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Keywords: Theodore Schwann, cell theory, animal cells, discovery of cells, cell nucleus
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Theodore Schwann (1810–1882), a German physician and physiologist (Fig. 1
), was assistant to Johannes Peter Müller (1801–1858), a prominent physiologist and pathologist in Berlin. Schwann discovered by microscopy that animal tissues, like plants, are composed of cells and that each cell has a nucleus. These are among the greatest scientific discoveries ever made.
Schwann published his observations in three articles in the January, February, and April 1838 issues of the Neue Not. Geb. Nat. Heilk. [1]. He described nucleated cells of notochordal tissue and cartilage in the larvae of toads, as well as in the kidney, liver, pancreas, salivary gland, and connective tissues of pig embryos. His articles were followed in 1839 by his landmark book: "Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen" (Microscopic investigations on the similarity of structure and growth of animals and plants) [2].
Schwann was a devout Catholic and despite the fact that his 270 page-book was largely a reiteration of what he had already published a year earlier, he submitted the manuscript to the Archbishop of Malines for approval before publication. He wrote in his Untersuchungen that "The tissues of animals are formed from cells. The globules of lymph, pus, and mucus are cells with their walls distinct and isolated from each other. Horny (squamous) tissues are cells with distinct walls, but united into coherent tissues; bone and cartilage are formed of cells whose walls have coalesced; fibrous tissue and tendon are cells which have split into fibers; and muscle, nerves and capillary vessels are cells of which both the walls and cavities have coalesced." He concluded that the "One common principle of development... for the most highly differentiated organisms (plants and animals)... is the formation of cells." (Fig. 2).
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Botanists had long known that plants are composed of cells. In 1665, an English botanist Robert Hooke (1635–1703) coined the term "cell" to designate the microscopic units in cork [3]. The cell nucleus was discovered in 1831 by a Scotch botanist and physician, Robert Brown (1773–1858) [4]. Jakob Henle (1809–1885), another assistant of Johannes Müller in Berlin, reached the conclusion in 1837 that there is some similarity between cells of plants and animals [5]. But, the predecessors of Schwann did not generalize their findings as Schwann did. Schwann presented his comparisons of plant and animal tissues in a concise manner. His success was mainly due to his ability to describe and illustrate his microscopic observations accurately, drawing conclusions from the work of others, and formulating his opinion in a most convincing way.
There is no question that Schwann’s published works laid the foundations for the cell theory, but it must be recognized that, as often stated, discoveries are not made in isolation. Nonetheless, the important discoverer is the one who presents his contribution in such a way that the discovery becomes accepted in the scientific world. This was eminently accomplished by Schwann.
It is amazing that Schwann accomplished almost all of his major discoveries before he was 30 years old. These dicoveries include the action of pepsin [6], the role of germs in fermentation and putrefaction [7], the importance of bile in digestion [8], the essentiality of air for embryonic development [9], and the description of the nerve sheath that is named after him [10].
In 1839, shortly after his Untersuchungen was published, Schwann was appointed as a professor at the Catholic University of Louvain, Belgium. (At the same university 300 years earlier, Andreas Vesalius had studied and performed his first anatomic dissections.) In 1848, Schwann became professor of anatomy and physiology at the University of Liége, near the Belgian town of Bastogne, a short distance from the German border. He remained in Liége and lectured in French, his adopted language, until his death at age 72.
Within a few years after Schwann had concluded that the cell is the ultimate morphological unit of tissues and organs, two atlas texts of histopathology were published independently in 1843 by Julius Vogel (1814–1880) in Germany [11] and in 1845 by Hermann Lebert (1813–1878) in France [12]. With these printed volumes, the cell theory, introduced and publicized by Schwann in 1838–1839, became universally accepted as scientific fact.
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