single cell with many nuclei. This rather unusual situation is the result of an event that occurs in the embryo when the cells that give rise to the fibers fuse together, pooling their nuclei in a common cytoplasm (the term cytoplasm is historically a crude term meaning the semi‐viscous ground substance of cells; we use the term to mean everything inside the plasma membrane except the nucleus). The mechanism of muscle contraction will be described in Chapter 13.
Cells multiply by division. In the human body an estimated 25 million cell divisions occur every second! These provide new cells for the blood and immune systems, for the repair of wounds and the replacement of dead cells. In complex tissues such as those described above, division is restricted to a small number of undifferentiated stem cells that are capable of dividing many times; some of their daughter cells then differentiate to become all of the other cells of the tissue. In the case of the intestine, folds in the surface epithelium form crypts, each of which contains ~250 cells (Figure 1.6). Mature cells at the top die and must be replaced by the division of between four and six stem cells near the base of the crypt. Each stem cell divides roughly twice a day, the resulting cells moving up the crypt to replace those lost at the surface. Benign (non‐cancerous) polyps can be formed in the intestine if this normal balance between birth and death is disturbed.
As in the intestine, stem cells in other tissues exist in specific locales, called niches, with environments that support their special and vital functions. In many tissues the requirement to replace dead cells is much less than it is in the intestine and in such cases the stem cell niche must maintain its occupants in a quiescent (nondividing) state until needed (for more on stem cells see In Depth 14.1 on page 234).
Many types of cell, particularly bacteria and plant cells, create a rigid case around themselves called a cell wall. For cells that live in an extracellular medium more dilute than their own cytosol, the cell wall is critical in preventing the cell bursting. For example, penicillin and many other antibiotics block the synthesis of bacterial cell walls with the result that the bacteria burst. Within trees, plant cells modify the cell wall to generate the woody trunk. Animal cells do not have cell walls.
Many different techniques have contributed to our understanding of the structure of cells but nothing can compare to actually seeing what is there. Microscopy, the visualization of small objects, began with Robert Hooke (1635–1703) who described the cella (open spaces) of plant tissues. But the colossus of this era of discovery was Anton van Leeuwenhoek (1632–1723), a Dutchman with no scientific training but with unrivaled talents as both a microscope maker and as an observer and recorder of the microscopic living world. Van Leeuwenhoek's microscope was a single glass lens that bent light rays to form a magnified image so it, and all the later instruments that use visible light to image small structures, are called light microscopes.
The Modern Light Microscope
Today's light microscopes, such as those one would find in a school laboratory, consist of a light source, which may be the sun or an artificial light, plus three glass lenses: a condenser lens to focus light on the specimen, an objective lens to form the magnified image, and a projector lens, usually called the eyepiece, to convey the magnified image to the eye (Figures 1.8 and 1.9). Since the image is formed by light passing through the specimen, this is a transmission light microscope. Depending on the focal length of the various lenses and their arrangement, a given magnification is achieved. In bright‐field microscopy, the image that reaches the eye consists of the colors of white light minus those absorbed by the cell. Most living cells have little color and are therefore largely transparent to light. This problem can be overcome by cytochemistry, the use of colored stains to selectively highlight particular structures and organelles. However, many of these compounds are highly toxic and to be effective they often require that the cell or tissue is subjected to a series of harsh chemical treatments. A different approach, and one that can be applied to living cells, is the use of phase contrast microscopy. This relies on the fact that light travels at different speeds through regions of the cell that differ in composition. The phase contrast microscope converts these differences in refractive index into differences in contrast, and considerably more detail is revealed (Figure 1.10). Transmitted light microscopes can distinguish objects as small as about half the wavelength of the light used, so about 250 nm (nm, 1 nm = 1/1000 μm). They can therefore be used to visualize the smallest cells and the major intracellular structures and organelles (Figure 1.11a).
The Transmission Electron Microscope
In principle the smaller the wavelength of the radiation used to image a structure, the better the resolution. This fact led to the invention in 1931 of the transmission electron microscope by Max Knoll and Ernst Ruska. An electron gun generates a beam of electrons by heating a thin, V‐shaped piece of tungsten wire to 3000 °C. A large voltage accelerates the beam down the microscope column, which is under vacuum because the electrons are slowed and scattered if they collide with air molecules. The beam passes through the specimen and is bent by powerful magnets to form a highly magnified image (Figure 1.11b). This image can be viewed on a fluorescent screen that emits light when struck by electrons. While the electron microscope offers enormous resolution, electron beams are potentially highly destructive, and biological material must be subjected to a complex processing schedule before it can be examined. The preparation of cells for electron microscopy is summarized in Figure 1.12. The transmission electron microscope produces a detailed image but one that is static, two‐dimensional, and highly processed (Figure 1.4). Often, only a small region of what was once a dynamic, living, three‐dimensional cell is revealed. Moreover, the picture revealed is essentially a snapshot taken at the particular instant that the cell was killed (“fixed”). Clearly, such images must be interpreted with great care. Also, electron microscopes are large, expensive, and require a skilled operator. Nevertheless, since they can resolve objects down to about 0.2 nm in size, they are the main source of information on the organization of the cell at