The German biologist Theodor Schwann (1810-1882) is considered a founder of the cell theory. He also discovered pepsin, the first digestive enzyme prepared from animal tissue, and experimented to disprove spontaneous generation.
Theodor Schwann was born at Neuss near Düsseldorf on Dec. 7, 1810. At the University of Bonn, which he entered in 1829, he met Johannes Müller, the physiologist, whom he assisted in his experiments. Schwann continued his medical studies at the University of Würzburg and later at the University of Berlin, from which he graduated in 1834. His doctoral dissertation dealt with the respiration of the chick embryo.
At the University of Berlin, Schwann again came into contact with Müller, who convinced him that he should follow a scientific career. Very soon after he began to work under Müller, he had his first success. From extracts which he made of stomach lining, Schwann demonstrated that a factor other than hydrochloric acid was operating in digestion. Two years later, in 1836, he succeeded in isolating the active principle, which he named pepsin.
Between 1834 and 1838 Schwann undertook a series of experiments designed to settle the question of the truth or falsity of the concept of spontaneous generation. His method was to expose sterilized (boiled) broth only to heated air in a glass tube, the result being that no micro-organisms were detectable and no chemical change (putre-faction) occurred in the broth. He was convinced that the idea of spontaneous generation was false. His sugar fermentation studies of 1836 also led to his discovery that yeast originated the chemical process of fermentation.
At Müller's suggestion, Schwann also began research on muscle contraction and discovered striated muscles in the upper portion of the esophagus. He also identified the delicate sheath of cells surrounding peripheral nerve fibers, which is now named the sheath of Schwann.
In 1838 Schwann became familiar with Matthias Schleiden's microscopic research on plants. Schleiden described plant cells and proposed a cell theory which he was certain was the key to plant anatomy and growth. Pursuing this line of research on animal tissues, Schwann not only verified the existence of cells, but he traced the development of many adult tissues from early embryo stages. This research and the cell theory which followed were summarized in Mikroskopische Untersuchungen ueber die Uebereinstimmung in der Struktur und dem Wachstum der Thiere und Pflanzen (1839; Microscopical Researches on the Similarity in the Structure and the Growth of Animals and Plants, 1847). This work, which in Schwann's own words demonstrated that "the great barrier between the animal and vegetable kingdoms, viz. the diversity of ultimate structure, thus vanishes," established the cell theory to the satisfaction of his contemporaries.
Schwann proposed three generalizations concerning the nature of cells: First, animals and plants consist of cells plus the secretions of cells. Second, these cells have independent lives, which, third, are subject to the organism's life. Furthermore, he realized that the phenomena of individual cells can be placed into two classes: "those which relate to the combination of the molecules to form a cell. These may be called plastic phenomena," and "those which result from chemical changes either in the component particles of the cell itself, or in the surrounding cytoblastema [the modern cytoplasm]. These may be called metabolic phenomena." Thus Schwann coined the term "metabolism," which became generally adopted for the sum total of chemical processes by which energy changes occur in living things.
Schwann also contributed to the understanding and classification of adult animal tissues. He classified tissues into five groups: separate independent cells, such as blood; compacted independent cells, such as skin; cells whose walls have coalesced, such as cartilage, bones, and teeth; elongated cells which have formed fibers, such as tendons and ligaments; and finally, cells formed by the fusion of walls and cavities, such as muscles and tendons. His conclusions were also basic to the modern concept of embryology, for he described embryonic development as a succession of cell divisions.
This generalization of the essential structural kinship of all living things had been denied for centuries by the old Aristotelian doctrine of vegetable and animal souls. Perhaps Schwann's findings were more disturbing than he liked to admit, since he realized that they supported an ultimate physical rather than a theological explanation. Schwann saw the implications of his discovery, and the idea of the world of life being nothing more than a machine appalled him. He found refuge in the Roman Catholic faith, choosing, as he said, a God "more sensitive to the heart than to reason."
In 1839 Schwann was appointed professor of anatomy at the University of Louvain, Belgium, where he remained until 1848, when he accepted a professorship at the University of Liège. He remained there until his retirement in 1880. After leaving the influence of Müller, Schwann's productivity practically ceased; in Belgium he published only one paper, on the use of bile. He was an excellent, conscientious teacher, loved and appreciated by his students.
Schwann's work was ultimately recognized by scientists in other countries, and in 1879 he was made a member of the Royal Society and also of the French Academy of Science. In 1845 he had received the Copley Medal. Death came to Schwann on Jan. 11, 1882, 2 years after his retirement, in Cologne.
Excerpts in English translation from Mikroskopische Untersuchungen are found in the following works: Forest Ray Moulton and Justus J. Schifferes, eds., The Autobiography of Science (1945; rev. ed. 1960); Augusto Pi Suñer, Classics of Biology (1955); Friedrich S. Bodenheimer, The History of Biology: An Introduction (1958); and George Schwartz and Philip W. Bishop, eds., Moments of Discovery (2 vols., 1958). There is no biography of Schwann. Gilbert Causey in The Cell of Schwann (1960) devotes the first chapter to a sparse recital of the essential details of Schwann's life. Erik Nordenskiöld, The History of Biology (1928; new ed. 1935), gives a brief biographic account, as does Gordon R. Taylor, The Science of Life (1963). A good treatment of the cell theory and Schwann's part in it is in William A. Locy, Biology and Its Makers (1908; rev. ed. 1915).