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Exercise 1.   The Discovery of Cells

D.   Schleiden, Schwann, Virchow and the Cell Theory

Matthias Schleiden & Theodor Schwann

While Hooke and Leeuwenhoek were the first to observe cells and are rightfully credited with the discovery of cells, neither one of them understood the role cells played in the organization of living systems or the universality of cells among living organisms. It wasn't until the 1800s, roughly 200 years after the discovery of cells, that the work was done that firmly established the Cell Theory and convincingly dispelled the Theory of Spontaneous Generation.

The notion that the cell is the basic unit of life, which is the core tenet of the Cell Theory, was first declared by Henri Dutrochet in 1824. Dutrochet was an accomplished French biologist credited with, among other things, early research into the development of embryos and determining the basis for osmosis. Following up on the work of Ludolph Treviranus and Johann Moldenhawer, who put forth the notion that cells were separable into individual units, Dutrochet proposed the idea that "The cell is the fundamental element of organization". But it wasn’t until the work of Matthias Scleiden and Theodore Schwann was published, about 15 years later, that a concise cell theory was formulated.

Matthias Jakob Schleiden (1804 – 1881) was a German botanist. He spent a significant part of his career studying plant structure under the microscope. In his monograph Beiträge zur Phytogenesis (Contributions of Phytogenesis), which was published in 1838, he described his observations that different parts of the plant are made of cells, and he concluded that all plant tissues are made of cells and that the embryonic plant grows from a single cell.

Theodor Schwann (1810 – 1882) was a German physiologist and a close associate of Schleiden. He studied the microscopic structure of various animal tissues and collaborated with Schleiden on studies of the microscopic structure of plants and animals. Among his many accomplishments is his discovery of the cells that form sheaths around neurons in the peripheral nervous system, now known as Schwann cells. In 1839, a year after Schleiden reported his studies on plants, Schwann published his manuscript entitled Microscopic Investigations on the Accordance in the Structure and Growth of Plants and Animals, in which he formally concludes that "All living things are composed of cells and cell products".

The Work of Robert Remak and Rudolf Virchow

Robert Remak & Rudolf Virchow

Robert Remak (1815-1865) was a Polish physician, biologist and contemporary of Schleiden and Schwann. His work in embryology resulted in his determination that adult animal tissues come from three embryonic germ layers, the ectoderm, mesoderm, and endoderm. Remak's experimental observations also showed, for the very first time, that cells come from pre-existing cells through a cell divsion process.

Rudolf Virchow (1821 - 1902) was a German physician, pathologist and contemporary of Remak. His microscopic study of diseases earned him the moniker "Father of Modern Pathology". Virchow popularized the maxim Omnis cellula e cellula ("Every cell originates from another existing cell like it."). His genuine experimental contribution to our knowledge about the origin of cells has been questioned. It has been authoritatively claimed that Virchow plagarized Remak's work. Moreover, the maxim Omnis cellula e cellula has its origins with two other scientists, Francesco Redi and François-Vincent Raspail. Francesco Redi carried out the landmark experiments that showed, for the first time, that fly larvae (maggots) only develop on decaying meat when living flies had access to the meat, leading Redi to conclude that life only comes from pre-existing life. He coined the phrase Omne vivum ex vivo ("Every living thing comes from life"). François-Vincent Raspail extended Redi's conclusion to the realm of cells and coined the phrase Omnis cellula e cellula. Thus, Virchow's genuine contributions to the notion that cells come from other cells should probably be limited to a recognition that he popularized Raspail's maxim.

Despite the controversy surrounding Virchow's work on the origin of cells, the scientific community credits Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow with the formulation of the Cell Theory. The theory has three basic tenets:

  • All living things are made of cells.
  • The cell is the basic unit of all living things.
  • Cells only come from pre-existing cells.

The Cell Theory is now considered one of the great unifying principles of biology. It establishes the cell as the basic structural and physiological unit of life and formally establishes the principle that life is not generated spontaneously but comes directly from pre-existing life. Thus, the formulation of the Cell Theory during the 1850s, based on work that began with the development of the microscope in the 1600s, finally and unequivocally dispelled the notion that life originated spontaneously from non-living materials.