A unifying concept: the history of cell theory
After the first observations of life under the microscope, it took two centuries of research before the ‘cell theory’, the idea that all living things are composed of cells or their products, was formulated. It proved even harder to accept that individual cells also make up nervous tissue.
ith theinvention of the microscope at the beginning of the seventeenth century, it became possible to take a first glimpse at the previously invisible world of microscopic life. A bewildering array of new structures appeared before the astonished eyes of the first microscopists. The Jesuit priest Athanasius Kircher (1601– 1680) showed, in 1658, that maggots and other living creatures developed in decayingtissues. In the same period, oval red-blood corpuscles were described by the Dutch naturalist Jan Swammerdam (1637–1680), who also discovered that a frog embryo consists of globular particles1,2. Another new world of extraordinary variety, that of microorganisms, was revealed by the exciting investigations of another Dutchman, Antoni van Leeuwen-
hoek (1632–1723). The particles that he sawunder his microscope were motile and, assuming that motility equates to life, he went on to conclude, in a letter of 9 October 1676 to the Royal Society, that these particles were indeed living organisms. In a long series of papers van Leeuwenhoek then described many specific forms of these microorganisms (which he called ‘‘animalcules’’), including protozoa and other unicellular organisms3–5. Butthe first description of the cell is generally attributed to Robert Hooke (1635– 1702), an English physicist who was also a distinguished microscopist (see photographs below). In 1665 Hooke published Micrographia, the first important work devoted to microscopical observation, and showed what the microscope could mean
for naturalists. He described the microscopic units that made up thestructure of a slice of cork and coined the term ‘‘cells’’ or ‘‘pores’’ to refer to these units. Cella is a Latin word meaning ‘a small room’ and Latin-speaking people applied the word Cellulae to the six-sided cells of the honeycomb. By analogy, Hooke applied the term ‘‘cells’’ to the thickened walls of the dead cells of the cork. Although Hooke used the word differently to later cytologists (he thoughtof the cork cells as passages for fluids involved in plant growth), the modern term ‘cell’ comes directly from his book6.
Bridge between life and ‘non-life’?
The existence of an entire world of microscopic living beings was seen as a bridge between inanimate matter and living organisms that are visible to the naked eye7. This seemed to support the old aristotelian doctrine of ‘spontaneousgeneration’, according to which water or land bears the potential to generate, ‘spontaneously’, different kinds of organism. This theory, which implied a continuity between living and non-living matter, natura non facit saltus, was disproved by the masterful experiments of the Italian naturalist Lazzaro Spallanzani (1729–1799)8. He and other researchers showed that an organism derives from anotherorganism(s) and that a gap exists between inanimate matter and life. (But it was a century later before the idea of spontaneous generation was definitively refuted, by Louis Pasteur, 1822–1895; ref. 9.) As a consequence, the search for the first elementary steps in the scala naturae was a motif in early-nineteenth-century biological thought: what could be the minimal unit carrying the potential forlife?
‘‘there is one universal principle of development for the elementary parts of organisms... and this principle is
Under the microscope: drawings of the instruments used by Robert Hooke (left) and the cellular structure of cork according to Hooke (right) (reproduced from Micrographia, 1665).
in the formation of cells’’
NATURE CELL BIOLOGY | VOL 1 | MAY 1999 | | cellbio.nature.com...