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". |
|
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
protozoa. Leeuwenhoek
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
Philosophical
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.
|
|
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
nucleus.
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.
It is upon the works of Hooke,
Leeuwenhoek, Oken,
and Brown that Schleiden
and Schwann built their
Cell Theory. It was the German professor of botany at the University
of Jena, Dr. M. J. Schleiden, who brought the
nucleus to popular attention, and to asserted its all-importance in the
function of a cell. Schleiden freely
acknowledged his indebtedness to Brown for
first knowledge of the nucleus, but he soon carried out his own observations
of the nucleus, far beyond those of Brown. He
came to believe that the nucleus is really the most important portion of the
cell, in that it is the original structure from which the remainder of the
cell is developed. He called it the cytoblast.
He outlined his views in an epochal paper published in Muller's Archives in
1838, under title of "Beitrage zur Phytogenesis." This paper is in itself of
value, yet the most important outgrowth of Schleiden's observations of the
nucleus did not spring from his own labors, but from those of a friend to
whom he mentioned his discoveries the year previous to their publication.
This friend was Dr. Theodor Schwann, professor
of physiology in the University of Louvain.
Schwann was puzzling over
certain details of animal histology which he could not clearly explain. He
had noted a strange resemblance of embryonic cord material, from which the
spinal column develops, to vegetable cells. Schwann
recognized a cell-like character of certain animal tissues.
Schwann felt that this similarity could not be
mere coincidence, and it seemed to fit when Schleiden
called his attention to the nucleus. Then at once he reasoned that if there
really is the correspondence between vegetable and animal tissues that he
suspected, and if the nucleus is so important in the vegetable cell as
Schleiden believed, the nucleus should also be
found in the ultimate particles of animal tissues. A closer study of animal
tissues under the microscope showed, in particular in embryonic tissues,
that the "opaque spots" that Schleiden
described were found in abundance. The location of these nuclei at
comparatively regular intervals suggested that they are found in definite
compartments of the tissue, as Schleiden had
shown to be the case with vegetables; indeed, the walls that separated such
cell-like compartments one from another were in some cases visible. Soon
Schwann was convinced that his original premise
was right, and that all animal tissues are composed of cells not unlike the
cells of vegetables. Adopting the same designation,
Schwann propounded what soon became famous as the
CELL THEORY. So expeditious was his
observations that he published a book early in 1839, only a few months
after the appearance of Schleiden's paper.
The main theme of his book was to unify vegetable and
animal tissues. Accepting cell-structure as the basis of all vegetable
tissues, he sought to show that the same is true of animal tissues.
And by cell Schwann meant,
as did Schleiden also, what the word ordinarily
implies--a cavity walled in on all sides. He knew that the cell might be
filled with fluid contents, but he regarded these as relatively subordinate
in importance to the nucleus and cell wall.
Their main thesis, the similarity of development of
vegetable and animal tissues and the cellular nature of life, was supported
almost immediately by a mass of carefully gathered evidence which a
multitude of microscopists confirmed. So Schwann's
work became a classic almost from the moment of its publication. Various
other workers disputed Schwann's claim to
priority of discovery, in particular an English microscopist,
Valentin, who asserted that he was working
closely along the same lines. So did many others, such as
Henle, Turpin,
Du-mortier, Purkinje,
and Muller, all of whom
Schwann himself had quoted in his work. Many physiologists had,
earlier than any of the above, foreshadowed the cell theory, including
Kaspar Friedrich Wolff around the close of the
previous century, and Treviranus in 1807.
But, as we have seen in the scientific method, it is
one thing to foreshadow a discovery, it is quite another to give it full
expression and make it the cornerstone of future discoveries. And when
Schwann put forward the explicit claim that
"there is one universal principle of development for the elementary parts,
of organisms, however different, and this principle is the formation of
cells," he enunciated a doctrine which was for all practical purposes
absolutely new and opened up a novel field for the microscopist to enter. A
most important era in Cell Biology dates from
the publication of his book in 1839.
|
