Cell theory

In biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from pre-existing cells. Cells are the basic unit of structure in all organisms and also the basic unit of reproduction. With continual improvements made to microscopes over time, magnification technology advanced enough to discover cells in the 17th century. This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientists. Most of these debates involved the nature of cellular regeneration, and the idea of cells as a fundamental unit of life. Cell theory was eventually formulated in 1839. This is usually credited to Matthias Schleiden and Theodor Schwann. However, many other scientists like Rudolf Virchow contributed to the theory. It was an important step in the movement away from spontaneous generation.

The three tenets to the cell theory are as described below:

  1. All living organisms are composed of one or more cells.
  2. The cell is the basic unit of structure and organization in organisms.
  3. Cells arise from pre-existing cells.

The first of these tenets is disputed, as non-cellular entities such as viruses are sometimes considered life-forms.[1]

HeLa cells stained with Hoechst 33258
Human cancer cells with nuclei (specifically the DNA) stained blue. The central and rightmost cell are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed.

Microscopes

Leeuwenhoek Microscope
Anton van Leeuwenhoek's microscope from the 17th century with a magnification of 300x. [2]
Hooke-microscope
Robert Hooke's microscope

The discovery of the cell was made possible through the invention of the microscope. In the first century BC, Romans were able to make glass, discovering that objects appeared to be larger under the glass. In Italy during the 12th century, Salvino D’Armate made a piece of glass fit over one eye, allowing for a magnification effect to that eye. The expanded use of lenses in eyeglasses in the 13th century probably led to wider spread use of simple microscopes (magnifying glasses) with limited magnification.[3] Compound microscope, which combine an objective lens with an eyepiece to view a real image achieving much higher magnification, first appeared in Europe around 1620[4][5] In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia. Hooke also used a simpler microscope with a single lens for examining specimens with directly transmitted light, because this allowed for a clearer image.[6]

Extensive microscopic study was done by Anton van Leeuwenhoek, a draper who took the interest in microscopes after seeing one while on an apprenticeship in Amsterdam in 1648. At some point in his life before 1668, he was able to learn how to grind lenses. This eventually led to Leeuwenhoek making his own unique microscope. His were a single lens simple microscope, rather than a compound microscope. This was because he was able to use a single lens that was a small glass sphere but allowed for a magnification of 270x. This was a large progression since the magnification before was only a maximum of 50x. After Leeuwenhoek, there was not much progress for the microscopes until the 1850s, two hundred years later. Carl Zeiss, a German engineer who manufactured microscopes, began to make changes to the lenses used. But the optical quality did not improve until the 1880s when he hired Otto Schott and eventually Ernst Abbe.[7]

Optical microscopes can focus on objects the size of a wavelength or larger, giving restrictions still to advancement in discoveries with objects smaller than the wavelengths of visible light. Later in the 1920s, the electron microscope was developed, making it possible to view objects that are smaller than optical wavelengths, once again, changing the possibilities in science.[7]

Discovery of cells

Cork Micrographia Hooke
Drawing of the structure of cork by Robert Hooke that appeared in Micrographia.

The cell was first discovered by Robert Hooke in 1665, which can be found to be described in his book Micrographia. In this book, he gave 60 ‘observations’ in detail of various objects under a coarse, compound microscope.[6] One observation was from very thin slices of bottle cork. Hooke discovered a multitude of tiny pores that he named "cells". This came from the Latin word Cella, meaning ‘a small room’ like monks lived in and also Cellulae, which meant the six sided cell of a honeycomb. However, Hooke did not know their real structure or function.[8] What Hooke had thought were cells, were actually empty cell walls of plant tissues. With microscopes during this time having a low magnification, Hooke was unable to see that there were other internal components to the cells he was observing. Therefore, he did not think the "cellulae" were alive.[9] His cell observations gave no indication of the nucleus and other organelles found in most living cells. In Micrographia, Hooke also observed mould, bluish in color, found on leather. After studying it under his microscope, he was unable to observe “seeds” that would have indicated how the mould was multiplying in quantity. This led to Hooke suggesting that spontaneous generation, from either natural or artificial heat, was the cause. Since this was an old Aristotelian theory still accepted at the time, others did not reject it and was not disproved until Leeuwenhoek later discovers generation is achieved otherwise.[6]

Anton van Leeuwenhoek is another scientist who saw these cells soon after Hooke did. He made use of a microscope containing improved lenses that could magnify objects almost 300-fold, or 270x.[9] Under these microscopes, Leeuwenhoek found motile objects. In a letter to The Royal Society on October 9, 1676, he states that motility is a quality of life therefore these were living organisms. Over time, he wrote many more papers in which described many specific forms of microorganisms. Leeuwenhoek named these “animalcules,” which included protozoa and other unicellular organisms, like bacteria.[7] Though he did not have much formal education, he was able to identify the first accurate description of red blood cells and discovered bacteria after gaining interest in the sense of taste that resulted in Leeuwenhoek to observe the tongue of an ox, then leading him to study "pepper water" in 1676. He also found for the first time the sperm cells of animals and humans. Once discovering these types of cells, Leeuwenhoek saw that the fertilization process requires the sperm cell to enter the egg cell. This put an end to the previous theory of spontaneous generation. After reading letters by Leeuwenhoek, Hooke was the first to confirm his observations that were thought to be unlikely by other contemporaries.[6]

The cells in animal tissues were observed after plants were because the tissues were so fragile and susceptible to tearing, it was difficult for such thin slices to be prepared for studying. Biologists believed that there was a fundamental unit to life, but were unsure what this was. It would not be until over a hundred years later that this fundamental unit was connected to cellular structure and existence of cells in animals or plants.[10] This conclusion was not made until Henri Dutrochet. Besides stating “the cell is the fundamental element of organization”,[11] Dutrochet also claimed that cells were not just a structural unit, but also a physiological unit.

In 1804, Karl Rudolphi and J.H.F. Link were awarded the prize for "solving the problem of the nature of cells", meaning they were the first to prove that cells had independent cell walls by the Königliche Societät der Wissenschaft (Royal Society of Science), Göttingen.[12] Before, it had been thought that cells shared walls and the fluid passed between them this way.

Cell theory

Schwann Theodore
Theodor Schwann (1810–1882)

Credit for developing cell theory is usually given to two scientists: Theodor Schwann and Matthias Jakob Schleiden.[13] While Rudolf Virchow contributed to the theory, he is not as credited for his attributions toward it. In 1839, Schleiden suggested that every structural part of a plant was made up of cells or the result of cells. He also suggested that cells were made by a crystallization process either within other cells or from the outside.[14] However, this was not an original idea of Schleiden. He claimed this theory as his own, though Barthelemy Dumortier had stated it years before him. This crystallization process is no longer accepted with modern cell theory. In 1839, Theodor Schwann states that along with plants, animals are composed of cells or the product of cells in their structures.[15] This was a major advancement in the field of biology since little was known about animal structure up to this point compared to plants. From these conclusions about plants and animals, two of the three tenets of cell theory were postulated.[10]

1. All living organisms are composed of one or more cells

2. The cell is the most basic unit of life

Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker.[7] In 1855, Rudolf Virchow added the third tenet to cell theory. In Latin, this tenet states Omnis cellula e cellula.[10] This translated to:

3. All cells arise only from pre-existing cells

However, the idea that all cells come from pre-existing cells had in fact already been proposed by Robert Remak; it has been suggested that Virchow plagiarized Remak and did not give him credit.[16] Remak published observations in 1852 on cell division, claiming Schleiden and Schawnn were incorrect about generation schemes. He instead said that binary fission, which was first introduced by Dumortier, was how reproduction of new animal cells were made. Once this tenet was added, the classical cell theory was complete.

Modern interpretation

The generally accepted parts of modern cell theory include:

  1. All known living things are made up of one or more cells[17]
  2. All living cells arise from pre-existing cells by division.
  3. The cell is the fundamental unit of structure and function in all living organisms.[18]
  4. The activity of an organism depends on the total activity of independent cells.
  5. Energy flow (metabolism and biochemistry) occurs within cells.[19]
  6. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm.[20]
  7. All cells are basically the same in chemical composition in organisms of similar species.[19]

Modern version

The modern version of the cell theory includes the ideas that:

  • Energy flow occurs within cells.[19]
  • Heredity information (DNA) is passed on from cell to cell.[19]
  • All cells have the same basic chemical composition.[19]

Opposing concepts in cell theory: history and background

The cell was first discovered by Robert Hooke in 1665 using a microscope. The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s. In this theory the internal contents of cells were called protoplasm and described as a jelly-like substance, sometimes called living jelly. At about the same time, colloidal chemistry began its development, and the concepts of bound water emerged. A colloid being something between a solution and a suspension, where Brownian motion is sufficient to prevent sedimentation. The idea of a semipermeable membrane, a barrier that is permeable to solvent but impermeable to solute molecules was developed at about the same time. The term osmosis originated in 1827 and its importance to physiological phenomena realized, but it wasn’t until 1877, when the botanist Pfeffer proposed the membrane theory of cell physiology. In this view, the cell was seen to be enclosed by a thin surface, the plasma membrane, and cell water and solutes such as a potassium ion existed in a physical state like that of a dilute solution. In 1889 Hamburger used hemolysis of erythrocytes to determine the permeability of various solutes. By measuring the time required for the cells to swell past their elastic limit, the rate at which solutes entered the cells could be estimated by the accompanying change in cell volume. He also found that there was an apparent nonsolvent volume of about 50% in red blood cells and later showed that this includes water of hydration in addition to the protein and other nonsolvent components of the cells.

Evolution of the membrane and bulk phase theories

Two opposing concepts developed within the context of studies on osmosis, permeability, and electrical properties of cells.[21] The first held that these properties all belonged to the plasma membrane whereas the other predominant view was that the protoplasm was responsible for these properties. The membrane theory developed as a succession of ad-hoc additions and changes to the theory to overcome experimental hurdles. Overton (a distant cousin of Charles Darwin) first proposed the concept of a lipid (oil) plasma membrane in 1899. The major weakness of the lipid membrane was the lack of an explanation of the high permeability to water, so Nathansohn (1904) proposed the mosaic theory. In this view, the membrane is not a pure lipid layer, but a mosaic of areas with lipid and areas with semipermeable gel. Ruhland refined the mosaic theory to include pores to allow additional passage of small molecules. Since membranes are generally less permeable to anions, Leonor Michaelis concluded that ions are adsorbed to the walls of the pores, changing the permeability of the pores to ions by electrostatic repulsion. Michaelis demonstrated the membrane potential (1926) and proposed that it was related to the distribution of ions across the membrane.[22] Harvey and Danielli (1939) proposed a lipid bilayer membrane covered on each side with a layer of protein to account for measurements of surface tension. In 1941 Boyle & Conway showed that the membrane of frog muscle was permeable to both K+
and Cl
, but apparently not to Na+
, so the idea of electrical charges in the pores was unnecessary since a single critical pore size would explain the permeability to K+
, H+
, and Cl
as well as the impermeability to Na+
, Ca+
, and Mg2+
. Over the same time period, it was shown (Procter & Wilson, 1916) that gels, which do not have a semipermeable membrane, would swell in dilute solutions. Loeb (1920) also studied gelatin extensively, with and without a membrane, showing that more of the properties attributed to the plasma membrane could be duplicated in gels without a membrane. In particular, he found that an electrical potential difference between the gelatin and the outside medium could be developed, based on the H+
concentration. Some criticisms of the membrane theory developed in the 1930s, based on observations such as the ability of some cells to swell and increase their surface area by a factor of 1000. A lipid layer cannot stretch to that extent without becoming a patchwork (thereby losing its barrier properties. Such criticisms stimulated continued studies on protoplasm as the principal agent determining cell permeability properties. In 1938, Fischer and Suer proposed that water in the protoplasm is not free but in a chemically combined form—the protoplasm represents a combination of protein, salt and water—and demonstrated the basic similarity between swelling in living tissues and the swelling of gelatin and fibrin gels. Dimitri Nasonov (1944) viewed proteins as the central components responsible for many properties of the cell, including electrical properties. By the 1940s, the bulk phase theories were not as well developed as the membrane theories. In 1941, Brooks & Brooks published a monograph, "The Permeability of Living Cells", which rejects the bulk phase theories.

Emergence of the steady-state membrane pump concept

With the development of radioactive tracers, it was shown that cells are not impermeable to Na+
. This was difficult to explain with the membrane barrier theory, so the sodium pump was proposed to continually remove Na+
as it permeates cells. This drove the concept that cells are in a state of dynamic equilibrium, constantly using energy to maintain ion gradients. In 1935, Karl Lohmann discovered ATP and its role as a source of energy for cells, so the concept of a metabolically-driven sodium pump was proposed. The tremendous success of Hodgkin, Huxley, and Katz in the development of the membrane theory of cellular membrane potentials, with differential equations that modeled the phenomena correctly, provided even more support for the membrane pump hypothesis.

The modern view of the plasma membrane is of a fluid lipid bilayer that has protein components embedded within it. The structure of the membrane is now known in great detail, including 3D models of many of the hundreds of different proteins that are bound to the membrane. These major developments in cell physiology placed the membrane theory in a position of dominance and stimulated the imagination of most physiologists, who now apparently accept the theory as fact—there are, however, a few dissenters.[citation needed]

The reemergence of the bulk phase theories

In 1956, Afanasy S. Troshin published a book, The Problems of Cell Permeability, in Russian (1958 in German, 1961 in Chinese, 1966 in English) in which he found that permeability was of secondary importance in determination of the patterns of equilibrium between the cell and its environment. Troshin showed that cell water decreased in solutions of galactose or urea although these compounds did slowly permeate cells. Since the membrane theory requires an impermanent solute to sustain cell shrinkage, these experiments cast doubt on the theory. Others questioned whether the cell has enough energy to sustain the sodium/potassium pump. Such questions became even more urgent as dozens of new metabolic pumps were added as new chemical gradients were discovered.

In 1962, Gilbert Ling became the champion of the bulk phase theories and proposed his association-induction hypothesis of living cells.

Types of cells

Endomembrane system diagram en (edit)
Eukaryote cell.

Cells can be subdivided into the following subcategories:

  1. Prokaryotes: Prokaryotes are relatively small cells surrounded by the plasma membrane, with a characteristic cell wall that may differ in composition depending on the particular organism.[23] Prokaryotes lack a nucleus (although they do have circular or linear DNA) and other membrane-bound organelles (though they do contain ribosomes). The protoplasm of a prokaryote contains the chromosomal region that appears as fibrous deposits under the microscope, and the cytoplasm.[23] Bacteria and Archaea are the two domains of prokaryotes.
  2. Eukaryotes: Eukaryotic cells are also surrounded by the plasma membrane, but on the other hand, they have distinct nuclei bound by a nuclear membrane or envelope. Eukaryotic cells also contain membrane-bound organelles, such as (mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, vacuoles).[24] In addition, they possess organized chromosomes which store genetic material.

Animals have evolved a greater diversity of cell types in a multicellular body (100–150 different cell types), compared with 10–20 in plants, fungi, and protoctista.[25]

See also

References

  1. ^ Villarreal, Luis P. (August 8, 2008) Are Viruses Alive? Scientific American
  2. ^ "A glass-sphere microscope". Funsci.com. Archived from the original on 11 June 2010. Retrieved 13 June 2010.
  3. ^ Atti Della Fondazione Giorgio Ronchi E Contributi Dell'Istituto Nazionale Di Ottica, Volume 30, La Fondazione-1975, page 554
  4. ^ Albert Van Helden; Sven Dupré; Rob van Gent (2010). The Origins of the Telescope. Amsterdam University Press. p. 24. ISBN 978-90-6984-615-6.
  5. ^ William Rosenthal, Spectacles and Other Vision Aids: A History and Guide to Collecting, Norman Publishing, 1996, page 391 - 392
  6. ^ a b c d Gest, H (2004). "The discovery of microorganisms by Robert Hooke and Antoni Van Leeuwenhoek, fellows of the Royal Society". Notes and Records of the Royal Society of London. 58 (2): 187–201. doi:10.1098/rsnr.2004.0055. PMID 15209075.
  7. ^ a b c d Mazzarello, P. (1999). "A unifying concept: the history of cell theory". Nature Cell Biology. 1 (1): E13–5. doi:10.1038/8964. PMID 10559875. Archived from the original on 2015-06-03.
  8. ^ Inwood, Stephen (2003). The man who knew too much: the strange and inventive life of Robert Hooke, 1635–1703. London: Pan. p. 72. ISBN 0-330-48829-5.
  9. ^ a b Becker, Wayne M.; Kleinsmith, Lewis J.; Hardin, Jeff (2003). The World of the Cell. Benjamin/Cummings Publishing Company. p. 1. ISBN 978-0-8053-4854-5.
  10. ^ a b c Robinson, Richard. "History of Biology: Cell Theory and Cell Structure". Advameg, Inc. Retrieved 17 March 2014.
  11. ^ Dutrochet, Henri (1824) "Recherches anatomiques et physiologiques sur la structure intime des animaux et des vegetaux, et sur leur motilite, par M.H. Dutrochet, avec deux planches"
  12. ^ Kalenderblatt Dezember 2013 – Mathematisch-Naturwissenschaftliche Fakultät – Universität Rostock. Mathnat.uni-rostock.de (2013-11-28). Retrieved on 2015-10-15.
  13. ^ Sharp, L. W. (1921). Introduction To Cytology. New York: McGraw Hill Book Company Inc.
  14. ^ Schleiden, M. J. (1839). "Beiträge zur Phytogenesis". Archiv für Anatomie, Physiologie und wissenschaftliche Medicin: 137–176.
  15. ^ Schwann, T. (1839). Mikroskopische Untersuchungen über die Uebereinstimmung in der Struktur und dem Wachsthum der Thiere und Pflanzen. Berlin: Sander.
  16. ^ Silver, GA (1987). "Virchow, the heroic model in medicine: health policy by accolade". American Journal of Public Health. 77 (1): 82–8. doi:10.2105/AJPH.77.1.82. PMC 1646803. PMID 3538915.
  17. ^ Wolfe
  18. ^ Wolfe, p. 5
  19. ^ a b c d e "The modern version of the Cell Theory". Retrieved 12 February 2015.
  20. ^ Wolfe, p. 8
  21. ^ Ling, Gilbert N. (1984). In search of the physical basis of life. New York: Plenum Press. ISBN 0306414090.
  22. ^ Michaelis, L. (1925). "Contribution to the Theory of Permeability of Membranes for Electrolytes". The Journal of General Physiology. 8 (2): 33–59. doi:10.1085/jgp.8.2.33. PMC 2140746. PMID 19872189.
  23. ^ a b Wolfe, p. 11
  24. ^ Wolfe, p. 13
  25. ^ Margulis, L. & Chapman, M.J. (2009). Kingdoms and Domains: An Illustrated Guide to the Phyla of Life on Earth ([4th ed.]. ed.). Amsterdam: Academic Press/Elsevier. p. 116.

Bibliography

Further reading

External links

Antidromic

An antidromic impulse in an axon refers to conduction opposite of the normal (orthodromic) direction. That is, it refers to conduction along the axon away from the axon terminal(s) and towards the soma. For most neurons, their dendrites, soma, or axons are depolarized forming an action potential that moves from the starting point of the depolarization (near the cell body) along the axons of the neuron (orthodromic). Antidromic activation is often induced experimentally by direct electrical stimulation of a presumed target structure. Antidromic ( cell theory) activation is often used in a laboratory setting to confirm that a neuron being recorded from projects to the structure of interest.

Antoine Béchamp

Pierre Jacques Antoine Béchamp (October 16, 1816 – April 15, 1908) was a French scientist now best known for breakthroughs in applied organic chemistry and for a bitter rivalry with Louis Pasteur. Béchamp developed the Béchamp reduction, an inexpensive method to produce aniline dye, permitting Perkin to launch the synthetic-dye industry. Béchamp also synthesized the first organic arsenical drug, arsanilic acid, from which Ehrlich later synthesized the first chemotherapeutic drug. Béchamp's rivalry with Pasteur was initially for priority in attributing fermentation to microorganisms, later for attributing the silkworm disease pebrine to microorganisms, and eventually over the validity of germ theory. Béchamp also disputed cell theory.

Claiming discovery that the "molecular granulations" in biological fluids were actually the elementary units of life, Béchamp named them microzymas—that is, "tiny enzymes"—and credited them with producing enzymes and were the builders of cells while "evolving" amid favorable conditions into bacteria. Denying that bacteria could invade a healthy animal and cause disease, Béchamp claimed instead that unfavorable host and environmental conditions destabilize the host's native microzymas, whereupon they decompose host tissue by producing pathogenic bacteria. While cell theory and germ theory gained widespread acceptance, granular theories became obscure. Béchamp's version, microzymian theory, has been retained by small groups, especially in alternative medicine and those competent in the use of darkfield microscopy.

Biogenesis

For the origin of life, see Abiogenesis.Biogenesis is the production of new living organisms or organelles. Conceptually, biogenesis is primarily attributed to Louis Pasteur and encompasses the belief that complex living things come only from other living things, by means of reproduction. That is, life does not spontaneously arise from non-living material, which was the position held by spontaneous generation. This is summarized in the phrase Omne vivum ex vivo, Latin for "all life [is] from life." A related statement is Omnis cellula e cellula, "all cells [are] from cells;" this conclusion is one of the central statements of cell theory.

Biology

Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution. Despite the complexity of the science, there are certain unifying concepts that consolidate it into a single, coherent field. Biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the creation and extinction of species. Living organisms are open systems that survive by transforming energy and decreasing their local entropy to maintain a stable and vital condition defined as homeostasis.Sub-disciplines of biology are defined by the research methods employed and the kind of system studied: theoretical biology uses mathematical methods to formulate quantitative models while experimental biology performs empirical experiments to test the validity of proposed theories and understand the mechanisms underlying life and how it appeared and evolved from non-living matter about 4 billion years ago through a gradual increase in the complexity of the system. See branches of biology.

Biomolecule

A biomolecule or biological molecule is a loosely used term for molecules and ions that are present in organisms, essential to some typically biological process such as cell division, morphogenesis, or development. Biomolecules include large macromolecules (or polyanions) such as proteins, carbohydrates, lipids, and nucleic acids, as well as small molecules such as primary metabolites, secondary metabolites, and natural products. A more general name for this class of material is biological materials. Biomolecules are usually endogenous but may also be exogenous. For example, pharmaceutical drugs may be natural products or semisynthetic (biopharmaceuticals) or they may be totally synthetic.

Biology and its subsets of biochemistry and molecular biology study biomolecules and their reactions. Most biomolecules are organic compounds, and just four elements—oxygen, carbon, hydrogen, and nitrogen—make up 96% of the human body's mass. But many other elements, such as the various biometals, are present in small amounts.

The uniformity of specific types of molecules (the biomolecules) and of some metabolic pathways as invariant features between the diversity of life forms is called "biochemical universals" or "theory of material unity of the living beings", a unifying concept in biology, along with cell theory and evolution theory.

Cancer stem cell

Cancer stem cells (CSCs) are cancer cells (found within tumors or hematological cancers) that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found in a particular cancer sample. CSCs are therefore tumorigenic (tumor-forming), perhaps in contrast to other non-tumorigenic cancer cells. CSCs may generate tumors through the stem cell processes of self-renewal and differentiation into multiple cell types. Such cells are hypothesized to persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumors. Therefore, development of specific therapies targeted at CSCs holds hope for improvement of survival and quality of life of cancer patients, especially for patients with metastatic disease.

Existing cancer treatments have mostly been developed based on animal models, where therapies able to promote tumor shrinkage were deemed effective. However, animals do not provide a complete model of human disease. In particular, in mice, whose life spans do not exceed two years, tumor relapse is difficult to study.

The efficacy of cancer treatments is, in the initial stages of testing, often measured by the ablation fraction of tumor mass (fractional kill). As CSCs form a small proportion of the tumor, this may not necessarily select for drugs that act specifically on the stem cells. The theory suggests that conventional chemotherapies kill differentiated or differentiating cells, which form the bulk of the tumor but do not generate new cells. A population of CSCs, which gave rise to it, could remain untouched and cause relapse.

Cancer stem cells were first identified by John Dick in acute myeloid leukemia in the late 1990s. Since the early 2000s they have been an intense cancer research focus.

Cell (biology)

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. 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 (1013) cells. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometres.Cells were discovered by Robert Hooke in 1665, who named them for their 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, and that all cells come from pre-existing cells. Cells emerged on Earth at least 3.5 billion years ago.

Cell biology

Cell biology (also called cytology, from the Greek κύτος, kytos, "vessel") is a branch of biology that studies the structure and function of the cell, which is the basic unit of life. Cell biology is concerned with the physiological properties, metabolic processes, signaling pathways, life cycle, chemical composition and interactions of the cell with their environment. This is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells. Knowing the components of cells and how cells work is fundamental to all biological sciences; it is also essential for research in bio-medical fields such as cancer, and other diseases. Research in cell biology is closely related to genetics, biochemistry, molecular biology, immunology and cytochemistry .

History of biology

The history of biology traces the study of the living world from ancient to modern times. Although the concept of biology as a single coherent field arose in the 19th century, the biological sciences emerged from traditions of medicine and natural history reaching back to ayurveda, ancient Egyptian medicine and the works of Aristotle and Galen in the ancient Greco-Roman world. This ancient work was further developed in the Middle Ages by Muslim physicians and scholars such as Avicenna. During the European Renaissance and early modern period, biological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were Vesalius and Harvey, who used experimentation and careful observation in physiology, and naturalists such as Linnaeus and Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms. Antonie van Leeuwenhoek revealed by means of microscopy the previously unknown world of microorganisms, laying the groundwork for cell theory. The growing importance of natural theology, partly a response to the rise of mechanical philosophy, encouraged the growth of natural history (although it entrenched the argument from design).

Over the 18th and 19th centuries, biological sciences such as botany and zoology became increasingly professional scientific disciplines. Lavoisier and other physical scientists began to connect the animate and inanimate worlds through physics and chemistry. Explorer-naturalists such as Alexander von Humboldt investigated the interaction between organisms and their environment, and the ways this relationship depends on geography—laying the foundations for biogeography, ecology and ethology. Naturalists began to reject essentialism and consider the importance of extinction and the mutability of species. Cell theory provided a new perspective on the fundamental basis of life. These developments, as well as the results from embryology and paleontology, were synthesized in Charles Darwin's theory of evolution by natural selection. The end of the 19th century saw the fall of spontaneous generation and the rise of the germ theory of disease, though the mechanism of inheritance remained a mystery.

In the early 20th century, the rediscovery of Mendel's work led to the rapid development of genetics by Thomas Hunt Morgan and his students, and by the 1930s the combination of population genetics and natural selection in the "neo-Darwinian synthesis". New disciplines developed rapidly, especially after Watson and Crick proposed the structure of DNA. Following the establishment of the Central Dogma and the cracking of the genetic code, biology was largely split between organismal biology—the fields that deal with whole organisms and groups of organisms—and the fields related to cellular and molecular biology. By the late 20th century, new fields like genomics and proteomics were reversing this trend, with organismal biologists using molecular techniques, and molecular and cell biologists investigating the interplay between genes and the environment, as well as the genetics of natural populations of organisms.

History of zoology through 1859

The history of zoology before Charles Darwin's 1859 theory of evolution traces the organized study of the animal kingdom from ancient to modern times. Although the concept of zoology as a single coherent field arose much later, systematic study of zoology is seen in the works of Aristotle and Galen in the ancient Greco-Roman world. This work was developed in the Middle Ages by Islamic medicine and scholarship, and in turn their work was extended by European scholars such as Albertus Magnus.

During the European Renaissance and early modern period, zoological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were the anatomist Vesalius and the physiologist William Harvey, who used experimentation and careful observation, and naturalists such as Carl Linnaeus and Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms. Microscopy revealed the previously unknown world of microorganisms, laying the groundwork for cell theory. The growing importance of natural theology, partly a response to the rise of mechanical philosophy, encouraged the growth of natural history (although it entrenched the argument from design).

Over the 18th and 19th centuries, zoology became increasingly professional scientific disciplines. Explorer-naturalists such as Alexander von Humboldt investigated the interaction between organisms and their environment, and the ways this relationship depends on geography—laying the foundations for biogeography, ecology and ethology. Naturalists began to reject essentialism and consider the importance of extinction and the mutability of species. Cell theory provided a new perspective on the fundamental basis of life. These developments, as well as the results from embryology and paleontology, were synthesized in Charles Darwin's theory of evolution by natural selection. In 1859, Darwin placed the theory of organic evolution on a new footing, by his discovery of a process by which organic evolution can occur, and provided observational evidence that it had done so.

John Goodsir

Dr John Goodsir (20 March 1814 – 6 March 1867) was a Scottish anatomist. He was a pioneer in the formulation of cell theory.

Matthias Jakob Schleiden

Matthias Jakob Schleiden (German pronunciation: [maˈtiːas ˈjaːkɔp ˈʃlaɪ̯dn̩]; 5 April 1804 – 23 June 1881) was a German botanist and co-founder of cell theory, along with Theodor Schwann and Rudolf Virchow.

Max Schultze

Max Johann Sigismund Schultze (25 March 1825 – 16 January 1874) was a German microscopic anatomist noted for his work on cell theory.

Neuron doctrine

The neuron doctrine is the concept that the nervous system is made up of discrete individual cells, a discovery due to decisive neuro-anatomical work of Santiago Ramón y Cajal and later presented by, among others, H. Waldeyer-Hartz. The term neuron (spelled neurone in British English) was itself coined by Waldeyer as a way of identifying the cells in question. The neuron doctrine, as it became known, served to position neurons as special cases under the broader cell theory evolved some decades earlier. He appropriated the concept not from his own research but from the disparate observation of the histological work of Albert von Kölliker, Camillo Golgi, Franz Nissl, Santiago Ramón y Cajal, Auguste Forel and others.

Preformationism

In the history of biology, preformationism (or preformism) is a formerly-popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development. It suggests that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation.

Epigenesis (or neoformism), then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to "strict" preformationism, it is the notion that "each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added" (Magner 2002, p. 154). This word is still used, on the other hand, in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words, epigenetic.

Furthermore, apart from those distinctions (preformationism-epigenesis and genetic-epigenetic), the terms preformistic development, epigenetic development and somatic embryogenesis are also used in another context, in relation to the differentiation of a distinct germ cell line. In preformistic development, the germ line is present since early development. In epigenetic development, the germ line is present, but it appears late. In somatic embryogenesis, a distinct germ line is lacking. Some authors call Weismannist development (either preformistic or epigenetic) that in which there is a distinct germ line.The historical ideas of preformationism and epigenesis, and the rivalry between them, are obviated by our contemporary understanding of the genetic code and its molecular basis together with developmental biology and epigenetics.

Rudolf Virchow

Rudolf Ludwig Carl Virchow (; German: [ˈfɪɐ̯ço] or German: [ˈvɪɐ̯ço]; 13 October 1821 – 5 September 1902) was a German physician, anthropologist, pathologist, prehistorian, biologist, writer, editor, and politician. He is known as "the father of modern pathology" and as the founder of social medicine and zoology, and to his colleagues, the "Pope of medicine". He received the Copley Medal in 1892. He was a foreign member of the Royal Swedish Academy of Sciences and was elected to the Prussian Academy of Sciences, but he declined to be ennobled as "von Virchow".

Virchow studied medicine at the Friedrich-Wilhelms Institute under Johannes Peter Müller. He worked at the Charité hospital under Robert Froriep, whom he succeeded as the prosector. His investigation of the 1847–1848 typhus epidemic in Upper Silesia laid the foundation for public health in Germany, and paved his political and social careers. From it, he coined a well known aphorism: "Medicine is a social science, and politics is nothing else but medicine on a large scale". He participated in the Revolution of 1848, which led to his expulsion from Charité the next year. He then published a newspaper Die medicinische Reform (Medical Reform). He took the first Chair of Pathological Anatomy at the University of Würzburg in 1849. After five years, Charité reinstated him to its new Institute for Pathology. He cofounded the political party Deutsche Fortschrittspartei, and was elected to the Prussian House of Representatives, and won a seat in the Reichstag. His opposition to Otto von Bismarck's financial policy resulted in an anecdotal "Sausage Duel", although he supported Bismarck in his anti-Catholic campaigns, which he named Kulturkampf ("culture struggle").A prolific writer, his scientific writings alone exceeded 2,000. Cellular Pathology (1858), regarded as the root of modern pathology, introduced the third dictum in cell theory: Omnis cellula e cellula ("All cells come from cells"). He was a co-founder Physikalisch-Medizinische Gesselschaft in 1849 and Deutsche Pathologische Gesellschaft in 1897. He founded journals such as Archiv für pathologische Anatomie und Physiologie und für klinische Medicin (with Benno Reinhardt in 1847, from 1903 under the title Virchows Archiv), and Zeitschrift für Ethnologie (Journal of Ethnology). The latter is published by German Anthropological Association and the Berlin Society for Anthropology, Ethnology and Prehistory, the societies which he also founded.Virchow was the first to describe and christen diseases such as leukemia, chordoma, ochronosis, embolism, and thrombosis. He coined medical names such as "chromatin", "neuroglia", "agenesis", "parenchyma", "osteoid", "amyloid degeneration", and "spina bifida"; terms such as Virchow's node, Virchow–Robin spaces, Virchow–Seckel syndrome, and Virchow's triad are named after him. His description of the life cycle of a roundworm Trichinella spiralis influenced the pratice of meat inspection. He developed the first systematic method of autopsy, and he was the first to use hair analysis in forensic investigation. He was critical of what he described as "Nordic mysticism" regarding the Aryan race. As an anti-evolutionist, he called Charles Darwin an "ignoramus" and his own student Ernst Haeckel a "fool". He described the original specimen of Neanderthal man as nothing but that of a deformed human.

Scientific theory

A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested and verified in accordance with the scientific method, using accepted protocols of observation, measurement, and evaluation of results. Where possible, theories are tested under controlled conditions in an experiment. In circumstances not amenable to experimental testing, theories are evaluated through principles of abductive reasoning. Established scientific theories have withstood rigorous scrutiny and embody scientific knowledge.The meaning of the term scientific theory (often contracted to theory for brevity) as used in the disciplines of science is significantly different from the common vernacular usage of theory. In everyday speech, theory can imply an explanation that represents an unsubstantiated and speculative guess, whereas in science it describes an explanation that has been tested and widely accepted as valid. These different usages are comparable to the opposing usages of prediction in science versus common speech, where it denotes a mere hope.

The strength of a scientific theory is related to the diversity of phenomena it can explain and its simplicity. As additional scientific evidence is gathered, a scientific theory may be modified and ultimately rejected if it cannot be made to fit the new findings; in such circumstances, a more accurate theory is then required. That doesn’t mean that all theories can be fundamentally changed (for example, well established foundational scientific theories such as evolution, heliocentric theory, cell theory, theory of plate tectonics etc). In certain cases, the less-accurate unmodified scientific theory can still be treated as a theory if it is useful (due to its sheer simplicity) as an approximation under specific conditions. A case in point is Newton's laws of motion, which can serve as an approximation to special relativity at velocities that are small relative to the speed of light.

Scientific theories are testable and make falsifiable predictions. They describe the causes of a particular natural phenomenon and are used to explain and predict aspects of the physical universe or specific areas of inquiry (for example, electricity, chemistry, and astronomy). Scientists use theories to further scientific knowledge, as well as to facilitate advances in technology or medicine.

As with other forms of scientific knowledge, scientific theories are both deductive and inductive, aiming for predictive and explanatory power.

The paleontologist Stephen Jay Gould wrote that "...facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts."

Stem cell theory of aging

The stem cell theory of aging postulates that the aging process is the result of the inability of various types of stem cells to continue to replenish the tissues of an organism with functional differentiated cells capable of maintaining that tissue's (or organ's) original function. Damage and error accumulation in genetic material is always a problem for systems regardless of the age. The number of stem cells in young people is very much higher than older people and this cause a better and more efficient replacement mechanism in the young contrary to the old. In other words, aging is not a matter of the increase of damage, but a matter of failure to replace it due to decreased number of stem cells. Stem cells decrease in number and tend to lose the ability to differentiate into progenies or lymphoid lineages and myeloid lineages.

Maintaining the dynamic balance of stem cell pools requires several conditions. Balancing proliferation and quiescence along with homing (See niche) and self-renewal of hematopoietic stem cells are favoring elements of stem cell pool maintenance while differentiation, mobilization and senescence are detrimental elements. These detrimental effects will eventually cause apoptosis.

There are also several challenges when it comes to therapeutic use of stem cells and their ability to replenish organs and tissues. First, different cells may have different lifespans even though they are originated from the same stem cells (See T-cells and erythrocytes), meaning that aging can occur differently in cells that have longer lifespans as opposed to the ones with shorter lifespans. Also, continual effort to replace the somatic cells may cause exhaustion of stem cells.

Theodor Schwann

Theodor Schwann (German pronunciation: [ˈteːodoːɐ̯ ˈʃvan]; 7 December 1810 – 11 January 1882) was a German physician and physiologist. His most significant contribution to biology is considered to be the extension of cell theory to animals. Other contributions include the discovery of Schwann cells in the peripheral nervous system, the discovery and study of pepsin, the discovery of the organic nature of yeast, and the invention of the term metabolism.

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