The cell theory, or cell doctrine, states that all organisms are composed of similar units of organization, called cells. The concept was formally articulated in 1839 by Schleiden & Schwann and has remained as the foundation of modern biology. The idea predates other great paradigms of biology including Darwin’s theory of evolution (1859), Mendel’s laws of inheritance (1865), and the establishment of comparative biochemistry (1940).
Formulation of the Cell Theory
In 1838, Theodor Schwann and Matthias Schleiden were enjoying after-dinner coffee and talking about their studies on cells. It has been suggested that when Schwann heard Schleiden describe plant cells with nuclei, he was struck by the similarity of these plant cells to cells he had observed in animal tissues. The two scientists went immediately to Schwann’s lab to look at his slides. Schwann published his book on animal and plant cells (Schwann 1839) the next year, a treatise devoid of acknowledgments of anyone else’s contribution, including that of Schleiden (1838). He summarized his observations into three conclusions about cells:
- The cell is the unit of structure, physiology, and organization in living things.
- The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
- Cells form by free-cell formation, similar to the formation of crystals (spontaneous generation).
We know today that the first two tenets are correct, but the third is clearly wrong. The correct interpretation of cell formation by division was finally promoted by others and formally enunciated in Rudolph Virchow’s powerful dictum, Omnis cellula e cellula…: “All cells only arise from pre-existing cells”.
The modern tenets of the Cell Theory include:
- All known living things are made up of cells.
- The cell is structural & functional unit of all living things.
- All cells come from pre-existing cells by division. (Spontaneous Generation does not occur).
- Cells contains hereditary information which is passed from cell to cell during cell division.
- All cells are basically the same in chemical composition.
- All energy flow (metabolism & biochemistry) of life occurs within cells.
Of course observations of cells go back further than Schleiden and Schwann. Prominently, there’s Robert Hooke and the ‘cells’ that he saw in cork, Anthony van Leeuwenhoek and the microscope he invented, among others:
1595 Jansen credited with 1st compound microscope
1626 Redi postulated that living things do not arise from spontaneous generation.
1655 Hooke described ‘cells’ in cork.
1674 Leeuwenhoek discovered protozoa. He saw bacteria some 9 years later.
1833 Brown descibed the cell nucleus in cells of the orchid.
1838 Schleiden and Schwann proposed cell theory.
1840 Albrecht von Roelliker realized that sperm cells and egg cells are also cells.
1856 N. Pringsheim observed how a sperm cell penetrated an egg cell.
1858 Rudolf Virchow (physician, pathologist and anthropologist) expounds his famous conclusion: omnis cellula e cellula, that is cells develop only from existing cells [cells come from preexisting cells]
1857 Kolliker described mitochondria.
1869 Miescher isolated DNA for the first time.
1879 Flemming described chromosome behavior during mitosis.
1883 Germ cells are haploid, chromosome theory of heredity.
1898 Golgi described the golgi apparatus.
1926 Svedberg developed the first analytical ultracentrifuge.
1938 Behrens used differential centrifugation to separate nuclei from cytoplasm.
1939 Siemens produced the first commercial transmission electron microscope.
1941 Coons used fluorescent labeled antibodies to detect cellular antigens.
1952 Gey and co-workers established a continuous human cell line.
1953 Crick, Wilkins and Watson proposed structure of DNA double-helix.
1955 Eagle systematically defined the nutritional needs of animal cells in culture.
1957 Meselson, Stahl and Vinograd developed density gradient centrifugation in cesium chloride solutions for separating nucleic acids.
1965 Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial scanning electron microscope.
1976 Sato and colleagues publish papers showing that different cell lines require different mixtures of hormones and growth factors in serum-free media.
1981 Transgenic mice and fruit flies are produced. Mouse embryonic stem cell line established.
1987 First knockout mouse created.
1998 Mice are cloned from somatic cells.
2000 Human genome DNA sequence draft.