Cell Theory

The notion that the cell is the smallest division of life and its attendant principles have been developed over the past three centuries and are collectively known as the cell theory.

Before the invention of the microscope, people studying living organisms saw whole and complete organisms and did not imagine that life was subdivided into smaller compartments. It is now known that the cell is the fundamental unit of life and that all living organisms are composed of cells. Because cells are microscopic, their existence was not discovered until the seventeenth century.

Discovery of the Cell

The discovery of the cell did not come about until the last half of the seventeenth century, after the Dutch inventor Antoni van Leeuwenhoek built the first light microscope. When looking at pond water using his light microscope in 1674, Leeuwenhoek saw many tiny creatures which were invisible to the naked eye. Leeuwenhoek assumed that these tiny “animalcules” were alive because he could see them moving.

However, the first description of the cell is attributed to the English scientist Robert Hooke. In 1665 Hooke first published Micrographia, a work devoted to observations made with his compound light microscope. Hooke examined the structure of cork, a dead plant tissue, by cutting cork into very thin slices and observing the slices under his light microscope.

Hooke saw the dead cells of the cork outlined by the thickened cell walls of that tissue and determined that cork was composed of a pattern of spaces which resembled small rooms, or “cells.” Interestingly, Hooke did not recognize the significance of the cells that he described and thought that the cork cells were merely channels for fluid conduction in the plant.

Cells as Globules

Partly because of problems with chromatic aberrations in early microscopes, scientists thought that living organisms were made of “globules.” As early as 1682, plant tissues were described as bladders clustered together.

In 1771 William Hewson performed one of the first cell biology experiments by confirming Leeuwenhoek’s earlier observation of blood and brain globules and by showing that the blood globules swelled and shriveled in different solutions. By 1812 Johann Jacob Paul Moldenhawer had shown that plant tissue was composed of independent cells, and in 1826 Henri Milne-Edwards determined that all animal tissues were formed from globules.

Finally, in 1824 Henri Dutrochet proposed that animals and plants had a similar cellular structure. Ironically, Thomas Hodgkin and Joseph Jackson Lister showed in 1827 that many of the previously observed globules were likely to have been optical artifacts that disappeared when the new achromatic microscope was used. However, the idea that life was made of tiny cells remained.

Cells Compose All Organisms

Matthias Schleiden, a German botanist, suggested in 1838 that all of the structural elements found in plant tissues were composed of cells or cellular products. In 1839 Theodor Schwann, a zoologist, reported that animal tissues were also composed of cells and suggested that all living organisms were actually composed of cells. Based on their contributions, Schleiden and Schwann are considered to have established the official “cell theory.”

Schwann also contributed to the description of the cell by defining a cell as having three essential components: a cell wall, a nucleus, and a fluid content. The Scottish botanist Robert Brown first described the nucleus of the cell as an essential component of living cells in 1831.

The work of both Schleiden and Schwann contested the notion of vitalism, the belief that no single part of an organism was alive but that some how the substances composing the whole organism together shared the characteristics of life. The idea of vitalism made sense prior to the invention of the microscope (because cells cannot be seen with the naked eye) but did not hold up when cells were observed in all plant and animal tissues.

Therefore, the idea that living organisms were made of cells was becoming widespread at this time. An 1830 plant anatomy textbook by Franz Julius Ferdinand Meyen contained a chapter on cell structure and discussed the idea that cells unite to form cellular tissues.

Cell Origins in Organisms

Interestingly, while Schleiden and Schwann realized that all living organisms were made of cells, they did not realize that all cells came from preexisting cells. Indeed, Schwann mistakenly thought that cells had an extracellular origin and could arise de novo.

Schwann originally observed plant cells in the endosperm of seeds, where the nuclei multiply before the cell walls form, and he generalized from this unusual system. This idea persisted for a time, even though Hugo von Mohl first described cell division in green algae in 1837.

Essentially, von Mohl investigated his hypothesis that all cells must start out very small and gradually grow to full size by observing a filamentous green algae. By studying the alga, Mohl discovered cell division as the algal cells divided and formed partitions between the newly formed daughter cells.

By 1858 the pathologist Rudolf Virchow had expressed the idea that all cells arise from preexisting cells. This idea was important because until that time, the idea of spontaneous generation was also popular.

Spontaneous generation was the mistaken idea that living organisms could arise spontaneously from non living material and was thought to explain such phenomena as the fact that frogs are found in ponds and maggots in rotten meat. In contrast, Virchow wrote: “Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and the plant only from a plant.”

Modern Cell Theory

The modern cell theory consists of several principles. As stated by Schleiden and Schwann, all living organisms are composed of one or more cells. Because cells are the smallest units of life, cells must be the site of the chemical reactions that sustain living organisms. Finally, it is clear that all cells arise from preexisting cells, as first stated by Virchow.

In order for cells to perform this replication, it is now apparent that all cells contain hereditary information and pass this information from parent cell to daughter cell. Additionally, this hereditary information is contained in the deoxyribonucleic acid (DNA) found in the nuclei of all cells, with this DNA being copied for each daughter cell to use.