The Cell
Part 1. Discovery

by Ian Magrath

Omnis cellula e cellula (all cells originate from other cells); Francois-Vincent Raspail, 1858 ¹

A microscope manufactured by Christopher Cocks of London, which is believed to have been used by Robert Hooke for his microscopic survey published in Micrographia. Courtesy of the Billings Microscope Collection, National Museum of Health and Medicine, Armed Forces Institute of Pathology, USA.

Robert Hooke (1835-1703), in the densely worded preface to Micrographia, his magnum opus, observed that mankind is unique in being able not only to behold the works of nature and to sustain our lives by them, but also has the power of considering, comparing, altering, assisting and improving them to various uses. "This ability," he remarked, "varied greatly among men, and was far from perfect because of the limitations of the senses coupled to a willingness to speculate, to accept widely held beliefs, or to listen uncritically to others." He suggested that:

"By the addition of such artificial Instruments and methods, there may be, in some manner, a reparation made for the mischiefs, and imperfection, mankind has drawn upon itself, by negligence, and intemperance, and a wilful and superstitious deserting the Prescripts and Rules of Nature, whereby every man, both from a deriv'd corruption, innate and born with him, and from his breeding and converse with men, is very subject to slip into all sorts of errors."

Hooke went on to emphasize the importance of Sense, the Memory, and Reason – i.e., the scientific method. He had worked as an assistant to Robert Boyle (1627-1691) in Oxford University – he built the pumps used to establish Boyle's gas law – and became, in 1662, the Curator of Experiments of the Royal Society of London, whose Royal Charter had been granted earlier the same year. Hooke was an excellent choice and Micrographia was published in 1665, just three years after he had taken up his new post. Among the more famous illustrations in the book is a drawing of cork, in which he described pore-like structures similar to those he found in the tissues of several plants and the stalks of feathers (Figure 1). Hooke coined the term cell (from the Latin "cellula" meaning a small room) for the structures he observed. He suspected that cells had an important role to play in both animals and plants, although recognizing the limitations of his simple microscope, he did not explore this further. On page 116, he wrote:

Figure 1. Hooke's illustration of the "cells" or "pores" of cork (Micrographia). Two sections are shown, cut transversely and vertically. Hooke comments that he found this same structure in many different kinds of trees, the stalks of vegetables and of feathers. He thought the cells in plants were related to the passage of fluids (sap), but could not prove this. He searched unsuccessfully for valves that might control the direction of flow, as occur in the heart and veins of animals.

"For in several of those vegetables, whil'st still green, I have with my Microscope, plainly enough discover'd these Cells or Pores fill'd with juice, and by degrees sweating them out; …"

Hooke could find no connections between the cells he observed, although he thought it likely that they were, in some way, controlling the passage of "liquor's" in both animal and plant tissue and that:

"though me thinks, it very probable that Nature has in these passages many appropriated instruments and contrivances whereby to bring her designs and ends to pass, which it is not improbable that some diligent Observer, if help'd with better microscopes, may in time detect."

In fact, it was almost two centuries before animal and plant cells became a topic of scientific pursuit, largely through microscopic observations, the importance of which, as a starting point, cannot be overestimated. Micrographia appears to have inspired a young Dutchman, Anthony van Leeuwenhoek (1632-1623), to undertake a similar microscopic survey. Unlike Hooke, Leeuwenhoek used single lenses that he made himself. He is reputed to have made more than 500, and achieved such skill as to reach a magnification of more than 200-fold (nine or so of his lenses and some of his specimens still survive) – some 10 times greater than that achieved by the earliest compound microscopes, such as that used by Hooke. Unfortunately, Leeuwenhoek kept to himself the methods he used to make his lenses and it was a century or so before such quality was achieved by others. Leeuwenhoek's survey was no more systematic than Hooke's, although he differed somewhat in his choice of subject matter. He examined, for example, the plaque between his own and other's teeth, including that of two old men who had never cleaned their teeth in their lives. In this material he observed bacteria for the first time. Subsequently he observed many "animalcules" in hay infusions and drops of lake water, his descriptions being sufficient to identify them as flagellates and ciliates (Figure 2), organisms we now know to be single celled. He also discovered spermatazoa, examined animal and plant tissue, and observed small multicellular organisms, some of which had been previously discovered, such as spirogyra, rotifers and nematode worms. Leeuwenhoek started writing about his observations to the Royal Society in 1674 and his letters were published in the society's Philosophical Transactions, the world's longest running scientific journal, eventually leading to his election as a Fellow of the society in 1680. Like Hooke, Leeuwenhoek could not possibly have recognized the full significance of the organisms he had discovered - indeed, he lacked a scientific education (he was trained as a draper and spoke no Latin or English; his letters to the Royal Society had to be translated from Dutch). Nevertheless, his meticulous observations established the existence of a vast world beyond the reach of the human senses - a world that would, centuries later, provide a focus for speculation on the origins of life itself.

Leeuwenhoek also probably discovered the cell nucleus. In 1682, he mentioned globules in "oval structures" in codfish blood, but it was not until 1831 that such globular structures, the existence of which had, meanwhile, been observed by several other microscopists, were referred to as the cell nucleus (from the Latin meaning a kernel or small nut) in a paper read to the Linnaean society in 1831 by a Scottish botanist, Robert Brown (1773-1858). Brown gave due credit to the botanical illustrations of the Austrian, Franz Bauer, in bringing attention to a structure that, he believed, must have functional significance. Robert Brown also observed the random movements of dust particles suspended in water (now known as Brownian motion) – a phenomenon later used by Einstein to unequivocally demonstrate the existence of molecules and atoms.

Figure 2. Some of Leeuwenhoek's animalcules. 1, Monads; 2, Forms assumed by the Ameoba; 3, Flask Animalcules, Enchelis; 4, Actinophrys sol; 5, Euglena viridis; 6, Gonium pectorale; 7, Trachelias anas; 8, Paramecium aurelia; 9, Navicula; 10, Vibrie Spirillum; 11, Vorticella Stentor — Goodrich, 1859. Source: S.G. Goodrich Animal Kingdom Illustrated Vol 2 (New York: Derby & Jackson, 1859) 2:646. Provided by Florida Center for Instructional Technology under its licence for educational use.

The Cell Theory

Little attention to cells appears to have been given until the early 19th century. Ludolph Treviranus (1779-1864), discussed plant cells in the first chapter of his book Vom inwendigen Bau der Gewächse, published in 1806, mentioning that they appear to contain "grains" (nuclei?) (Körner in den Zellen) that others had also observed and suggested that cells are separated from each other by an intervening space (Zwischenräume der Zellen). Johann Moldenhawer (1766-1827) was able to disrupt the cell walls of plants and thus separate individual cells from each other. He also showed that the pores of leaves and plant stems – critical for controlling transpiration and gaseous exchange - are comprised of a pair of cells, and identified several important plant structures, including vascular bundles (xylem and phloem), and the cambium, from which the vascular bundles develop and which permits lateral growth of ligneous plants (e.g., of the trunks of trees). He also recognized the significance of tree rings. His pioneering work was brought together in the now classical Beyträge zur Anatomie der Pflanzen, published in Kiel in 1812.

The French botanist, Henri Dutrochet (1776-1847) discussed plant cells in 1821 in an article entitled Recherches sur l'accroissement et la reproduction des végétaux, published in the Mémoires du museum d'histoire naturelle, for which he was awarded the French Academy's prize for experimental physiology. In 1824 in Recherche anatomique et physiologic sur la structure intime des animaux et des végétaux et sur leur motilité, Dutrochet proposed that cells are the "fundamental unit of organization" of both animals and plants. He believed that cells in different organs secrete different substances, presumably giving the organs their distinct characteristics. In 1826, he described and named osmosis, showing that cells are surrounded by a cell membrane that permits the passage of fluids into or out of cells (endosmosis or exosmosis respectively), the flow being controlled by the internal and external salt concentration. This body of work and its interpretation would seem to qualify Dutrochet for primacy in the genesis of the cell theory, but this is not the case. In those days, the greater emphasis on books summarizing many years of work, and the small number of scientific journals, limited communication and slowed progress. Henri Dutrochet commented on this in the preface to his Mémoires pour servir a l'histoire anatomique et physiologique des végétaux et des animaux, published in 1837. He bewailed the lack of familiarity of plant physiologists with animal physiology and believed – almost heretically for the times - that fundamental natural phenomena would be revealed by demonstrating similarities between animals and plants.

The cell theory is usually ascribed to Matthias Schleiden (1804-1881) and Theodor Schwann (1810-1882). Schleiden had studied with the pioneering physiologist, Johannes Müller (1801-1858), in Berlin, many of whose students would subsequently make major contributions to medicine and the natural sciences. He subsequently became Professor of Botany at the University of Jena. Schwann was a zoologist working at the time as an assistant to Müller. He later moved to Belgium, where he was professor at the University of Louvain before moving to Liège. The insights of Schleiden and Schwann regarding the fundamental nature of cells are said to have derived from a discussion they had over dinner in 1837 (the year, incidentally, in which the Czech physiologist, Jan Purkinje (1787-1869) discovered giant cells in the cerebellum, and commented, as had Dutrochet more than ten years earlier, on the fact that cells were present in both plants and animals. Schleiden was working on plant embryos, which he recognized as consisting of nucleated cells (he referred to the nucleus as the cytoblast, since he believed that new plant cells arose from the nuclei of old cells). In his work, which was first published in the journal Müller's Archives in 1838, he also referred to the frequent presence in the nuclei of some cells of internal "spots" of various sizes that Schwann later referred to as nucleoli. Schwann had, with Müller, identified nucleated cells in the chorda dorsalis (notochord) of tadpoles, and realized in the course of his discussion with Schleiden that they were basically similar to plant cells. Schwann's book, Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstum der Thiere und Pflanzen. ("Microscopic Researches into the Accordance in Structure and Growth of Animals and Plants") was published in 1839, making copious reference to Schleiden's work as well as to that of Brown and others. Schwann put great emphasis on the hypothesis that "all living things are composed of cells and cell products" and whether or not Schleiden and Schwann should be given credit as the originators of the cell theory, it was Schwann's book that drew attention to the remarkable similarity between plants and animals at a microscopic level.

The Cellular Origin of Cells

The theory, however, was incomplete since the origin of cells remained unknown. There were two main schools of thought – that cells budded off from the nuclear or external membranes of other cells, or that they arose from the intercellular matrix (or both). Schwann proposed that cells emerged from the intracellular matrix (which he referred to as the cytoblastema), in a manner analogous, although clearly very different, to the formation of crystals. Even when cell division had been observed, it was not seen as the exclusive mode of cellular proliferation. Barthelemy Dumortier (1797-1798), for example, in spite of the fact that he was the first to observe "binary fission" of plant cells (in 1832), continued to believe that cells could originate from the intercellular substance. Karl Wilhelm von Nägeli (1817-1891) described cell division in pollen (in 1842), while working, at the age of 25, in Schleiden's laboratory. Shortly afterwards he reported binary fission in unicellular algae, but still believed that, with this exception, new cells generally arose from the nucleus. Eventually the tide turned and by the mid-1940s Nägeli was convinced that all plant cells proliferated via a process of cell division.

Lepidus: Your Serpent of Egypt, is bred now of your mud by the operation of your Sun: so is your Crocodile. William Shakespeare Anthony and Cleopatra Act 2 Scene 7

It was yet another of Müller's students, Robert Remak (1815-1865), who provided unequivocal evidence that in vertebrates, all cells arise from a single original cell – the fertilized ovum (Untersuchungen über die Entwickelung der Wirbelthiere, 1855). Remak, a Polish physician interested in embryology, first observed cell division in blood cells while studying the development of blood vessels in chick embryos in 1841. He subsequently showed that the fertilized frog ovum repeatedly divides as it develops into a multicellular animal with many different tissues' and postulated that all cells arise as a consequence of cell division. He also suggested that cell division permitted the spread of tumors, stating:

"These findings are as relevant to pathology as they are to physiology. I make bold to assert that pathological tissues are not, any more than normal tissues, formed in an extracellular cytoblastema (spontaneously) but are the progeny or products of normal tissue in the organism."

Remak was not initially supported in his conclusions by his friend, the eminent Rudolph Virchow (1821-1902), professor at the well-known Charité Hospital in Berlin. Often given credit with Schleiden and Schwann for the cell theory, it was some years before Virchow accepted Remak's work as demonstrating that all cells arise from pre-existing cells by cell division. While this may seem surprising, acceptance of the full cell theory required overcoming an intellectual obstacle first propounded by Greek philosophers more than 2000 years before and summarized by Aristotle. According to this theory, life emerged spontaneously - usually from putrefying organic material. The spontaneous generation theory, discussed by Augustine of Hippo in The City of God and accepted by the Christian church, was widely (although not universally) believed well into the Renaissance period and beyond (Shakespeare referred, for example, to the widely held belief that snakes and crocodiles arise from the mud of the river Nile). Scientific experiments and observation for most of human history - even up to the present time - have not been considered the final arbiter of truth, and remain today open to different interpretations - quite apart from the possibility of specific instances that cannot be generalized. Some 200 years before the emergence of the cell theory, for example, the Italian, Francesco Redi (1626-1697), conducted many experiments relating to the spontaneous generation of insects and showed that the exclusion of flies prevented the appearance of maggots in rotting meat and that flies develop from maggots that hatch, in turn, from flies' eggs. Meanwhile, Leeuwenhoek's ability, around the same time, to observe animalcules in infusions of hay was taken by some (but not by Leeuwenhoek) to support evidence for spontaneous generation. In 1859, experiments conducted by Louis Pasteur, which improved on the design of earlier experiments carried out by others, demonstrated that microorganisms present in the atmosphere explain the putrescence of meat broth. His experiments are considered to have finally dispelled the spontaneous generation theory.

Rudolph Virchow' although originally skeptical of Remak's conclusions' eventually became a convert in the late 1850s and popularized Raspail's epigram Omnis cellula e cellula (although Raspail, like Dutrochet' was of the opinion that new cells arose from within old ones, not by cell division). Virchow expounded the idea that all diseases are disorders of cellular function in a series of lectures given in Berlin in the spring of 1858. These lectures were subsequently assembled into a single volume entitled Cellularpathologie (1859), which emphasized the origin of cancer from cells and was, therefore, an essential step toward the recognition that cancers could be differentiated from each other by histological examination. In spite of the promise of understanding, or at least, of classifying cancer at the level of cells, decades were to pass before histology became the primary approach to classification – for long it was simply one of the several descriptive characteristics of any given tumor. This is again rapidly becoming the case as more definitive molecular techniques evolve. Remarkably, in spite of his critical contributions, Remak was not mentioned in Cellularpathologie. Instead, the book was dedicated to Sir John Goodsir (1814-1867), a Scottish anatomist who had performed extensive work on cells in various tissues since the late 1830s. Virchow is still generally credited, falsely, with being the first to recognize that all cells are derived from other living cells by a process of cell division. His notion that disorders of cells give rise to disease was, however a new idea, although not totally at odds with Giovanni Morgagni's (1682–1771) much earlier postulate that organs are the seat of human disease, or Marie-Francois-Xavier Bichat's (1771–1802) idea that diseases arose in tissues. Although partly right, all of these statements are oversimplifications arising from incomplete knowledge.

Animalculists and Ovists

Recognition that plants and animals are comprised of cells raised the question of their relationship (or vice versa) to the rapidly expanding world of microorganisms discovered by the Dutch draper, Leeuwenhoek, almost 200 years before. These miniscule life-forms were considered by some to be an intermediate stage between the animate and inanimate, "explaining" how the spontaneous generation of life was possible. However, the discovery of spermatozoa, also referred to by Leeuwenhoek as animalcules, along with the other tiny creatures he saw through his lenses, also raised the possibility that individuals (whether animals or human) were generated from tiny copies of the adult form (homunculi) present in the spermatozoa. Supporters of this notion became known as animalculists while others (such as William Harvey), who believed that all animals develop from eggs (also from homunculi), even if such eggs (as was the case in humans) had not been identified, were referred to as ovists. The role of the egg was all too apparent in many animals - chick embryos, for example, had been studied by Malpighi (1628-1694) in the early 1670s - and the ovists saw no obvious use for sperm, except perhaps, to stimulate the egg to develop. These theories of preformation, as opposed to spontaneous generation, postulated either a male or a female origin of the offspring – an error of the kind referred to by Hooke in Micrographia, but due entirely to a complete lack of understanding of the process of inheritance. The animalculists assumed that microorganisms must be tiny animals and plants, but various theories existed about their origin and nature. In 1765 Horace Bénédicte de Saussure (1740-1799) demonstrated that animalcules derived from plant infusions were able to divide into two equal parts, suggesting that these tiny organisms were not transmuted from vegetables, or gave rise to other organisms, but that the offspring were derived from and resembled their parents. Although never published, Saussure's experiments were widely disseminated, being described in detail, for example, in a letter written by Saussure to Bonnet in 1769, as recorded in Animal Biography; or, Authentic Anecdotes of the Lives, Manners and Creation of Animals, by the Reverend W. Bingly (3rd Edition, volume III, 1806). Many, however, greeted these results with skepticism.

Ernst Von Baer (1792-1876) is credited with the discovery of the mammalian ovum. His famous letter, Epistola de Ovo Mammalium et Hominis Genesi, published in 1827, described the microscopic appearance of the ovum, large enough in mammals to be seen with the human eye and contained within the Graafian follicules of the ovary, first observed in 1672 by Reinier von Graaf (1641-1673) and believed by him to be the ova themselves. In contrast, spermatozoa (except, in some algae) are tiny. It was Rudolf Albert von Kölliker (1807-1905), a friend of von Siebold' who realized in 1841 that spermatozoa are cells, likening them to pollen. Von Kölliker made major contributions to embryology but his recognition of the importance of the microscope to his studies stimulated him to improve the preparation of tissue for microscopic examination. He developed techniques of hardening, sectioning and staining of tissues (a field to which Paul Ehrlich (1854-1915) also made major contributions) which greatly improved the ability to separate cell and tissue types under the microscope. Progress in histology, based to a large degree on the pioneering work of von Kölliker' contributed greatly to medical diagnosis, particularly of cancer. In 1861, Karl Gegenbaur (1826-1903) proposed that all ova are single cells.

Surprisingly, recognizing both sperm and ova as cells did not immediately lead to the idea that fusion of the heritable material contained in each (although it was still unknown where in the cell it resided) would be necessary for the formation of a new individual organism – doubtless due to the almost total lack of understanding of the methods and nature of inheritance. Even Gregor Mendel's (1822-1884) laws of inheritance, discovered through a series of experiments conducted between 1856 and 1863, were ignored or actively rejected until their rediscovery in the 20th century.

A Third Kingdom?

Karl Theodor Ernst von Siebold (1804-1884), like so many of his contemporaries, studied and taught a number of subjects, including zoology, physiology and anatomy. Von Siebold was co-editor of the two volumes of the Lehrbuch der Vergleichenden Anatomie (Manual of Comparative Anatomy, 1846-48) with Hermann Friedrich Stannius (1808-1883), a physiologist and entomologist. This work contributed greatly to the recognition of the relationships between various life forms, although in the absence of an overarching theory remained largely descriptive. Von Siebold was responsible for introducing the terms Arthropoda (insects and crustaceans) while limiting the use of the term Protozoa (from the Greek, meaning the first animals) to single celled organisms (although the latter were still considered to be tiny animals). The term Protozoan is believed to have been introduced by Georg Goldfuss (1782-1848) in 1817 in his book Über die Entwicklungsstufen des Thieres (The Developmental Stages of Animals), prior to any knowledge of cells. It originally included a variety of "simple" animals including microscopic organisms, such as those observed by Leeuwenhoek in plant infusions and lake water, as well, for example, as sponges, corals and jellyfish. Ernst von Haeckel (1834–1919), who was responsible for the introduction of the terms phylum and ecology and strongly supported Charles Darwin's (1809-1882) theory of natural selection (published in 1859 in The Origin of Species by Natural Selection) in Germany, believed that single celled organisms were worthy of their own kingdom, which he proposed, in 1866, should be called Protista (single celled) rather than each organism being arbitrarily labeled as either an animal or plant - terms which, he suggested, should now be confined to multicellular organisms, or metazoans. Since the latter develop from single cells, whether ova or sperm, the cell theory - that all life is comprised of cells and derived from cells - implies that life also began in primordial cells which, through the process of natural selection, evolved into the myriad life forms that subsequently developed. Modern molecular studies of a broad range of life forms suggest, in fact, that life, in all of its tremendous diversity, derives from a single cell that arose some three and a half to four billion years ago, within a billion years of the formation of our planet.

In 1665, Hooke had predicted that the "designs and ends" of nature might well be hidden in the cells he had discovered and that they might someday be uncovered by a diligent observer helped with better microscopes. Prescient though this statement was, he could surely have had no idea quite how vital to the designs and ends of nature the cells he had discovered would prove to be.

Part 2 of The Cell will be published in the next edition of NETWORK.

¹ This epigram is usually ascribed to Virchow, but in fact Virchow quoted it, without reference, from Raspail