beings. Thus, the cell retains a dual existence – as a distinct entity and as a building block in the construction of organisms: unicellular organisms correspond to a single and autonomous cell, whereas multicellular organisms are made of two up to several billions of often highly differentiated cells. The second concept of the cellular theory is that ‘every cell stems from another cell’, and was formulated by Rudolf Virchow in 1855 (Virchow, 1855).