Putting the Life in Cells
Several main themes in the nineteenth and into the twentieth century help illuminate the difference understandings. First is understanding of Schleiden and Schwann’s cell theory, then fertilization, the relative roles of nucleus and cytoplasm, cell lineage and development, cell-cell interactions, regeneration, and cell culture outside of the organism. Here, we can look briefly at each of these points.
Schleiden and Schwann in 1838 gave cell theory a name and declared first that cells exist and are constituents of living organisms, and second that the theory might help explain individuality of organisms as clusters of connected cellular units. As Henry Harris aptly puts it, Schleiden’s long article on cells in plants “does not make pleasant reading.” Fortunately, Harris helps us digest the key points. Schleiden, like Schwann, saw the nucleus (which he called the cytoblast) as centrally important. They each held that the nucleus is the structure that appears first and then generates the cell. From the moment he had encountered Schleiden’s ideas during a conversation at dinner one evening, Schwann claimed, “I devoted all my energies to demonstrating the pre-existence of nuclei in the formation of cells.” Sometimes the nucleus exists alone and the cell crystallizes around it, sometimes the cells divide and each has a nucleus.
Since this is a central point in the reasoning, it is worth quoting Schleiden at some length, and Harris provides an excellent translation explaining how cells arise:
As soon as the cytoblasts have attained their full size, a delicate, transparent vesicle is formed on their surface. This is the young cell which to begin with appears as a very flat segment of a sphere, with its planar side constituted of the cytoblast and its convex side by the young cell which is superimposed on it much like a watch glass on a watch. ... Little by little the whole cell now grows out over the edge of the cytoblast and soon becomes so big that the latter eventually appears as no more than a small body enclosed within one of the parietal walls.
Schwann accepted Schleiden’s interpretation, even though he saw cell division in addition to what he interpreted as crystallization in the animal cells he studied. Therefore, both emphasized the role of the nucleus as a “cytoblast,” or literally cell- developer. Each individual cell emerged and cells then served as structural parts of the organisms in which they resided. And yet, Harris emphatically notes, “I think it is fair to say that no part of the scheme proposed by Schleiden turned out to be correct.” Harris provides suggestions about why these German cytologists gained so much attention at the time and later, but for our purposes, the point is that they emphasized the structure of the cells and saw them as not the only, but some of the structural parts of the living organisms. In fact, much of what has been credited to Schleiden and Schwann came later.
The decades following brought a great deal of additional observation as well as interpretation. Those years also brought improvements in both microscopes and microscopic techniques, as Hughes discusses in detail. For studying cells, it makes a big difference what one can see and how well one can see it; and making sure that others can see the same thing is especially important. Better lenses reduced chromatic aberrations, and better fixing, staining, and slicing methods improved standardization of specimens to improve consistency of observation. But it wasn’t just being able to see more that mattered. It was also looking more carefully, and with a more open mind than Schleiden and Schwann seem to have had.