Drawing extensively on previous work by other microanatomists, Schwann proposed that the single, indivisible structural unit of all living organisms was the cell. Further investigations into cellular behavior led Virchow to theorize that all living cells derive from other living cells. Collectively, these concepts form the cell theory, the cornerstone of modern cell biology.
Most cells that make up living plants, animals, and microorganisms are too small for direct observation by the human eye, so researchers could not turn their attention to the finer details of biological structure until the microscope was invented in the early seventeenth century. In 1665, using a compound microscope he designed, Robert Hooke published his observations of thin slices of cork. Under the microscope the cork looked like the small rooms of a monastery, known in Latin as cella, or cells. Hooke applied this term to the geometrically regular compartments he saw and was the first to do so, even though he observed not living cells but the cellulose walls that remain after the internal, organic, cellular material has deteriorated.
Rudolf Virchow.
The first descriptions of living cells were made by Nehemiah Grew, who published his work in 1672, and Marcello Malphigi, whose 1675 handiwork was only the first of many fine monographs on the microstructure of plants and animals. The detailed work of these two fine microanatomists established the foundation of the modern understanding of the fine structure of plants, but it also led most scientists of the time to view plants as aggregates of cells.
Microanatomical work on animals by Malphigi, by Dutch spectacle-maker and scientist Antoni van Leeuwenhoek (1632-1723), and by Dutch physiologist and anatomist Jan Swammerdam (1637-1680) showed definitively that animal tissues were composed of cells, but the more fibrous appearance of animal tissues and the lack of the geometric regularity observed in plant tissues made animal tissues more difficult to interpret. In 1683, Leeuwenhoek reported his use of a simple microscope to view what he called “animalcules” from the plaque of his own teeth, making him the first to view live bacteria. In 1768, Lazzaro Spallanzani
Near the end of the eighteenth century and the beginning of the nineteenth century, chemists
During the 1830’s, improvements in optical instruments and in the skill of those who used them generated a noticeable increase in the quality of microscopic observations. In 1832, Jan Evangelista Purkynê,
In 1835, Johannes Peter Müller
From conversations with Schleiden, Schwann became convinced of the overall similarity between plant and animal cells, because both cell types had a nucleus and a nucleolus. Schwann wholeheartedly adopted Schleiden’s theory of cell generation in his monograph Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen (1839; Microscopical Researches, 1847), but he also brought together a massive collection of evidence, from observations of animal cells and the work of Schleiden, to argue for the similarities between plant and animal cells and between plant and animal fine structure.
It must be noted that only a few of the observations of Schleiden and Schwann were original. According to Schwann, since plants are largely aggregates of cells, animals must be constructed in the same manner. Thus, Schwann argued that cells are the elementary units of structure, function, and organization in living things, and that cells exist as distinct entities, the building blocks for the construction of organisms. Even though Schwann failed to acknowledge his predecessors and contemporaries, his conclusions reached further than any other conclusion or postulates to that time, and they were readily accepted and utilized by scientists throughout Europe.
Schleiden and Schwann’s theory of cell generation was strongly disputed and was clearly rebutted by many scientific observations, but mostly by the meticulous illustrations and descriptions of the stages of mitosis
The question of cell origins was put to rest once and for all by another student of Müller, Rudolf Virchow. Drawing on data from Robert Remak and his own research on the biology of tumors, Virchow wrote “omnis cellula e cellula” (every cell from a cell) in Die cellular Pathologie in ihrer Begründung auf physiologische und pathologische Gewebelehre (1858; Cellular Pathology as Based upon Physiological and Pathological Histology, 1860). The aphorism is not original to Virchow; it was coined by French natural scientist and politician François Vincent Raspail
This theory of the cell created distinct problems for the doctrine of vitalism, which asserted that no single part of an organism could exist apart from the whole organism, since the organism was alive as a result of its indwelling vital principle. The cell theory of Schwann and Schleiden specified that individual cells were alive even apart from the body, which negated a major precept of vitalism. However, this same theory still advocated that something outside cells dictated their very creation. Virchow’s emendation to the cell theory of Schwann and Schleiden essentially placed cells as self-contained and self-generating units of fundamental structure in all biological organisms that worked together to form the phenomenon known as life. This effectively issued the decisive finishing blow to vitalism and opened the field of biology to avenues of investigation unimaginable at that time.
The development of the cell theory was nothing less than momentous. Its rapid acceptance and its classic negation of vitalism inspired biologists to elucidate the fine structure of living organisms and the processes that drive cell, tissue, and organ functions. Because a nonmaterial vital principle that could not be measured or directly examined was no longer considered responsible for life and its processes, scientists began to open all aspects of life to empirical inquiry. The cell theory would lead scientists to investigate the function of subcellular structures, eventually leading to molecular studies of development, gene regulation, and cell trafficking.
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Baker, John. The Cell Theory: A Restatement, History, and Critique. New York: Garland, 1988. A scholarly, historical reassessment of the development of the cell theory and the reactions of the scientific community. -
Becker, Wayne, Lewis Kleinsmith, and Jeff Hardin. The World of the Cell. 6th ed. New York: Pearson/Benjamin Cummings, 2006. A comprehensive textbook study of the cell and its scientific history. Includes photographs. -
Harris, Henry. The Birth of the Cell. New Haven, Conn.: Yale University Press, 1999. A comprehensive and erudite exposition of the history of cell biology. -
Nurse, Paul. “The Great Ideas of Biology.” Clinical Medicine 3 (2001): 560-568. A brief but thorough and brilliant history of the development of cell biology and genetics by one of the top molecular cell biologists of modern times. -
Scott, Iain, and Logan David. “The Birth of Cell Biology.” New Phytologist 163 (2004): 7-9. A delightful summary of the work of Schleiden and Schwann and the work that led up to their cell theory. -
Wagner, Robert. “Rudolph Virchow and the Genetic Basis of Somatic Ecology.” Genetics 151 (1999): 917-920. An assessment of the work of Rudolf Virchow from the perspective of modern molecular biology.
Saussure Publishes His Research on Plant Metabolism
Semmelweis Develops Antiseptic Procedures
Pasteur Begins Developing Germ Theory and Microbiology
Mendel Proposes Laws of Heredity
Roux Develops the Theory of Mitosis
Metchnikoff Advances the Cellular Theory of Immunity
Koch Announces His Discovery of the Tuberculosis Bacillus
Ramón y Cajal Shows How Neurons Work in the Nervous System
Abel and Takamine Isolate Adrenaline
Ross Establishes Malaria’s Transmission Vector
Beijerinck Discovers Viruses
Ferdinand Julius Cohn; Ernst Haeckel; Robert Koch; Gregor Mendel; Louis Pasteur.