CELL THEORY: HISTORY, POSTULATES AND AUTHORS - BIOLOGY - 2024

The cell theory is the theory that proposes that all living things are made up of cells. It was proposed by Matthias Schleiden, Theodor Schwann and Rudolph Virchow between the years 1838 and 1859, and is considered a key theory for the birth of cell biology.

The advent of this theory definitively discarded the Aristotelian conception that life could arise by spontaneous generation from inert or non-living matter, an idea maintained in the scientific world for many centuries.

Cells in a living tissue of a plant leaf (Source: Des_Callaghan via Wikimedia Commons)

Today it is not crazy to think that organisms as different as animals, plants and bacteria, for example, are made up of equivalent basic units like cells, but hundreds of years ago these ideas seemed a bit far-fetched.

With a simple microscope observation of the leaves of a plant, the skin of an amphibian, the hairs of a mammal or a colony of bacteria, it can be quickly affirmed that they are all composed of a basic unit with a similar organization and composition.; the cell.

Eukaryotic unicellular organisms of different kinds and the cells of complex animal tissues such as the brain or muscle, for example, are radically different in both structure and function, yet they all have a membrane that surrounds them, a cytosol that it houses a nucleus and organelles that have certain functional capacities.

Animal eukaryotic cell. Source: By Nikol valentina romero ruiz, from Wikimedia Commons

Although it was established as a theory by three main authors, the cell theory took place thanks to much knowledge, observations and previous contributions from different authors, who provided the pieces of the puzzle that Schleiden, Schwann and Virchow would later put together, and that others would refine later.

Background and history of cell theory

The formulation of the cell theory by Schleiden, Schwann and Virchow would not have been possible without the previous invention of the microscope, which took place in the mid-17th century.

Two important figures were involved in the first microscopic observations of cells and in the manufacture of the first rudimentary microscopes: Robert Hooke, in 1665, and later, Antoni van Leeuwenhoek.

However, there are reports of the observations of Athanasius Kircher who, in 1658, observed living creatures (in addition to worms) forming on decaying tissues. Around the same time, the German Swammerdam described globular “corpuscles” in the blood and realized that frog embryos were also made up of globular “particles”.

Robert Hooke was the one who coined the term "cell" to describe the cells he observed when looking at a cork sheet through a microscope; while Leeuwenhoek devoted himself fervently to the manufacture of microscopes and to the repeated observation of samples from different places, affirming the existence of minute life.

Both Hooke and Leeuwenhoek could be considered the “fathers” of microbiology, since they were the first to report the existence of microscopic organisms in different natural environments (bodies of water, scrapings of dirt from dentures, semen, etc.).

Two other authors of the time, Marcello Malpighi and Nehemiah Grew, studied some plant tissues in detail. The publications of Malpighi (1671) and Grew indicate that both authors identified the structure of the cells during their observations, but referred to these as "cells", "pores" or "saccules".

Plant eukaryotic cell

Disproving theories about spontaneous generation

For many centuries, the scientific community held the position that life could be spontaneously generated from inanimate matter (inert, not living), based on the "vital force" or the "potential" of elements such as water and earth. to generate life.

However, these postulates were refuted by the experiments carried out by the Italian Lazzaro Spallanzani, who showed in 1767 that when water from ponds or wells was boiled, said “vital force” disappeared, which implied that what existed in the water were living organisms.

Therefore, his works were the pioneers for the demonstration that life can only arise from pre-existing life or, what is the same, that all cells come from other cells and not from inert matter.

About a century after Spallanzani's work, the Frenchman Louis Pasteur set the precedent with his own experiments, definitively showing that spontaneous generation had no place in the scientific world.

Postulates of cell theory

One of the postulates of the cell theory is that cells come from cells that already existed

Although the cell theory was formulated based on observations made in "higher" organisms, it is valid for all living beings, even single-celled organisms such as some parasites and bacteria.

The main postulates of the cell theory are three:

1- All living beings are made up of cells

The botanist M. Schleiden and the zoologist T. Schwann proposed this postulate, stating that, at the microscopic level, plants and animals were made up of cells.

2- Cells are the basic units of all living beings

This principle was also postulated by Schleiden and Schwann and is a basic principle to define a living being; All living things are made up of cells, whether they are unicellular or multicellular.

3- Cells can only come from pre-existing cells and not by spontaneous generation

This principle was established by Rudolph Virchow.

Later, another author, A. Weismann, added the following corollary to the theory:

- The cells that we know today ("modern") originated from a small group of "ancestral" cells

A corollary that can be evidenced thanks to the similarities found between some complex proteins found in all cells, cytochrome being one of the best examples of these proteins, since it is "conserved" in terms of structure and function in both bacteria and in plants and animals.

Main authors

Although M. Schleiden, T. Schwann and R. Virchow were the main protagonists in the formulation of the cell theory as we know it today, many were the scientists who participated, directly or indirectly, in the definitive establishment of it.

Robert Hooke (1635-1702)

Portrait of Robert Hooke (Source: Gustav VH, via Wikimedia Commons)

This virtuous English scientist not only made discoveries in the field of biology, but was also interested in physics and astronomy.

In 1665 he presented to the Royal Society of London his book entitled "Micrography or Some Physiological Descriptions of Miniature Bodies through a Magnifying Glass" (from the English Micrographia Or Some Physiological Descriptions of Miniature Bodies by Magnifying Glass).

In this book, Hooke highlights the observations he made on a sheet of cork, in which he identified units similar to "cells" that he called "cells." At just a 30-fold magnification, Hooke observed the same pattern in other plants and in the bones of some animals, suggesting that living tissues were made up of the same "pores" or "cells."

Antoni van Leeuwenhoek (1632-1723)

Portrait of Antoni van Leeuwenhoek (Source: Jan Verkolje (1650-1693) via Wikimedia Commons)

Contemporary with Robert Hooke, the Dutch A. Leeuwenhoek dedicated part of his life to the manufacture of microscopes and to the observation of samples through them. He was the first author to show living cells (Hooke only saw dead cells from the bark of some trees and the bone of some animals).

In addition, the design of his microscopes allowed him to appreciate cellular structures in much more detail, and led him to the discovery of many single-celled organisms that he called "animalcules," which today are known to be both single-celled animals and plants.

In 1674, Leeuwenhoek first described red blood cells and sperm in his own semen.

Matthias Schleiden (1804-1881)

Portrait of Matthias Schleiden (Source: Fæ, via Wikimedia Commons)

This German scientist, professor of botany, was the one who "formulated" the cell theory based on his observations in plant tissues. In addition, he was really interested in the origin of cells, so he dedicated himself to its study using embryos from plant tissues.

Schleiden dared to propose that cells developed "de novo" from a mass of tiny granules within cells, which formed a "nucleus" whose progressive growth became a new cell.

Theodor Schwann (1810-1882)

Portrait of Theodor Schwann (Source: Fæ, via Wikimedia Commons)

This German author was in charge of "generalizing" the cell theory for all living organisms, including plants and animals.

Schwann described nucleated cells in various tissues: in notochord and cartilage cells, in toad larvae, in the liver, kidneys, pancreas, salivary glands and connective tissue of pig embryos.

His results were reported in 1838 in his "Field Notes on Nature and Medicine." This author also made important contributions to neuroscience, as he was the first to describe the membranous covering that surrounds the processes of nerve cells.

Robert Brown (1773-1858)

This Scottish botanist and physician was the first (in 1831) to recognize the nucleus as an essential part of living cells, thanks to his microscopic observations on orchid leaves. Brown was the one who coined the term "nucleus" to describe a "single opaque circular areola" in the center of cells.

Rudolf Virchow (1821-1902)

Portrait of Rudolf Virchow (Source: http://ihm.nlm.nih.gov/images/B25666 via Wikimedia Commons)

This German doctor and pathologist was commissioned to publish in writing, in 1855, the idea that every cell comes from a pre-existing cell (omnis cellula e cellula), ruling out the possibility of spontaneous generation.

A few years earlier he declared that: "the cell, as the simplest form of manifestation of life, which nevertheless represents the idea of ​​life, is the organic unity, the indivisible living being."

Louis Pasteur (1822-1895)

Portrait of Louis Pasteur (Source: Paul Nadar via Wikimedia Commons)

It was this French microbiologist who definitively discarded the theory of spontaneous generation, thanks to the experiments he carried out in the 1850s, in which he demonstrated that the multiplication of unicellular organisms occurred from already existing organisms.

His firm conviction led him to design an experimental procedure by which he demonstrated that a "meat broth" could be sterilized by boiling it in a "gooseneck" flask, capable of "trapping" dust particles and other contaminants before they reach the bottom of the container.

Pasteur showed that if the broth was boiled and then the neck of the flask was broken and it was left exposed to the air, it eventually became contaminated, acquiring a cloudy appearance due to microbial contamination.

It is important to note that other authors such as Carl Benda (1857–1933) and Camilo Golgi (1843–1926) (among others) later made important contributions regarding the clarification of the internal structure of eukaryotic cells, describing their main organelles and their functions..

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, AD, Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland Science.
2. Mazzarello, P. (1999). A unifying concept: the history of cell theory. Nature Cell Biology, 1 (1), E13.
3. Nabors, MW (2004). Introduction to botany (No. 580 N117i). Pearson.
4. Ribatti, D. (2018). An historical note on the cell theory. Experimental cell research, 364 (1), 1-4.
5. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.
6. Villanueva, JR (1970). The living cell.
7. Willey, JM, Sherwood, L., & Woolverton, CJ (2008). Prescott, Harley, and Klein's microbiology. McGraw-Hill Higher Education.