May 1, 2017History of Medicine
Editor's note: The evolution of the cell is one of Science's most awesome areas of study, leading to the origins of life itself. It is beyond our comprehension, why anyone able to contribute to the funding of research, inquiring into the mystery of life, and its pathologies, which cell biology does, why anyone would not be eager to do so. Perhaps Americans should vote to require that all politicians have an education high enough to enable them to understand the worlds of science, math, technology, engineering, and all the arts (which remind us of our humanity).
There are several theories about the origin of small molecules that led to life on the early Earth. They may have been carried to Earth on meteorites (see Murchison meteorite), created at deep-sea vents, or synthesized by lightning in a reducing atmosphere (see Miller-Urey experiment). There is little experimental data defining what the first self-replicating forms were. RNA is thought to be the earliest self-replicating molecule, as it is capable of both storing genetic information and catalyzing chemical reactions (see RNA world hypothesis), but some other entity with the potential to self-replicate could have preceded RNA, such as clay or peptide nucleic acid.
Cells emerged at least 3.5 billion years ago. The current belief is that these cells were heterotrophs. The early cell membranes were probably more simple and permeable than modern ones, with only a single fatty acid chain per lipid. Lipids are known to spontaneously form bilayered vesicles in water, and could have preceded RNA, but the first cell membranes could also have been produced by catalytic RNA, or even have required structural proteins before they could form. The eukaryotic cell seems to have evolved from a symbiotic community of prokaryotic cells. DNA-bearing organelles like the mitochondria and the chloroplasts are descended from ancient symbiotic oxygen-breathing proteobacteria and cyanobacteria, respectively, which were endosymbiosed by an ancestral archaean prokaryote. There is still considerable debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.
The cell (from Latin cella, meaning small room is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the building blocks of life. The study of cells is called cell biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometers.
The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cells inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, that all cells come from preexisting cells, and that all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. Cells are of two types, eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic cells, and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the domains of life, bacteria and archaea.
Three co-founders of cell theory:
Matthias Jakob Schleiden (1804-1881) was a German botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow. Born in Hamburg, Schleiden was educated at University of Jena, then practiced law in Heidelberg, but soon developed his love for botany into a full-time pursuit. Schleiden preferred to study plant structure under the microscope. While a professor of botany at the University of Jena, he wrote Contributions to our knowledge of phytogenesis (1838), in which he stated that all parts of the plant organism are composed of cells. Thus, Schleiden and Schwann became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He also recognized the importance of the cell nucleus, discovered in 1831 by the Scottish botanist Robert Brown, and sensed its connection with cell division. Schleiden was one of the first German biologists to accept Charles Darwin's theory of evolution. He became professor of botany at the University of Dorpat in 1863. He concluded that all plant parts are made of cells and that an embryonic plant organism arises from the one cell. He died in Frankfurt am Main on 23 June 1881. It was during the four years spent under the influence of Muller in Berlin, that Schwann's most valuable work was done. Muller was at this time preparing his great book on physiology, and Schwann assisted him in the experimental work required. Schwann observed animal cells under the microscope, noting their different properties. Schwann found particular interest in the nervous and muscular tissues. He discovered the cells that envelope the nerve fibers, now called Schwann cells in his honor. Schwann discovered the striated muscle in the upper esophagus and initiated research into muscle contraction, since expanded upon greatly by Emil du Bois-Reymond and others. Muller directed Schwann's attention to the process of digestion, and in 1837 Schwann isolated an enzyme essential to digestion, which he called pepsin. Schwann became chair of anatomy at the Belgian Catholic University of Leuven in 1839. Here he produced little new scientific work, the exception being a paper establishing the importance of bile in digestion. He nonetheless proved to be a dedicated and conscientious professor. In 1848, his compatriot Antoine Frederic Spring convinced him to transfer to the University of Liege, also in Belgium. At Liege, he continued to follow the latest advances in anatomy and physiology without himself contributing. He became something of an inventor, working on numerous projects including a human respirator for environments where the surroundings are not breathable. In his later years, Schwann found growing interest in theological issues. Three years after retiring, Schwann died in Cologne on 11 January 1882. There is a bronze statue of Theodor Schwann at the entrance of the Institute of Zoology, University of Liege, Belgium.
In 1837, Matthias Jakob Schleiden viewed and stated that new plant cells formed from the nuclei of old plant cells. While dining that year with Schwann, the conversation turned on the nuclei of plant and animal cells. Schwann remembered seeing similar structures in the cells of the notochord (as had been shown by Muller) and instantly realized the importance of connecting the two phenomena. The resemblance was confirmed without delay by both observers, and the results soon appeared in Schwann's famous Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants, in which he declared that All living things are composed of cells and cell products. This became cell theory or cell doctrine. In the 19th century, physiological knowledge began to accumulate at a rapid rate, in particular with the 1838 appearance of the Cell theory of Matthias Schleiden and Theodor Schwann. It dramatically stated that organisms are made up of units called cells. At this time, cell theory was considered a radical idea. In the course of his verification of cell theory, Schwann proved the cellular origin and development of the most highly differentiated tissues including nails, feathers, and tooth enamel. Schwann established a basic principle of embryology by observing that the ovum is a single cell that eventually develops into a complete organism.
In 1857, pathologist Rudolf Virchow posed the maxim Omnis cellula e cellula - that every cell arises from another cell. By the 1860s, cell doctrine became the conventional view of the elementary anatomical composition of plants and animals. Schwann's theory and observations became the foundation of modern histology.
Timeline: History of Cell Biology
- 1632-1723: Antonie van Leeuwenhoek teaches himself to make lenses, constructs basic optical microscopes and draws protozoa, such as Vorticella from rain water, and bacteria from his own mouth.
- 1665: Robert Hooke discovers cells in cork, then in living plant tissue using an early compound microscope. He coins the term cell (from Latin cella, meaning small room) in his book Micrographia (1665).
- 1839: Theodor Schwann and Matthias Jakob Schleiden elucidate the principle that plants and animals are made of cells, concluding that cells are a common unit of structure and development, and thus founding the cell theory.
- 1855: Rudolf Virchow states that new cells come from pre-existing cells by cell division (omnis cellula ex cellula).
- 1859: Louis Pasteur (1822-1895) contradicts the belief that life forms can occur spontaneously (generatio spontanea) (although Francesco Redi had performed an experiment in 1668 that suggested the same conclusion).
- 1931: Ernst Ruska builds the first transmission electron microscope (TEM) at the University of Berlin. By 1935, he has built an EM with twice the resolution of a light microscope, revealing previously unresolvable organelles.
- 1953: Watson and Crick made their first announcement on the double helix structure of DNA on February 28.
- 1981: Lynn Margulis published Symbiosis in Cell Evolution detailing the endosymbiotic theory