After the first observations of life under the microscope, it took two
centuries of research before the 'cell theory', the idea that all living things
are composed of cells or their products, was formulated. It proved even harder
to accept that individual cells also make up nervous tissue.
With the invention of the microscope at the beginning of the seventeenth
century, it became possible to take a first glimpse at the previously invisible
world of microscopic life. A bewildering array of new structures appeared
before the astonished eyes of the first microscopists. The Jesuit priest Athanasius
Kircher (1601−1680) showed, in 1658, that maggots and other living creatures
developed in decaying tissues. In the same period, oval red-blood corpuscles
were described by the Dutch naturalist Jan Swammerdam (1637−1680), who
also discovered that a frog embryo consists of globular particles1,
2.
Another new world of extraordinary variety, that of microorganisms, was
revealed by the exciting investigations of another Dutchman, Antoni van Leeuwenhoek
(1632−1723). The particles that he saw under his microscope were motile
and, assuming that motility equates to life, he went on to conclude, in a
letter of 9 October 1676 to the Royal Society, that these particles were indeed
living organisms. In a long series of papers van Leeuwenhoek then described
many specific forms of these microorganisms (which he called "animalcules"),
including protozoa and other unicellular organisms3,
4,
5.
Under the microscope: drawings of the instruments used by Robert Hooke
(left) and the cellular structure of cork according to Hooke (right) (reproduced
from Micrographia, 1665).
But the first description of the cell is generally attributed to Robert
Hooke (1635−1702), an English physicist who was also a distinguished
microscopist (below). In 1665 Hooke published
Micrographia, the first important work devoted to microscopical observation,
and showed what the microscope could mean for naturalists. He described the
microscopic units that made up the structure of a slice of cork and coined
the term "cells" or "pores" to refer to these units. Cella is a Latin
word meaning 'a small room' and Latin-speaking people applied the word
Cellulae to the six-sided cells of the honeycomb. By analogy, Hooke applied
the term "cells" to the thickened walls of the dead cells of the cork. Although
Hooke used the word differently to later cytologists (he thought of the cork
cells as passages for fluids involved in plant growth), the modern term 'cell'
comes directly from his book6.
Bridge between life and 'non-life'? The existence of an entire world of microscopic living beings was seen
as a bridge between inanimate matter and living organisms that are visible
to the naked eye7. This seemed to support the old aristotelian
doctrine of 'spontaneous generation', according to which water or land bears
the potential to generate, 'spontaneously', different kinds of organism. This
theory, which implied a continuity between living and non-living matter,
natura non facit saltus, was disproved by the masterful experiments of
the Italian naturalist Lazzaro Spallanzani (1729−1799)8.
He and other researchers showed that an organism derives from another organism(s)
and that a gap exists between inanimate matter and life. (But it was a century
later before the idea of spontaneous generation was definitively refuted,
by Louis Pasteur, 1822−1895; ref. 9.) As
a consequence, the search for the first elementary steps in the scala naturae
was a motif in early-nineteenth-century biological thought: what could
be the minimal unit carrying the potential for life?
The cell theory Hints at the idea that the cell is the basic component of living organisms
emerged well before 1838−39, which was when the cell theory was officially
formulated. Cells were not seen as undifferentiated structures. Some cellular
components, such as the nucleus, had been visualized, and the occurrence of
these structures in cells of different tissues and organisms hinted at the
possibility that cells of similar organization might underlie all living matter.
The abbot Felice Fontana (1730−1805) glimpsed the nucleus in epithelial
cells in 1781, but this structure had probably been observed in animal and
plant cells in the first decades of the eighteenth century7,
10.
The Scottish botanist Robert Brown (1773−1858) was the first to recognize
the nucleus (a term that he introduced) as an essential constituent of living
cells (1831). In the leaves of orchids Brown observed "a single circular areola,
generally somewhat more opake than the membrane of the cell... This areola,
or nucleus of the cell as perhaps it might be termed, is not confined to the
epidermis, being also found not only in the pubescence of the surface... but
in many cases in the parenchyma or internal cells of the tissue"11.
Brown recognized the general occurrence of the nucleus in these cells and
apparently thought of the organization of the plant in terms of cellular constituents.
Meanwhile, technical improvements in microscopy were being made. The principal
drawback of microscopes since van Leeuwenhoek's time was what we now call
'chromatic aberration', which diminishes the resolution power of the instrument
at high magnifications. Only in the 1830s were achromatic microscopes introduced,
allowing more precise histological observations. Improvements were also made
in tissue-preservation and -treating techniques.
In 1838, the botanist Matthias Jakob Schleiden (1804−1881) suggested
that every structural element of plants is composed of cells or their products12. The following year, a similar conclusion was elaborated for animals
by the zoologist Theodor Schwann (1810−1882). He stated that "the elementary
parts of all tissues are formed of cells" and that "there is one universal
principle of development for the elementary parts of organisms... and this
principle is in the formation of cells"13. The conclusions of
Schleiden and Schwann are considered to represent the official formulation
of 'cell theory' and their names are almost as closely linked to cell theory
as are those of Watson and Crick with the structure of DNA4,
14.
According to Schleiden, however, the first phase of the generation of cells
was the formation of a nucleus of "crystallization" within the intracellular
substance (which he called the "cytoblast"), with subsequent progressive enlargement
of such condensed material to become a new cell. This theory of 'free cell
formation' was reminiscent of the old 'spontaneous generation' doctrine (although
as an intracellular variant), but was refuted in the 1850s by Robert Remak
(1815−1865), Rudolf Virchow (1821−1902) and Albert Kölliker
(1817−1905) who showed that cells are formed through scission of pre-existing
cells7. Virchow's aphorism omnis cellula e cellula (every
cell from a pre-existing cell) thus became the basis of the theory of tissue
formation, even if the mechanisms of nuclear division were not understood
at the time.
Cell theory stimulated a reductionistic approach to biological problems
and became the most general structural paradigm in biology. It emphasized
the concept of the unity of life and brought about the concept of organisms
as "republics of living elementary units"7.
As well as being the fundamental unit of life, the cell was also seen as
the basic element of pathological processes. Diseases came to be considered
(irrespective of the causative agent) as an alteration of cells in the organism.
Virchow's Cellularpathologie was the most important pathogenic concept
until, in this century, the theory of molecular pathology was developed.
Protoplasmic constituents After Schleiden and Swann's formulation of cell theory, the basic constituents
of the cell were considered to be a wall or a simple membrane, a viscous substance
called "protoplasm" (a name now replaced by Kölliker's term "cytoplasm"),
and the nucleus. It soon became evident that the protoplasm was not a homogeneous
fluid. Some biologists regarded its fine structure as fibrillary, whereas
others described a reticular, alveolar or granular protoplasmic architecture.
This discrepancy resulted partly from artefactual and illusory images attributable
to fixation and staining procedures that caused a non-homogeneous precipitation
of colloidal complexes.
However, some staining of real cellular components led to the description
of differentiated elements, which were subsequently identified. The introduction
of the oil-immersion lens in 1870, the development of the microtome technique
and the use of new fixing methods and dyes greatly improved microscopy. Towards
the end of the nineteenth century, the principal organelles that are now considered
to be parts of the cell were identified. The term "ergastoplasm" (endoplasmic
reticulum) was introduced in 1897 (ref. 15);
mitochondria were observed by several authors and named by Carl Benda (1857−1933)
in 1898 (ref. 16), the same year in which Camillo
Golgi (1843−1926) discovered the intracellular apparatus that bears
his name17.
The protoplasm was not the only structure to have a heterogeneous appearance.
Within the nucleus, the nucleolus and a stainable substance could be seen.
Moreover, a number of structures (ribbons, bands and threads) appeared during
cell division. As these structures could be heavily stained, they were called
"chromatin" by Walther Flemming (1843−1905), who also introduced the
term "mitosis" in 1882 and gave a superb description of its various processes18. Flemming observed the longitudinal splitting of salamander chromosomes
(a term introduced only in 1888 by Wilhelm Waldeyer, 1836−1921) during
metaphase and established that each half-chromosome moves to the opposite
pole of the mitotic nucleus18. This process was also observed
in plants, providing further evidence of the deep unity of the living world.
The neuron theory There was, however, a tissue that seemed to belie cell theory —
nervous tissue. Because of its softness and fragility, it was difficult to
handle and susceptible to deterioration. But it was its structural complexity
that prevented a simple reduction to models derived from the cell theory.
Nerve-cell bodies, nervous prolongations and nervous fibres were observed
in the first half of the nineteenth century. However, attempts at reconstructing
a three-dimensional structure of the nervous system were frustrated by the
impossibility of determining the exact relationships between cell bodies (somas),
neuronal protoplasmic processes (dendrites) and nervous fibres.
A book by Karl Deiters (1834−1863), published posthumously in 1865,
contains beautiful descriptions and drawings of nerve cells studied by using
histological methods and microdissections made with thin needles under the
microscope (on next page)19.
Deiters's nerve cells were characterized by a soma, dendrites and a nerve
prolongation (axon) which showed no branching. Kölliker, in the fifth
edition of his important book on histology, published in 1867, proposed that
sensory and motor cells of the right and left halves of the spinal cord were
linked "by anastomoses" (direct fusion)20.
Left, drawing of an isolated neuron by Karl Deiters (reproduced from ref. 19). Right, isolated neuron obtained with the Deiters
microdissection technique, using thin needles under the microscope (courtesy
of G. Merico). The long axon in both cases does not appear ramified because
branchings were disrupted during the procedure.
In 1872, the German histologist Joseph Gerlach (1820−1896) expanded
Kölliker's view and proposed that, in all of the central nervous system,
nerve cells established anastomoses with each other through a network formed
by the minute branching of their dendrites. According to this concept, the
network or reticulum was an essential element of grey matter that provided
a system for anatomical and functional communications, a protoplasmic continuum
from which nerve fibres originated21.
The most important breakthrough in neurocytology and neuroanatomy came
in 1873 when Golgi developed the 'black reaction'22, which he
announced to a friend with these few words, "I am delighted that I have found
a new reaction to demonstrate, even to the blind, the structure of the interstitial
stroma of the cerebral cortex. I let the silver nitrate react with pieces
of brain hardened in potassium dichromate. I have obtained magnificent results
and hope to do even better in the future." This reaction provided, for the
first time, a full view of a single nerve cell and its processes, which could
be followed and analysed even when they were at a great distance from the
cell body. The great advantage of this technique is that, for reasons that
are still unknown, a precipitate of silver chromate randomly stains black
only a few cells (usually from 1 to 5%), and completely spares the others,
allowing individual elements to emerge from the nervous puzzle.
Aided by the black reaction, Golgi discovered the branching of the axon
and found that, contrary to Gerlach's theory, dendrites are not fused in a
network. Golgi, however, failed to go beyond the 'reticularistic paradigm'.
He believed that the branched axons stained by his black reaction formed a
gigantic continuous network along which the nervous impulse propagated. In
fact, he was misled by an illusory network created by the superimposition
and the interlocking of axons of separate cells. Golgi's network theory was,
however, a substantial step forward because it emphasized, for the first time,
the function of branched axons in connecting nerve cells.
According to Gerlach and Golgi, the nervous system represented an exception
to cell theory, being formed not by independent cells but rather by a gigantic
syncytium. Its unique structure and function could well justify an infringement
of the general rule.
Matters changed quickly in the second half of the 1880s. In October 1886,
the Swiss embryologist Wilhelm His (1831−1904) put forward the idea
that the nerve-cell body and its prolongations form an independent unit23,
24. In discussing how the axons terminate at the motor plate and
how sensory fibres originate at peripheral receptors such as the Pacinian
corpuscles, he suggested that a separation of cell units might be true of
the central nervous system. The nervous system began to be considered, like
any other tissue, as a sum of anatomically and functionally independent cells,
which interact by contiguity rather than by continuity.
Similar conclusions were reached, at the beginning of 1887, by another
Swiss scientist, the psychiatrist August Forel (1848−1931), and, in
1891, Waldeyer introduced the term "neurons" to indicate independent nerve
cells25,
26. Thereafter, cell theory as applied to the nervous
system became known as the 'neuron theory'.
Ironically, it was by using Golgi's black reaction that the Spanish neuroanatomist
Santiago Ramón y Cajal (1852−1934) became the main supporter
and indefatigable champion of the neuron theory. His neuroanatomical investigations
contributed to the foundations of the basic concepts of modern neuroscience.
However, definitive proof of the neuron theory was obtained only after the
introduction of the electron microscope, which allowed identification of synapses
between neurons21. When the nervous system was also found to
be made up of independent units, cell theory obtained its final triumph.
The missing link With the theory of evolution, the cell theory is the most important generalization
in biology. There is, however, a missing link between these theories that
prevents an even more general and unifying concept of life. This link is the
initial passage from inorganic matter to the primordial cell and its evolution
— the origin of life. If it everproves possible to recreate in the laboratory
the prebiotic physicochemical conditions required for the spontaneous generation
of life, the link between these two generalizations will be finally at hand
and a unifying paradigm will explain all biological phenomena. The theory
of spontaneous generation would then be vindicated.
Singer, S. A Short History of Biology (Clarendon, Oxford, 1931).
Westfall, R. S. Hooke, Robert in Dictionary of Scientific Biography Vol. 7 (ed. Gillespie, C.) 481−488 (Scribner, New York, 1980).
Mayr, E. The Growth of the Biological Thought (Belknap, Cambridge, MA, 1982).
Spallanzani, L. Opuscoli di Fisica Animale e Vegetabile (Società Tipografica, Modena, 1776).
Pasteur, L. A. Ann. Sci. Nat. (part. zool.)16, 5−98 (1861).
Fontana, F. Traité sur le Vénin de la Vipère sur les Poisons Américains sur le Laurier-cerise et sur Quelques Autres Poisons Végetaux (Firenze, 1781).
Brown, R. Trans. Linnean Soc. Lond.16, 685−742 (1833).
Schleiden, M. J. Arch. Anat. Physiol. Wiss. Med.13, 137−176 (1838).
Schwann, T. Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstum der Tiere und Pflanzen (Sander'schen Buchhandlung, Berlin, 1839).
Harris, H. The Birth of the Cell (Yale Univ. Press, New Haven, 1998).
Garnier, C. Bibliogr. Anat.5, 278−289 (1897).
Benda, C. Arch. Anat. Physiol.73, 393−398 (1898).
Golgi, C. Boll. Soc. Med. Chir. Pavia13, 3−16 (1898); partial transl. Geller Lipsky, N. J. Micros.155, 3−7 (1989). | ISI |
Flemming, W. Zellsubstanz, Kern und Zelltheilung (FCW Vogel, Leipzig, 1882).
Deiters, O. F. K. Untersuchungen über Gehirn und Rückenmark des Menschen und der Säugetiere (Braunschweig, Vieweg, 1865).
Kölliker, A. Handbuch der Gewebelehre des Menschen 5th edn (Engelmann, Leipzig, 1867).
Shepherd, G. M. Foundations of the Neuron Doctrine (Oxford Univ. Press, New York, 1991).
Mazzarello, P. La Struttura Nascosta. La Vita di Camillo Golgi (Cisalpino−Monduzzi, Bologna, 1996); transl. Buchtel, H. & Badiani, A. The Hidden Structure. The Life of Camillo Golgi. (Oxford Univ. Press, Oxford, in the press).
