Cell Theory and How it Came to Be
Around 1655, using a compound microscope, Robert Hooke was the first
person to see the
building blocks of life.
He drew the room-like
structures he saw in
thin slices of cork, and
coined the term “cells”
to describe them.
However, for many
years, people did not
know what cells were,
or what they did. It
took 200 years of
investigation by many
scientists to work out
an explanation of the
nature of cells. This is
Hooke’s drawing of cork cells, from his book Micrographia
the story of the development of cell theory, and it begins with the invention of
the tool which made cells visible.
Hans and Zacharias Janssen: The Invention of the Microscope
Magnifying devices were known since Roman times, when crystals which
were thicker in the middle than at
the sides were used to make
things appear larger, or to start
fires. Eyeglasses were invented
in the 13th century. The first
microscopes were invented
One of the Janssens'
microscopes, from around
1595
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around 1590 by Hans and Zacharias Janssen, a father and son team of Dutch
eyeglass makers. The fist microscope was a tube with a lens in each end. The
Janssens’ microscopes magnified objects from 3x to 9x. Although not very
powerful, the early microscope led to the invention of more powerful
microscopes, and also to the invention of the telescope.
Robert Hooke: Micrographia
Englishman Robert Hooke (1635-1703) was a one of the most important
scientists and inventors of his time. He invented the balance spring, which made
more accurate clocks possible. He invented an early prototype of the respirator.
He improved the design of barometers and anemometers. He worked out an
equation describing elasticity (“Hooke’s Law”). He correctly described
combustion. He helped rebuild London after the Great Fire of 1666. And, of
interest to biologists, he was among the first to recognize the potential of the
microscope as a scientific tool.
Hooke improved upon the Janssens’ design of the microscope.
His microscope was many times more powerful than the older
models, and Hooke used his
microscope to observe many
things, including cork cells,
Hooke’s
microscope
fleas, insects, sponges, mites
and birds feathers. He
published his wonderfully detailed drawings
and observations in a book titled Micrographia
in 1665. It became a best seller.
Hooke discovered cells, but he did not
work out a theory of what they were or how
they worked. However, his very popular book
inspired others, including a certain Dutch
fabric dealer, to investigate further.
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Hooke’s drawing of a flea, which
he described as "...adorn'd with a
curiously polish'd suite of sable
Armour, neatly jointed. . ."
Anton van Leeuwenhoek: The Gifted Observer
Anton van Leeuwenhoek (1632-1723) was an unlikely scientist. He was
born in Delft, Holland, the son of a
basketmaker. He attended grade school,
but received little or no scientific
education, and he did not go to college.
He ran a draper’s shop (a fabric store) in
Delft. He also served as a minor public
official, a surveyor and a wine assayer.
Part of his job as a fabric merchant called
for him to count the number of threads per
Anton van Leeuwenhoek
inch in fabric, which required the use of a
magnifying glass. He taught himself how
to grind exceptionally fine lenses.
After seeing a copy of Hooke’s book,
Micrographia, van Leeuwenhoek was inspired to perform
his own investigations into the world of very small
things. Van Leeuwenhoek devised his own style of
microscope. Unlike the compound (two-lens) microscope
used by Hooke, his microscope consisted of a single lens
in the middle of a metal plate. The observer peers
through the lens to see the object being observed, which
is held on the other side of the plate on the tip of a pin.
There are two adjustment screws, one of which moves the pin up or down, and the
other which moves the pin closer or further away. Adjusting the two screws
focuses the object. His microscope is really just a very powerful magnifying glass,
but it was the most powerful microscope of its time, magnifying objects up to
300X. Van Leeuwenhoek made over 500 of these microscopes during his life, the
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smallest about the size of a rectangular postage stamp, and the largest about 4
inches long. He gave one of his microscopes to Hooke. (Hooke said that Van
Leeuwenhoek’s microscope was very powerful, but he found it very difficult to
use.)
Van Leeuwenhoek’s curiosity drove him to observe many things with his
microscope. He observed tiny creatures swimming in a drop of water from a lake.
He was the first to see tiny one-cell organisms which he dubbed “animalcules”.
He helped provide evidence against the theory of spontaneous generation by
showing that ants were hatched from tiny eggs, instead of arising from the soil.
He accurately described the life cycle of ants (egg, larva, pupa, adult). He was the
first to see yeast cells, and the first to see the circulation of blood cells in
capillaries.
He once scraped plaque ("a little white matter, which is as thick as if 'twere
batter.") from his own teeth, and those of his wife and daughter. In a letter, he
described what he saw:
I then most always saw, with great wonder, that in the said
matter there were many very little living animalcules, very
prettily a-moving. The biggest sort. . . had a very strong and
swift motion, and shot through the water (or spittle) like a pike
does through the water. The second sort. . . oft-times spun
round like a top. . . and these were far more in number."
He also obtained plaque scrapings from two old men who had never
brushed their teeth in their lives. In the mouth of one of these old men, he
found...
... an unbelievably great company of living animalcules, aswimming more nimbly than any I had ever seen up to this
time. The biggest sort. . . bent their body into curves in going
forwards. . . Moreover, the other animalcules were in such
enormous numbers, that all the water. . . seemed to be alive.
These were the first observations ever made of bacteria.
Although he was not a gifted artist, he hired one to make drawings of the
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things he observed, and sent over a hundred letters of his observations (and the
specimens) to the most prominent scientists in the world, at the Royal Society of
London and the French Academy in Paris. His letters, written in Dutch, were
translated into English and Latin, and circulated among scientists. He was visited
by scientists curious to see the things he was observing. He also was visited by
Queen Anne of England and Czar Peter the Great of Russia. For Peter the Great,
he demonstrated the flow of blood in the capillaries of an eel. His observations
opened up the microscopic world to a generation of scientists. He once wrote,
...my work, which I've done for a long time, was not pursued
in order to gain the praise I now enjoy, but chiefly from a
craving after knowledge, which I notice resides in me more
than in most other men. And whenever I found out anything
remarkable, I have thought it my duty to put down my
discovery on paper, so that all ingenious people might be
informed thereof.
After Van Leeuwenhoek, a few biologists began to look at the plants and
animals they were studying at the microscopic level. They wanted to know what
they would discover when they looked at plant and animal tissues close up.
Schwann and Schleiden: Every living thing is made of cells.
Matthias Jakob Schleiden (1804-1881) believed that there was more to
botany than just naming and describing plants. He used microscopes to look at the
tissues of a great many plants. Every plant he looked at was made of cells. In
1837, he stated that all plants were made of cells, and that the growth of plants
came about through the production of new cells. He was correct on both counts.
Schleiden’s friend, Theodor Schwann (1810-1882) was conducting similar
investigations in the field of zoology at the same time. He applied Schleiden’s
findings on plant cells to his study of animal tissues. He found that every animal
tissue he looked at was made of cells, or the products of cells. He published his
results in 1838.
In 1839, Schwann and Schleiden came up with the term cell theory to
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describe their findings about cells. Their cell theory stated that all living things
are made of cells, and that the cell is the smallest form of life. The cell theory of
Schwann and Schleiden focused attention on the cell as the fundamental unit of
life. Scientists soon discovered that the life processes they observed in animals
and plants - growth, movement, respiration, and reproduction - were also carried
out by cells.
However, Schwann and Schleiden were wrong about how new cells are
produced. They believed that new cells grow in the spaces between existing cells,
in a way similar to the way in which crystals grow. They stated that the nucleus
of an existing cell provided the materials from which the new cell began to
assemble itself. Many scientists rejected this concept of cell growth, because it
seemed very similar to the idea of spontaneous generation: living things coming
from nonliving things.
Virchow: “Every cell from a cell”
In 1855, the German doctor Rudolph Virchow correctly described how new
cells are produced. He rejected Schwann and Schleiden’s idea that new cells grow
outside of existing cells. He stated that every cell comes from an existing cell.
He correctly claimed that the cell divides to make two new cells. This discovery
is in harmony with Redi’s statement that living things only come from other
living things. Virchow showed that this was true on a cellular level. The idea that
cells come from other cells completed cell theory.
Cell theory was born of the work of many scientists, from the Janssens',
who could not have imagined what microscopes would someday reveal, to Hooke
and Leeuwenhoek, who observed and described cells, to Schwann, Schleiden and
Virchow, who saw patterns in their observations, and formulated the theory. The
three main ideas of cell theory are...
1. All living things are made of cells.
2. The cell is the smallest form of life.
3. Cells only come from other living cells.
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