The beginnings of cell theory: Schleiden, Schwann, and Virchow - Hektoen International (2024)

JMS Pearce
Hull, England

Figure 1. Robert Hooke’s pores (cells) of the cork oak. Wellcome Collection. CC BY 4.0.

Every schoolchild is taught in biology about cells and their elemental importance. Students of biological and medical sciences also learn about the Schwann cell sheath that invests nerve fibers. What is less well known is how these two are related. Schwann, a physician by training, and Schleiden, a botanist from Hamburg, were the founders of the original concept of cell theory, the fundamental scientific basis of biology. This concept stated that cells are the structural and functional units of all living organisms, forming tissues, organs, and the entire plant or animal.

Cell theory had its roots in the physicist and astronomer Robert Hooke’s first depiction published in Micrographia in 1665. Before Hooke, in the seventeenth century, all living forms were thought to depend on vitalism. A force distinct from all physical and chemical forces peculiar to living organisms, vitalism controlled form and function. Living forms and their cells arose from spontaneous generation (abiogenesis) or from a pre-existing form (preformationism).

Hooke’s Micrographia contained a collection of copper-plate illustrations of many objects he had seen with his own compound microscope, including the porous cells of Quercus suber, the cork tree (Fig 1). He stated:

… these pores, or cells…were indeed the first microscopical pores I ever saw, and perhaps, that were ever seen, for I had not met with any Writer or Person, that had made any mention of them before this…

In 1670, Antony van Leeuwenhoek built a simple one-lens microscope and observed single-celled bacteria, which he reported to the Royal Society as animalcules.¹ He also observed protozoa, plant cells, algae, blood cells, and living sperm cells. In his time, simple and compound microscopes developed and improved; scientists driven by curiosity used them to examine the structures of diverse living and non-living objects. Schleiden and Schwann rejected preformationism in favor of a theory of epigenesis, a sequence in which cells differentiate to form tissues and organs. To demonstrate the existence of cells was of obvious importance, but no elementary coherent theory of cellular organization or cellular mechanisms was known. Cell theory started with Theodor Schwann and Matthias Jakob Schleiden.

Figure 2. Matthias Jakob Schleiden (right) and Theodor Schwann (left). From Robinson V., Pathfinders in Medicine.4 Public domain.

Matthias Jakob Schleiden (1804–1881)

Schleiden was a lawyer. When his practice dwindled, in desperation he shot himself. Fortunately, he recovered and turned his attention to botany, first at the University of Göttingen, then in Berlin in the thriving research laboratory of zoologist Johannes Müller. There he met Theodor Schwann, MD. Much of Schleiden’s work was done using the advanced Zeiss microscope with which he examined the structure of many plant species. He knew of the famous Scottish naturalist Robert Brown’s (1773–1858) description of the cell nucleus (syn. cytoblast) in orchids in 1831. Schleiden came to realize that cells were structural units common to all plants, which, although now obvious, was not understood in his time. In 1838, he published his original findings in his first paper, “Contributions to Phytogenesis”,² and in his 1842 textbook on botany, Grundzüge der wissenschaftlichen Botanik.

Shortly after Schleiden’s discoveries in plants, Müller’s favorite pupil, Theodor Schwann, showed a similar cellular structure in all animal species.³ Schleiden and Schwann were musing over Brown’s cell nuclei one day over a drink when Schleiden remarked:

The discovery of the nucleus was a great piece of work—it will yield tremendous results. To the health of Robert Brown! Really, I think I can trace back every plant embryo to a single nucleated cell. What you say, said Schwann, reminds me that I have seen a similar organ in the cells of the dorsal cord upon which Professor Muller has put me to work. Besides, in the vertebrate notochord Muller has pointed out the existence of cells resembling those of the plant. Let me see these cells, said Schleiden. They hastened to the laboratory, and looking thru the microscope the eager eye of Schleiden saw that the nuclei of the chorda dorsalis were similar to the nuclei of vegetable tissue. Identical, exclaimed Schleiden. Then, mused Schwann, it must follow in consequence of this identity that your conception must be extended also to the animal world. This means that the cell is the unit of all organic structure.⁴

Figure 3. T. Schwann, Mikroskopische Untersuchungern. Wellcome Collection. CC BY 4.0.

Their joint conclusion was that plants and animals shared an “analogous” primary structure, the cell.⁵ They thought the formation of new cells was responsible for growth but mistakenly believed that new cells were formed from crystalized remnants of old cell nuclei. However, they quickly recognized the importance of Brown’s nucleus (syn. cytoblast) in cell division by mitosis, which was later understood as replication of chromosomes.

From their collaborative investigations, Schwann and Schleiden formulated cell theory that states:

• All living things are made up of one or more cells.
• The cell is the basic unit of structure of all organisms.
• Cells arise from pre-existing cells.

To this is now added:⁶

• Cells carry genes, the basis of heredity.
• Evolution by natural selection is a characteristic of life.
• Biological organization is based on logical and informational processes and structures.

Barthelemy Dumortier, another botanist, had observed reproduction by cell division of pre-existing cells in plants in 1832. He called the process “binary fission.” Robert Remak, a neurologist, was able to confirm cell division and reproduction in animal cells as early as 1844. However, many scientists disputed their findings. Rudolf Virchow, the charismatic, eminent pathologist and statesman, confirmed and finally published Remak’s early findings in 1855. Though not the originator, Virchow advanced cell theory when he stated that all cells develop from existing cells: Omnis cellula e cellula. He also applied cell theory to disease and showed that when cells malfunction, they may result in diseased tissues.

Schleiden received his doctorate in botany in 1839 at the University of Jena. In 1863, he was appointed Professor of Botany at Jena. He was one of the first to support Darwinism. He eventually became a professor of anthropology at Dorpat, Russia in 1863, but quickly returned to Frankfurt where he died in in 1881. Among his last published works was The Sciences Among the Jews, Before and During the Middle Ages, a well-known but, at the time, controversial study.

Theodor Schwann (1810–1882)

Schwann was born in Neuss in the Rhineland.⁷ His early education was at the Jesuit Gymnasium of Cologne. A deeply religious, non-confrontational, modest man, he attended the universities of Bonn and Würzburg. In Bonn, he was taught by Johannes Müller and followed him to Berlin.

His research was original, versatile, and scrupulously conducted. In his inaugural dissertation of 1834, he showed that air was necessary for the hen’s embryo to survive. He discovered pepsin in gastric juice, the first enzyme demonstrated in human tissues. He was the first to apply physical laws to the investigation of striated muscle contraction in the esophagus. At Leuven he showed that yeast spores were living cells, which fermented sugar and starch as the result of a living processes. This was the “germ theory” of alcoholic fermentation, later made famous by Louis Pasteur. Schwann also coined the word metabolism.

The eponymous Schwann cell sheath of the axis-cylinder of nerves also was published in Microscopic investigations on the similarity of structure and growth of animals and plants, 1839.³ There he recognized that the outer membrane of nerve axons was a separate cell, not, as generally held, a part of the surrounding connective tissue or neurilemma. Virchow also experimented with myelin, a substance that he likened to bone marrow.⁸ Each Schwann cell comprises a single myelin sheath investing an axon.

Rejected by Prussian academia, Schwann was appointed to the Chair in anatomy at the Catholic University of Louvain; he then moved to Liège. Although his scientific investigations had dwindled, he received the Royal Society’s prestigious Copley Medal in 1845. Schwann died after a stroke. A bronze statue is sited at the museum entrance of the Institute of Zoology, University of Liège.

References

1. Leeuwenhoek M. Philosophical Transactions, VIII (94), 6037-6038 as: A Specimen of some Observations made by a Microscope, contrived by M. Leeuwenhoek in Holland, lately communicated by Dr. Regnerus de Graaf; cited by Ford, Brian J. “From Dilettante to Diligent Experimenter, a Reappraisal of Leeuwenhoek as microscopist and investigator.” Biology History 1992; 5(3): 3–21.
2. Schleiden Matthias Jacob. “Beiträge über Phytogenesis.” Archiv für Anatomie, Physiologie und wissenschaftliche Medicin 1838; 137–176. www.biodiversitylibrary.org/item/43441#page/263/mode/1up.
3. Schwann, Theodor. Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen 1839. Translated by Henry Smith as Microscopic investigations on the similarity of structure and growth of animals and plants. London: The Sydenham Society, 1847. www.biodiversitylibrary.org/item/43441#page/9/mode/1up.
4. Robinson V. Pathfinders in Medicine. New York, Medical Review of Reviews 1912; p. 293.
5. Schwann T. Ueber die Analogie in der Structur und dem Wachsthum der Thiere und Pflanzen. Neue Not Geb Nat Heil 1838;33–36; 1838;25–29; 1838;21–23.
6. Nurse P. “The great ideas of biology.” Clinical Medicine 2003; Vol 3 No 6 560–568.
7. Causey G. “Theodor Schwann (1810-1882).” In Haymaker W., Schiller F. (eds.): The founders of neurology. Springfield, Charles C Thomas, 1970, pp. 77–80.
8. Virchow R. “Ueber das ausgebreitete Vorkommen einer dem Nervenmark analogen Substanz in den thierischen Geweben.” Archiv fur pathologische Anatomie und Physiologie und fur klinische Medicin. Sechster Band. 1854.

JMS PEARCE is a retired neurologist and author with a particular interest in the history of medicine and science.

Summer 2022 | Sections | Science