CELL THEORY, or cell doctrine, states
that all organisms are composed of similar units of organization, called
cells. The concept was formally
articulated in 1839 by Schleiden &
Schwann and has remained as the
foundation of modern biology. The idea predates other great paradigms of
biology including Darwin's
theory of evolution (1859),
Mendel's laws of inheritance (1865), and the
establishment of comparative biochemistry
Formulation of the Cell Theory
Theodor Schwann and
Matthias Schleiden were enjoying after-dinner coffee and talking
about their studies on cells. It has been suggested that when
Schwann heard Schleiden
describe plant cells with nuclei, he was struck by the similarity of these
plant cells to cells he had observed in animal tissues. The two scientists
went immediately to Schwann's lab to look at
his slides. Schwann published his book on
animal and plant cells (Schwann 1839) the next year, a treatise devoid of
acknowledgments of anyone else's contribution, including that of
Schleiden (1838). He summarized his
observations into three conclusions about cells:
We know today that the first two tenets are correct, but the third is clearly wrong. The correct interpretation of cell formation by division was finally promoted by others and formally enunciated in Rudolph Virchow's powerful dictum, "Omnis cellula e cellula"... "All cells only arise from pre-existing cells".
The modern tenets of the Cell Theory include:
1. all known living things are made up of cells.
2. the cell is structural & functional unit of all living things.
3. all cells come from pre-existing cells by division.
(Spontaneous Generation does not occur).
4. cells contains hereditary information which is passed from
cell to cell during cell division.
5. All cells are basically the same in chemical composition.
6. all energy flow (metabolism & biochemistry) of life occurs
any theory, its tenets are based upon previous observations and facts, which
are synthesized into a coherent whole via the scientific method. The
Theory is no different being founded upon the observations of many. (Landmarks
in the Study of Cells)
for the first compound (more than one lens) microscope is usually given to
Zacharias Jansen, of Middleburg, Holland, around the year 1595. Since
was very young at that time, it's possible that his father Hans made the
first one, but young Jansen perfected the production. Details about the
first Jansen microscopes are not clear, but there is some evidence which
allows us to make some guesses about them (Jansen
an English scientist, Robert Hooke, discovered cells in a piece of cork,
which he examined under his primitive microscope (figures).
Actually, Hooke only observed cell walls because cork cells are dead and without
cytoplasmic contents. Hooke drew the cells he saw
and also coined the word CELL. The word cell is derived from the
'cellula' which means small compartment. Hooke published his findings in his famous work,
Physiological Descriptions of Minute Bodies
made by Magnifying Glasses (1665).
Ten years later
Anton van Leeuwenhoek (1632-1723), a Dutch businessman and a contemporary of
Hooke used his own (single lens)
monocular microscopes and was the first person to observe bacteria and
is known to have made over 500 "microscopes," of which fewer than ten have
survived to the present day. In basic design, probably all of Leeuwenhoek's
instruments were simply powerful magnifying glasses, not compound
microscopes of the type used today. Leeuwenhoek's
skill at grinding lenses, together with his naturally acute eyesight and
great care in adjusting the lighting where he worked, enabled him to build
microscopes that magnified over 200 times, with clearer and brighter images
than any of his colleagues at that time. In 1673,
Leeuwenhoek began writing letters to the newly formed
Royal Society of
London, describing what he had seen with his lenses. His first
letter contained some observations on the stings of bees. For the next fifty
years he corresponded with the Royal Society. His observations, written in
Dutch, were translated into English or Latin and printed in the
Transactions of the Royal Society.
Leeuwenhoek looked at animal and plant tissues,
at mineral crystals, and at fossils. He was the first to see microscopic
single celled protists with shells, the foraminifera,
which he described as "little cockles. . . no bigger than a coarse
sand-grain." He discovered blood cells, and was the first to see
living sperm cells of animals. He discovered microscopic animals such as
nematodes (round worms) and
rotifers. The list of his discoveries is long.
Leeuwenhoek soon became famous as his letters were published and
translated. In 1680 he was elected a full member of the Royal Society.
After his death on August 30, 1723, a member of the Royal Society wrote...
"Antony van Leeuwenhoek considered that what is
true in natural philosophy can be most fruitfully investigated by the
experimental method, supported by the evidence of the senses; for which
reason, by diligence and tireless labour he made with his own hand certain
most excellent lenses, with the aid of which he discovered many secrets of
Nature, now famous throughout the whole philosophical World". No
truer definition of the scientific method may be found.
Between 1680 and the early 1800's it appears that not much was accomplished
in the study of cell structure. This may be due to the lack of quality lens
for microscopes and the dedication to spend long hours of detailed
observation over what microscopes existed at that time. Leeuwenhoek did not
record his methodology for grinding quality lenses and thus microscopy
suffered for over 100 years.
natur-philosopher and microscopist, Lorenz Oken
had been trained in medicine at Freiburg University. He went on to become a
renown philosopher and thinker of the 19th century. It is reported
that in 1805 Oken stated that "All
living organisms originate from and consist of cells"... which may have
been the first statement of a cell theory.
Around 1833 Robert Brown reported the discovery
of the nucleus. Brown was a naturalist who
visited the "colonies of Australia" from 1801 through 1805, where he
cataloged and described over 1,700 new species of plants.
Brown was an accomplished technician and an
extraordinarily gifted observer of microscopic phenomena. It was
Brown who identified the naked ovule in the
gymnospermae. This is a difficult observation to make even with a modern
instrument and the benefit of hindsight. But it was with the observation of
the incessant agitation of minute suspended particles that
Brown's name became inextricably linked. The
effect, since described as Brownian Movement,
was first noticed by him in 1827. Having worked on the ovum, it was
natural to direct attention to the structure of pollen and its
Brown interrelationship with the pistil.
In the course of his microscopic studies of the epidermis of orchids,
discovered in these cells "an opaque spot," which he named the
Doubtless the same "spot" had been seen often enough before by other
observers, but Brown was the first to recognize
it as a component part of the vegetable cell and to give it a name. This
nucleus (or areola as he called it) of the cell, was not confined to the
epidermis, being also found, in the pubescence of the surface and in the
parenchyma or internal cells of the tissue. This nucleus of the cell was not
confined to only orchids, but was equally manifest in many other
monocotyledonous families and in the epidermis of dicotyledonous plants, and
even in the early stages of development of the pollen. In some plants, as
Tradascantia virginica, it was uncommonly distinct, especially in the tissue
of the stigma, in the cells of the ovum, even before impregnation, and in
all the stages of formation of the grains of pollen.
"Mikroskopische Untersuchungen über die
Übereinstimmung in der Struktur und dem Wachstum der Thiere und Pflanzen"
For the first 150 years, the cell theory was primarily a structural idea. This structural view, which is found in most
textbooks, describes the components of a cell and their fate in cell
reproduction. Since the 1950's, however, cell biology has focused on DNA and its informational
features. Today we look at the cell as a unit of self-control. ie., the description of a cell
must includes ideas about how
genetic information is converted to structure.
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Last Update - 11 Feb 2008
Landmarks in Study of Cell Biology
|1595||Jansen credited with 1st compound microscope SCROLL DOWN|
|1626||Redi postulated that living things do not arise from spontaneous generation.|
|1655||Hooke described 'cells' in cork.|
|1674||Leeuwenhoek discovered protozoa. He saw bacteria some 9 years later.|
|1833||Brown descibed the cell nucleus in cells of the orchid.|
|1838||Schleiden and Schwann proposed cell theory.|
|1840||Albrecht von Roelliker realized that sperm cells and egg cells are also cells.|
|1856||N. Pringsheim observed how a sperm cell penetrated an egg cell.|
|1858||Rudolf Virchow (physician, pathologist and anthropologist) expounds his famous conclusion: omnis cellula e cellula , that is cells develop only from existing cells [cells come from preexisting cells]|
|1857||Kolliker described mitochondria.|
|1869||Miescher isolated DNA for the first time.|
|1879||Flemming described chromosome behavior during mitosis.|
|1883||Germ cells are haploid, chromosome theory of heredity.|
|1898||Golgi described the golgi apparatus.|
|1926||Svedberg developed the first analytical ultracentrifuge.|
|1938||Behrens used differential centrifugation to separate nuclei from cytoplasm.|
|1939||Siemens produced the first commercial transmission electron microscope.|
|1941||Coons used fluorescent labeled antibodies to detect cellular antigens.|
|1952||Gey and co-workers established a continuous human cell line.|
|1953||Crick, Wilkins and Watson proposed structure of DNA double-helix.|
|1955||Eagle systematically defined the nutritional needs of animal cells in culture.|
|1957||Meselson, Stahl and Vinograd developed density gradient centrifugation in cesium chloride solutions for separating nucleic acids.|
|1965||Ham introduced a defined serum-free medium. Cambridge Instruments produced the first commercial scanning electron microscope.|
|1976||Sato and colleagues publish papers showing that different cell lines require different mixtures of hormones and growth factors in serum-free media.|
|1981||Transgenic mice and fruit flies are produced. Mouse embryonic stem cell line established.|
|1987||First knockout mouse created.|
|1998||Mice are cloned from somatic cells.|
|2000||Human genome DNA sequence draft. BACK to text|
Prior to 1931 when the first
electron microscope (Fig. 1.9) was developed,
magnification of microscopes was limited to about 2 000 times. The small cell
structures did not show up well or remained invisible. The electron microscope
not only showed more detail of previously known parts of the cell but also
revealed new parts. Cells and cell structures can now be examined at
magnifications of up to 500 000 times and more.