The cell, queen of the living world
In the early 1800s, the basic unit of biology was still tissue. Until Theodor Schwann proposed his cell theory in 1839, making the cell the fundamental structural and functional unit of all living beings. And making him one of the great names in the history of biology.
F
rom the 15th to the 19th century, the history of the earth and of life is marked by the discovery of new worlds. The first idea that often springs to mind is the exploration of America from 1492 onwards. From a scientific point of view, this "new world", which had evolved independently of Europe, abounded in animal and plant species unknown to Europeans. Enough to satisfy the curiosity and thirst for knowledge of many a scientist! But the new "geographical" worlds were not the only ones opening up to researchers at this time. Enormous advances in scientific instruments and equipment meant that microscopy, for example, enabled us to delve into the heart of the small. Microscopes became ever more powerful, enabling observations that had previously been impossible. This development revolutionized biology by making the cellular world accessible. It was in the 17th century that "the cell" was described by one of the greatest experimental scientists of that time: Robert Hooke. " At that time, the unit in biology was the tissue, but we didn't know what made up these tissues ", explains Vincent Geenen, Professor of Embryology at the University of Liège.
The cell, a star long left in the shadows
In 1667, Robert Hooke was the first to use the word "cell" after examining slices of plant cork. " He found that the cork contained a series of structures, all the same, which resembled monks' chambers" , explains Vincent Geenen, "and he called them cells ". Although Robert Hooke was responsible for the discovery of the cell, it was not until much later that the notion of the cell came into its own. " At the time of Robert Hooke, this notion remained at the description stage. It was not until almost two centuries later that Theodor Schwann proposed his cell theory, making the cell the basic unit of biology ". Born into a German Catholic family, Theodor Schwann was a shy, introverted pupil with an early interest in the natural sciences. During his medical studies at the University of Bonn, he met Johannes Peter Müller, a leading professor of physiology, who became his mentor and introduced him to research. His early career was marked by numerous scientific contributions in various fields. " In his doctoral thesis in medicine, he demonstrated that oxygen was indispensable to the hen's egg, and also conducted a companion thesis aimed at challenging an idea that was very much in vogue at the time: spontaneous generation. He was working on a physico-chemical explanation of life," explains Vincent Geenen. " He also showed that digestion in the stomach relies essentially on the enzyme pepsin . Theodor Schwann also introduced the term "metabolism" in the late 1830s.
A brilliant thought
But what undeniably elevated Theodor Schwann to the ranks of the world's great scientists was his cell theory. In 1837, a fellow professor of botany at Jena University in Germany, Matthias Jakob Schleiden, discovered that structures similar to the monk cells described by Robert Hooke were present in all the plant tissues he examined. Over lunch, Matthias Jakob Schleiden shared his findings with Theodor Schwann, who had himself observed this type of structure in animal tissue. Schwann understood the importance of linking these observations, and in 1839 wrote the famous manuscript "Recherches microscopiques sur la similité de structure et de développement des cellules animales et végétales" (1). Cell theory was born. The cell became the basic unit of biology. " With this theory, which launched the idea that the cell is the common origin of all living things, Theodor Schwann fought against vitalism, a commonly accepted theory at the time," explains Vincent Geenen. " According to vitalism, life appears thanks to an unknown force or energy that animates the living world . Theodor Schwann would say that " the cell is the origin of life ". Thus, Theodor Schwann's work is above all an act of thought that led him to define the cell as the basic unit of life. A quotation from the great Hungarian physiologist Szent Gyorgyi illustrates things well: "Genius is seeing what everyone else sees, but thinking what no one else has thought ", says Vincent Geenen. Twenty years later, his theory was widely accepted. Theodor Schwann was awarded the Copley Medal in 1845 for this work. At the time, this was the most prestigious award given by the Royal Society in London.
The cell, the starting point of every living being
Before and after he developed his cellular theory, Theodor Schwann spent a considerable amount of time observing animal tissues under the microscope. He examined cartilage taken from the gills of tadpoles, bones, bird eggs, the first rudiments of embryos, teeth, feathers, tendons, muscles, nerves and many other structures that make up animal organisms. In all of these, he noted the presence of cells consisting of a cavity delimited by a wall and containing a nucleus (then called a "cytoblast") with a nucleolus. In addition to the structure as such, he also observed the formation of young cells from existing ones. This confirms that the most diverse elementary parts of animals and plants share a common mode of development: their origin is always a cell.
Theodor Schwann's "Recherches microscopiques sur la conformité de structure et de croissance des animaux et des plantes", published in 1839, was the birth of cell theory. The German physiologist shows that the cell is the elementary structure of all living organisms, whether animal or plant, simple or complex.
Theodor Schwann went on to divide animal tissues into five classes according to the cells of which they are composed: independent and isolated cells swimming in liquids or located close to each other and mobile (blood corpuscles, lymphatic corpuscles, purulent corpuscles, mucous corpuscles...), independent cells strongly attached to others (epithelium, nail, lens, feathers..), cells whose walls but not cavities are fused together (cartilage, teeth, etc.) or fibrous cells that elongate to form bundles of fibers (tendons, etc.) or cells whose walls and cavities are fused together (muscles, nerves, etc.). Today, animal tissues are grouped into four main categories according to their morphology: epithelial, connective, muscular and nervous tissues.
The ancestor of the diving suit created at the University of Liège
" In the year he wrote his manuscript, Theodor Schwann was, curiously enough, experiencing a crisis of mysticism, a revival of religious feeling and abandoning his scientific rationalism ", says Vincent Geenen. From 1848 onwards, he devoted most of his time to teaching and his students. First at the Catholic University of Louvain, which he joined in 1848. " During these years, he studied the role of bile in digestion," continues Vincent Geenen. Then, in 1849, he moved to the University of Liège, where he held the Chair of Anatomy. He taught there until 1881. One of his last major scientific contributions, made at the University of Liège, was an answer to the question posed by the Académie des Sciences: "How can we live in an unbreathable environment? Theodor Schwann's answer was to invent a device for breathing in an oxygen-free environment (2). "At the time, he was particularly interested in medical engineering and, thanks to his physiological knowledge of gas exchange, he created the ancestor of the oxygen tank and the diving suit ", explains Vincent Geenen.
Prototype rebreather/scaphander built in 1853 by Prof. Theodor Schwann at the University of Liège
The advent of cytology, i.e. the study of isolated cells, stems from the cellular theory proposed by Theodor Schwann. "This was the starting point for understanding cell function, embryonic development and pathologies linked to cell dysfunction ," explains Vincent Geenen. Theodor Schwann's thinking opened up a boulevard that enabled young scientists of the time, such as his colleagues Edouard van Beneden and Léon Fredericq, and their successors, to make considerable advances in understanding cellular mechanisms.
Written by Audrey Binet
Scientific references
(1)Microscopic studies on the correspondence in the structure and growth of animals and plants. Berlin, Sander'sche Buchhandlung, 1839 Recherches microscopiques sur l'analogie de structure et de croissance entre les animaux et les plantes".
(2) Description of two devices for living in an unbreathable environment. Liège, De Thier (Liège), 1878.
Read Theodor Schwann's biography
Consult Theodor Schwann's scientific publications
Vincent Geenen
Vincent Geenen is Director of Research at the F.R.S.-FNRS in Belgium, Professor of Embryology and History of Biomedical Research at the University of Liège, and Head of the Endocrinology Clinic at Liège University Hospital. Since January 2012, he has headed the Centre d'Immunoendocrinologie at the GIGA-R research institute. He is secretary of the Fonds Léon Fredericq for biomedical research at Liège University Hospital and a member of the SVS-4 commission of the F.R.S.-FNRS.
For almost 30 years, Vincent Geenen and his team have been working on the thymus, the central lymphoid organ of the immune system. His research has shown that the thymus plays a unique role in educating the immune system to recognize and tolerate neuroendocrine functions, and that thymus dysfunction is involved in the development of selective autoimmunity in type 1 diabetes. Currently, the Center for Immunoendocrinology is developing a new type of negative/tolerogenic vaccination against type 1 diabetes.
Vincent Geenen was also coordinator of the FP6 integrated project Eurothymaide (2004-2008). This consortium of 20 academic laboratories and 5 SME biotech companies worked to develop new diagnostic and therapeutic approaches to autoimmune diseases, based on new knowledge of the natural tolerogenic mechanisms of the thymus.
Glossary
Vitalism: vitalism is a doctrine according to which the vital phenomena of the organism (growth, development of the hereditary plan, nutrition, etc.), far from being explained by the simple play of physico-chemical forces, are due to the action of a vital principle conceived either in the form of an intelligent soul, or in the form of a subaltern archaea (astral body).
Nucleolus: sub-compartment of the nucleus of eukaryotic cells where transcription and maturation of ribosomal RNA take place, as well as the first stages of ribosome assembly.
