Discovery of Cell
The 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 (1940).
First Cells Seen in Cork
While the invention
of the telescope made the Cosmos accessible to human observation, the microsope
opened up smaller worlds, showing what living forms were composed of. The cell
was first discovered and named by Robert Hooke in 1665. He remarked
that it looked strangely similar to cellula or small rooms which monks
inhabited, thus deriving the name. However what Hooke actually saw was the dead
cell walls of plant cells (cork) as it appeared under the microscope. Hooke’s
description of these cells was published in Micrographia. The cell walls observed by Hooke gave no indication of
the nucleus and other organelles found in most living cells. The first man to
witness a live cell under a microscope was Anton won Leeuwenhock, who in
1674 described the algae Spirogyra. Van Leeuwenhoek probably also saw bacteria.
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| Microscopic view of cork |
Formulation of the Cell Theory
In 1838, 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:
1.
The cell is the
unit of structure, physiology, and organization in living things.
2.
The cell retains a
dual existence as a distinct entity and a building block in the construction of
organisms.
3.
Cells form by
free-cell formation, similar to the formation of crystals (spontaneous
generation).
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”.
Modern Cell Theory
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 within cells.
As with the rapid
growth in molecular biology in th mid 20th centuary, cell biology research
exploded in the 1950’s. It became possible to maintain, grow, and manipulate
cells outside of living organisms. The first continuous cell line to be so
cultured was in 1951 by George Otto Gey and coworkers, derived from cervical
cancer cells taken from Henrietta Lacks, who died from her cancer in 1951. The
cell line, which was eventually referred to as Hela cells, have been the
watershed in studying cell biology in the way that the structure of DNA was the
significant breakthrough of molecular biology.
In an avalanche of
progress in the study of cells, the coming decade included the characterization
of the minimal media requirements for cells and development of sterile cell
culture techniques. It was also aided by the prior advances in electron
microscopy, and later advances such as development of transfection methods,
discovery of green fluorescent protein in jellyfish, and discovery of small
interfering RNA (siRNA), among others.
A Timeline Struggles
1595 – Jansen
credited with 1st compound microscope
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.
1879 – Flemming described chromosome behavior during
mitosis.
1883 – Germ cells are haploid, chromosome theory of
heredity.
1898 – Golgi described the golgi apparatus.
1938 – Behrens used differential centrifugation to
separate nuclei from cytoplasm.
1939 – Siemens produced the first commercial transmission
electron microscope.
1952 – Gey and coworkers established a continuous human
cell line.
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.
1995 – Tsien identifies mutant of GFP with enhanced
spectral properties
1998 – Mice are cloned from somatic cells.
1999 – Hamilton and Baulcombe discover siRNA as part of
post-transcriptional gene silencing (PTGS) in plants.
