The cell’s journey: from metaphorical to literal factory Andrew Reynolds Department of Philosophy and Religious Studies, Cape Breton University, Sydney, Nova Scotia, Canada B1P 6L2 The concept of the cell has been based on metaphor since its inception, and the history of cell theory has continued to rely on metaphor and analogy. In the nine- teenth century, cells were most popularly conceived either as building stones or elementary autonomous organisms from which larger organisms are composed. With advances in physiology and the rise of modern biochemistry in the early twentieth century, the chemi- cal factory or laboratory became the dominant metaphor for this biological unit. Today in the twenty-first century, the metaphorical imagery has become a reality, with cells acting as chemical factories for the synthesis of commercially valuable bio-products. The history of the cell shows how metaphors act as conceptual tools, with particular strengths for facilitating different sorts of questions and experimental techniques. In the beginning The proposition that all living things are composed of small fundamental vital units, otherwise known as cells, ranks next to Darwin’s theory of evolution as one of the great unifying theses of biological science. But like most important concepts in the history of science the cell has undergone an interesting evolution or development of its own. Today it is not at all unusual to run across some version of the following description of what scientists understand a cell to be: ‘All living things are made from cells, the chemical factories of life. Cells act as chemical factories, taking in materials from the environment, processing them, and producing ‘‘finished goods’’ to be used for the cell’s own maintenance and for that of the larger organism of which they may be part. In a complex cell, materials are taken in through specialized receptors (‘‘loading docks’’), processed by chemical reactions governed by a central information system (‘‘the front office’’), carried around to various locations (‘‘assem- bly lines’’) as the work progresses, and finally sent back via those same receptors into the larger organ- ism. Far from being a shapeless blob of protoplasm, the cell is a highly organized, busy place, whose many different parts must work together to keep the whole functioning’ [1]. It is tempting to dismiss such overt metaphorical language as fulfilling a purely communicative or rhetorical role, existing only within the confines of the popular science genre. But this would be a mistake. The history of cell theory offers a rich lesson in the use of metaphor and analogy in scientific thought. The first account of the cell likened it to an empty room, but it has also been conceptualized through the metaphors of a building stone (Baustein), an elementary organism (Elementarorganismus), a chemical laboratory or factory, a motor and a machine. Each of these ways of seeing the cell helped to emphasize certain features under inves- tigation and served to promote particular methodological approaches to further studies. Some metaphors are prim- arily morphological and concerned with structure (for instance, the cell as empty chamber or building stone) while others are primarily physiological (the cell as chemical laboratory or factory, electric motor or machine). The elementary organism The dominant metaphor in the second half of the nine- teenth century described the body as a ‘society’ or ‘state’ of cells (Zellenstaat). Cells were ‘citizens’ arranged into sep- arate classes or professions according to their functions, together making up the ‘economy of the organism’. There are hints of this metaphor in the writings of Matthias Schleiden (1804–1881) and Theodor Schwann (1810– 1882), who are credited with founding the cell theory in the 1830s (Figure 1). It was, however, the German anat- omist and pathologist Rudolf Virchow (1821–1902) who explicitly introduced such language (Figure 2). Using the physiologist Ernst Bru ¨ cke’s (1819–1892) proposal that cells be considered ‘elementary organisms’ (itself an analogy to the way the chemical elements came together to form com- plex molecules), evolutionary zoologist Ernst Haeckel (1834–1919) gave the cell-state metaphor a Darwinian spin: higher plants and animals, he argued, were evolved into colonies of these elemental organisms and humans were little more than a complex colony of protozoan-like cells with a highly evolved division of labor [2]. This ‘theory of the cell state’, as it was called, was of great significance to the fields of anatomy, embryology, phylogenetics and physiology [3]. Early in the twentieth century, this notion was strength- ened still further, as tissue-culture techniques demon- strated that cells removed from the body of a living animal could continue to survive and reproduce in isolation. Yet the thesis that the organism is merely the sum of its cellular parts did not go unchallenged. Criticisms from prominent figures like Thomas H. Huxley (1825–1895), Anton de Bary (1831–1888), Julius Sachs (1832–1897), Charles Otis Whitman (1842–1910), Adam Sedgwick (an embryologist and grandson of the celebrated geologist; 1854–1918) and Clifford Dobell (1886–1949) fell into two Review Endeavour Vol.31 No.2 Corresponding author: Reynolds, A. (Andrew_Reynolds@capebretonu.ca). Available online 29 June 2007. www.sciencedirect.com 0160-9327/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.endeavour.2007.05.005