What is cytology?
Cytology is the study of the appearance of cells, the fundamental units that make up all living organisms. Cells are complex structures constructed from many different subcomponents that work together in a precisely regulated fashion. Each cell must also cooperate with neighboring cells within the organism. A cell is like a complex automobile: Many separate components must be synchronized, and the cell (or car) must follow a strict order of function to coordinate successfully with its neighbors. Because illness results from the malfunction of cells, physicians must be able to measure key cell functions accurately. The normal and abnormal function of cells can be evaluated in many different ways; cytology is the study of cells using microscopes. A sophisticated collection of cytological techniques is available to pathologists; with these a precise diagnosis of cellular malfunction is possible.
All cells share several basic features. They are surrounded by a membrane, a flexible, sheetlike structure which encloses the fluid contents of the cell but allows required materials to move into the cell and waste products to move out of it. The complex salty fluid contained by the membrane is the cytoplasm; the other subcomponents of the cell, called organelles, are suspended in this substance. Each cell contains a set of genes, located on chromosomes, which function as blueprints for all other structures of the cell; the genes are inherited from an individual's parents. In plant and animal cells, the chromosomes are contained in a prominent organelle called the nucleus, which is surrounded by its own membrane inside the cell. Cells must also have a collection of enzymes used to convert food into energy to power the cell. In the cells of animals and plants, these enzymes are packaged into organelles called mitochondria. Membranes, the nucleus, and the mitochondria are the most prominent parts of a cell that are visible with a microscope, but cells also contain a variety of other specialized parts that are required for them to function properly. In addition, cells can also export (secrete) a variety of materials. For example, secreted materials make up bone, cartilage, tendons, mucus, sweat, and saliva.
Despite these basic features, the different types of cells have very distinct appearances. The cells of bacteria, plants, and animals are easily distinguishable from one another using a microscope. Bacterial cells are simplified, lacking organized nuclei and mitochondria. Different kinds of bacteria can be precisely distinguished; for example, strep throat is caused by spherical bacteria that form chains, like beads of a necklace. Some dangerous bacteria can be colored with dyes that do not stain harmless bacteria. Because so many human diseases are caused by bacteria, highly accurate procedures have been developed for their identification.
The adult human body is made up of approximately sixty to ninety trillion individual cells. Although much larger than bacteria, all of these are far too small to be seen with the naked eye (typically about 20 microns in diameter). Each organ of the body—the brain, liver, kidney, skin, and so on—is made up of several kinds of cells, specialized for particular functions. They must cooperate closely: Mistakes in the activities of any of these many cells can cause disease. A pathologist is able to recognize small changes in the appearance of each of many different cell types.
A few of the characteristic cell types in the human body include nerve, muscle, secretory, and epithelial cells. Nerve cells are designed to pass information throughout the nervous system. The nerve cells function much like electrical wires, so they have slender wirelike extensions that can be several feet long. Defects in the wiring circuits—for example, in patients with Alzheimer’s disease—can be readily detected. Muscle cells are easily identified because they are elongated cylinders packed with special fibers that cause muscular contraction. Secretory cells produce and release such substances as digestive enzymes. Such cells are often filled with membrane-bound packets of their specialized product, ready to be released from the cell. The skin and the surfaces of various internal organs are encased in a cell type called epithelium. Epithelial cells are tilelike and are often fastened tightly to neighboring epithelial cells by special kinds of connectors. Numerous other specialized cell types are found in the body as well, but these four types represent the most common cell designs.
Cells are sophisticated and delicate structures that carry out specific functions efficiently. The structure and function of normal cells are stable and predictable. If significant numbers of cells are somehow damaged, disease is the result. Such defective cells change in their appearance in characteristic ways. Therefore, cytology is an important element in the diagnosis of many diseases and for monitoring the cellular response to therapy.
Many different types of stress can cause cell damage. One of the most common stresses is oxygen deprivation, known as hypoxia. Even a brief interruption of oxygen can cause irreversible damage to cells because it is needed for energy production. Since oxygen is transported in the blood, the most common cause of hypoxia is loss of blood supply, which can occur with trauma, blockage by blood clots or narrowed blood vessels, and several kinds of lung or heart problems. Carbon monoxide poisoning results from interference with the blood’s ability to absorb and carry oxygen, while the poison cyanide interferes with a cell’s ability to make use of oxygen.
Poisons such as cyanide can damage cells in many other ways, as can drugs and alcohol. Prolonged use of barbiturates or alcohol can damage liver cells. These cells are also sensitive to common chemicals such as carbon tetrachloride, once used widely as a household cleaning agent. The liver is where foreign chemicals are changed to harmless forms, which explains why the liver cells are often damaged. Even useful chemicals, however, can cause harm to cells in some circumstances. Constant high levels of glucose, a sugar used by all cells, may overwork certain cells of the pancreas to the point where they become defective. Some foods (especially fats) and certain food additives, if they are eaten in excess, can interfere with cell function.
Physical damage to cells—caused, for example, by blows to the body—can dislocate parts of cells, preventing their proper coordination. Extreme cold can interfere with the blood supply, causing hypoxia; extreme heat can cause cells to speed up their rate of metabolism, again exceeding the oxygen-carrying capacity of the circulatory system. The “bends,” the affliction suffered by surfacing deep-sea divers, results from tiny bubbles of nitrogen that block capillaries. Various kinds of radiant energy, such as radioactivity or ultraviolet light, can damage specific chemicals of cells, causing them to malfunction. Electrical energy generates extreme local heat within the body, which can damage cells directly.
Many small living organisms can interfere with cellular function as well. Viruses are effective parasites of cells, using cells for their own survival. This relationship can result in cell death, as in poliomyelitis; in depressed cell function, as in viral hepatitis; or in abnormal cell growth, as in cancers. (Cancer occurs when the genetic material of a cell becomes damaged, causing mutations that lead to uncontrolled cell growth and division. Several viruses are known to cause cancer, including the human papillomavirus, or HPV.) Bacteria can also live as parasites, releasing toxins that interfere with cellular function in a variety of ways. Malaria is caused by a single-celled animal that damages blood cells, athlete’s foot is caused by a fungus, and tiny worms called nematodes can invade cells and cause them to work improperly.
All cell types are not equally sensitive to damage by each agent. Liver cells are particularly sensitive to damage by toxic chemicals. Nerve and muscle cells are the first to be injured by hypoxia. Kidney cells are also easily damaged by loss of blood supply. Lung cells are affected by anything that is inhaled.
Before cells can be successfully observed, they must be prepared through several steps. First, it is necessary to select a relatively small sample of a particular organ for closer scrutiny. Such a sample is called a biopsy when it is collected by a physician who wishes to test for a disease. The biopsy must then be preserved, or fixed, so that its parts will not deteriorate. Next, the specimen must be encased within a solid substance so that it can be handled without damage. Most often, the fixed specimen is soaked in melted paraffin, which then is allowed to solidify in a mold. For some kinds of microscopes, harder plastic materials are used. Next, the specimen must be thinly sliced so that the internal details can be seen. The delicate slices are mounted on a support, typically a thin glass slide for light microscopy. Finally, the parts of the cell must be colored, or stained. Without this coloring, the cell parts would be transparent and thus unobservable.
The basic tool of the cytologist is the light microscope. It can magnify up to about one thousand times. Numerous sophisticated methods are used with light microscopy. Specific stains have been developed for distinguishing the different molecules that make up cells. For example, Alcian blue is a dye that stains a type of complex sugar that accumulates outside certain abnormal cells, making it easier to identify these cells. Also, specially prepared antibodies can recognize particular proteins within cells. Disease-causing proteins, including the proteins of dangerous viruses and bacteria, can be precisely identified in this way.
A major advance in cytology is the electron microscope . It forms images in essentially the same way as a light microscope does, but using electrons rather than visible light. Because of the properties of electrons, this type of microscope can magnify up to one million times beyond life size. A wide range of new cell features has been revealed with the electron microscope. The details of how genes work, how materials enter and leave cells, how energy is produced, and how molecules are synthesized have been made clearer. The steps for preparing specimens for electron microscopy are delicate, time-consuming, and demanding. Furthermore, the electron microscope itself is complex and expensive. Considerable skill is required to use it effectively. For these reasons, electron microscopy is not commonly used for routine medical diagnoses.
Cell injury causes predictable changes in cells that can be interpreted by a pathologist to suggest the underlying cause of the damage and how best to treat it. Almost all forms of reversible injury cause changes in the size and shape of cells. Cellular swelling is an obvious symptom that almost always reflects a serious underlying problem. Such cells also have a characteristic cloudy appearance. Swelling and cloudiness indicate loss of energy reserves and abnormal uptake of water into the cell through improperly functioning cell-surface membranes. An indication of serious damage is the accumulation within the cell of vacuoles—small, fluid-filled sacs that have a characteristic clear appearance when viewed through a microscope. More severe injury can cause the formation of vacuoles that contain fat, giving the cells a foamy appearance. Such damage is most often seen in cells of the heart, kidneys, and lungs. These changes appear to reflect both membrane abnormalities and the defective metabolism of fats.
Cells that are damaged beyond the point of repair will die, a process called necrosis. The two key processes in necrosis are the breakdown and mopping up of cellular contents, and large changes in structure of cellular proteins in ways that can be identified using a microscope. The most conspicuous and reliable indicators of necrosis are changes in the appearance of the nucleus, which can shrink or even break into pieces and which eventually disappears completely. Ultimately, the entire cell disappears.
Cancer provides a good illustration of how cytology is employed in the diagnosis of a specific disease. A skilled cytologist can detect cells at an early stage of cancer development and, with accuracy, can gauge how dangerous a cancer cell is or is likely to become. Cancer is a disease of abnormal growth. Cancer cells may have few abnormal features other than their improper growth; tumors made up of such cells are generally not dangerous and so are labeled benign. Malignant tumor cells, on the other hand, are highly abnormal. They can damage and invade other parts of the body, making these cells much more dangerous.
The cells of benign tumors may have nearly the same appearance as the cells of the normal tissue from which they arose. Benign cancers of skin, bone, muscle, and nerve maintain the obvious structures that allow these highly specialized cell types to carry out their normal functions. Ironically, however, continued normal function can itself become a problem, because there are too many cells producing specialized products. For example, tumors in tissues that produce hormones can result in massive excesses of such hormones, causing severe imbalances in the function of the body’s organs. Malignant tumor cells, on the other hand, have lost some or all of the functional and cytological features of their parent normal cells. They have a simpler and more primitive appearance, termed anaplasia by pathologists. The degree of anaplasia is one of the most reliable hallmarks of how malignant a cell has become.
Almost any part of the cell can become anaplastic. A common change is in the chromosomes of a cancer cell. The number, size, and shape of chromosomes change, and detailed analysis of these changes is often important in diagnosis, as in leukemia. Many malignant tumor cells secrete enzymes that attack surrounding connective tissue, changing its appearance in characteristic ways. Membrane systems of anaplastic cells are also abnormal, with serious consequences. The movement of materials in and out of cells becomes defective, and energy production mechanisms are upset, causing the characteristic changes in appearance described above. A general feature of tumors made up of anaplastic cells is the variability among individual cells. Some cells can appear virtually normal, while other tumor cells nearby can appear highly abnormal in several ways.
The cells of benign tumors remain where they arose. The cells of malignant tumors, however, have the ability to spread through the body (metastasize), penetrating and damaging other organs in the process. These abilities, to invade and metastasize, have serious effects on the rest of the body. Invading cells often can be identified easily with a microscope. Extensions of the tumor cells may reach into surrounding normal organ parts. Tumor cells can be observed penetrating into blood and lymph vessels and other body cavities, such as the abdominal cavity and air pockets in the lung. Small clusters of tumor cells can be found in blood and identified in distant organs. These cells can begin the process of invasion all over again, producing so-called secondary tumors in other organs. How malignant cancer cells can cause so much harm becomes clear.
Of the diagnostic procedures that are available to physicians, cytologic techniques are among the most effective. Because the cells being examined are so tiny, the microscopes used must be able to magnify the cells enough to allow observation of their characteristics. Historically, the use of cytology in medical practice has closely paralleled the development of adequate microscopes and methods for preparing specimens.
Magnifying lenses by themselves lack the power required for observing cells. A microscope of adequate power must use several such lenses stacked together. The first crude microscopes with this design appeared late in the sixteenth century. During the next several hundred years, microscopes were mostly used to observe cells of plant material because the woody parts of plants can be thinly sliced and then observed directly, without the need for further preparation. The word “cell” was first employed by Robert Hooke (1635–1703) in a paper published in 1665. He observed small chambers in pieces of cork, which were where cells had been located in the living cork tree. These chambers reminded Hooke of monks’ cells in a monastery, hence the name.
The great anatomist Marcello Malpighi (1628–1694) may have been the first to observe mammalian cells, within capillaries. The real giant of this era, however, was the Dutch microscopist Antoni van Leeuwenhoek (1632–1723), who greatly improved the quality of microscopes and then used them to observe single-celled animals, bacteria, sperm, and the nuclei within certain blood cells. Although most progress continued to be made with plants, numerous observations accumulated during the seventeenth and eighteenth centuries which suggested that animals are made up of tiny saclike units, and Hooke’s word “cell” was applied to describe them. This concept was clearly stated in 1839 by Theodor Schwann (1810–1882); his idea that all animals are composed of cells and cellular products quickly gained acceptance. At this time, however, there was essentially no comprehension of how cells work. Without an understanding of normal cell function, cytology was still of little use in identifying and understanding disease.
During the late nineteenth and early twentieth centuries, the appearance of different cell types was carefully described. The main organelles of cells were identified, and such fundamental processes as cell division were observed and understood. At last it was possible to utilize cytology for medical purposes. The principles of medical cytology were established by the great pathologist Rudolf Virchow (1821–1902), who suggested for the first time that diseases originate from changes in specific cells of the body.
Rapid progress in cytology was made in the 1940s and 1950s, for two reasons. First, improved microscopes were developed, allowing greater accuracy in observing cell structure. The second reason—rapid progress in genetics and biochemistry—greatly increased the knowledge of how cells function and of the significance of specific changes in their appearance. Because cells are the basic units of life, scientists will continue to study them in detail, and the medical world will benefit directly from further, improved understanding in this field.
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