The examination of such similarities can lead us still further. Studies of sections of such retinal rods have confirmed Dr. Fritiof Sjöstrand’s earlier work (on isolated fragments) and shown that the stacks of plates, when sectioned in the proper plane, appear as very regular, platelike lamellae. This arrangement of the photoreceptors has also been confirmed as the structure of the chlorophyll-containing grana, or granules, of the chloroplast, where plants manufacture their starch. The very regular lamellar construction of photoreceptors—in animal and plant alike—is a striking feature that bears further investigation.
Long ago, an ingenious investigator showed that if electrodes are placed on two spots of a green leaf, and one of the electrodes is shaded from the sun, the electrode in the light becomes electronegative to the electrode in the shade. This reaction can also be obtained when electrodes are placed on the green and white parts of a variegate leaf—the green part is negative to the white when light falls on both. Since the white part of the leaf has no chlorophyll, this electrical evidence has been interpreted as a demonstration that chlorophyll is necessary for the leaf’s perception of light. Similar electrical patterns can be recorded from the retinal rods of animals when the retina is stimulated by a flash of light; and we know, as well, that vitamin A is essential for the function of these rods. Recently, some electron microscope studies on chloroplasts from white leaves (the plants were grown in the dark) showed that the usual lamellated appearance of the grana was not present. When the plants were put in the light and recovered their green color, the lamellae of the grana also reappeared. It was suggested, therefore, that chlorophyll is in some way necessary for the appearance of the lamellae of the grana. If the lamellae are really uniformly arranged molecules of protein and fat, as has been suggested, we now have to answer this question: was it the light, or the chlorophyll, which caused the very regular appearance of the layered structure?
Trying to answer such a question is something like trying to find out what the shade of an unexposed bit of photographic paper is. If one turns on the light to see, paper’s shade may be changed by the action of the light needed to observe it. Perhaps one way of getting at this problem would be to look at the photoreceptors of animals raised in the dark. If their retinal rods showed a lamellated structure, then perhaps some essential metabolite—such as vitamin A—might be removed from their diet. If vitamin A-deficient animals raised in the dark showed a nonregular structure of the light receptors, then some animals could be given the metabolite to see whetherit would cause the reappearance of the lamellae. Others could merely be allowed to see light, to determine what effect this energy had in aligning the molecules.
Another interesting structure within cells which has been the subject of electron microscopic studies is the Golgi complex. The Golgi complex has been the matter of controversy for fifty years—some investigators bitterly contend that all evidences for this juxtanuclear apparatus are artifact, to which others retort that the only artifacts present were those caused by the complaining experimenters themselves because they are not using the technique correctly. With the help of careful correlation between living and fixed cells studied with the light microscope, and electron microscope studies on the same material, it seems that the controversy is at least partially resolved. A system of membranes, granules and vacuoles has been seen near the nucleus of almost every cell type examined in the electron microscope, and it seems justifiable to conclude that this structure must be the basis for what was described earlier by light microscopists.
Apart from the similar appearance of the membrane component of the Golgi complex in many different cell types, it is becoming obvious that the other components, the vacuoles and granules, are different in different cell types. In the absorbing cells of the kidney tubule, there are often large vacuoles between the membranes; in the absorbing cells of the intestine, these same vacuoles often contain small black granules. Could they represent some difference in hat the Golgi complex is metabolizing? The same large vacuoles are present in the zymogen-secreting cells of the pancreas, but they do not exist in the epithelial cells of the cornea, which functions only as a transparent covering for our eyes, and so far as we know does not produce or absorb any foodstuffs for our general use. In these same cells of the pancreas, large granules of the digestive enzyme zymogen are produced, and these granules seems to be most intimately associated with the Golgi membranes. In fact, a whole range of different-sized granules can be seen in and around the Golgi complex. We wonder if these large and small granules represent various stages in the development of the enzyme, which is then discharged in this form to where it helps digest foodstuffs.
Interpreting electron micrographs can lead the most careful investigators astray, if they are not aware of the many factors to be considered before they can draw summary conclusions. One of the most difficult problems is to reconstruct what we know are dynamic processes from the static pictures we obtain in the electron microscope. Also, before any study is complete, it is necessary to know something of the variations which can be manifested even in supposedly “normal” biological material. This means that any comprehensive investigation must include a study of many samples from different organisms. Only then is it time to alter conditions experimentally in an effort to see how infections, injury or different functional states may be reflected by changes in the structure of the tissue.
It has often been remarked that the best experiment is the simple experiment. Once systematic electron microscope investigations of “normal” tissues have been completed, it should be possible to begin experiments where, in so far as possible, only a single variable has been introduced. Using specific enzyme poisons or removing an essential metabolite (such as a vitamin) should cause changes to appear which, while they are not easily seen with the light microscope, would be obvious at the macromolecular level. Such approaches should yield new information in which morphological and biochemical knowledge can be correlated. Dr. Lindestrom-Lang, working at the Carlsberg laboratories in Copenhagen, showed very clearly how such correlations could be achieved at the level of the light microscope when he cut alternate sections from a frozen cylinder of tissue and did chemical studies on one slice and microscopy on the next. A similar study with the electron microscope could help to identify the dense and non-dense structures seen in electron micrographs. Another technique for correlating biochemical properties with morphological appearance may be found in applying the principle that different substances absorb light of different wave lengths. Professor Caspersson, in Stockholm, has devised methods which allow him to make not only qualitative but quantitative determinations on particles as small as one-tenth of a square micron. All of these techniques offer new possibilities to the biologist who would better understand the complex nature of life.
In the future, the electron microscope will be used on an infinite number of problems—ranging from what may be the causes for cancer to what is the structure of an enzyme. But, at the moment, the most inviting problems seem to lie in correlating some of the new information we have already gleaned about biological material with the functional properties of these submicroscopic structures. The most difficult thing to do is to ask biological questions in such a way that they can be answered. We hope the electron microscope may be used as a new key with which to unlock some of these still-closed doors.