What is pathology? |

Science and Profession

The science of pathology seeks to identify accurately the etiology of a disease and its development in the human body, which in turn leads to other studies focused on the diagnosis, treatment, and prevention of the disease.

A pathologist may be a medical doctor or hold a doctoral degree in a related field such as cell biology or microbiology. He or she may be employed in one of several different types of work, such as research, clinical, surgical, and forensic pathology. The human body is a complex structure and, as in all living organisms, its basic unit of function is the cell. Similar cells are organized into tissues, tissues form organs, and organs are grouped into systems. All systems in the body must work together to maintain homeostasis. If this internal stability is changed too drastically, a disease results. To understand a disease thoroughly—and ultimately to diagnose, prevent, or treat it—its pathogenesis must be understood. This term includes the cause of the disease, its method of damaging the body, and the changes resulting from its presence. A specific disease may have many causes, and its symptoms and severity may vary in different patients. Nevertheless, it is convenient to place a disease into one of seven groups based on its primary cause or manifestation in the body: genetic defects, infections, immune disorders, nutritional disorders, traumas, toxins, and cancers.

Genetic disorders are those caused by a defect in one or more genes. Genes, which are found on chromosomes, are duplicated and passed from one generation to another. Each gene is responsible for directing the manufacture of a protein. Every protein has a characteristic three-dimensional structure, on which its function depends. If a mutation occurs in a gene, the protein may function poorly or not at all. Although the pathogenesis of a genetic disease may involve devastating effects in the body, its origin may be traced to the function of a single protein. A person with hemophilia may bleed to death because of a lack of the gene for blood-clotting protein. In sickle cell disease, hemoglobin molecules are abnormal, which results in abnormal red blood cells and difficulty in transporting oxygen throughout the body. Changes in gene function occur throughout a person’s life. Many degenerative changes associated with aging are believed to be caused by aging genes and the subsequent loss of cell, tissue, and organ function.

Infections are those diseases that are caused by other organisms, usually microorganisms such as viruses, bacteria, protozoa, or fungi. Such organisms, called pathogens, are parasitic on body tissues or fluids. The damage from pathogens may be direct, resulting from tissue destruction, or may be caused by the toxins that they produce. Entamoeba histolytica, for example, is a protozoan that is ingested in contaminated water or food. It begins feeding on the tissues of the intestine and may cause ulcerlike lesions in the intestinal wall. Clostridium botulinum is the bacterial species that causes botulism. Its deadly effects are attributable not to tissue destruction but to its production of a poison that attacks the nervous system.

Immune disorders include immune deficiencies, autoimmune diseases, and allergies. The body’s immunity involves a complex system of checks and balances to protect against invasion by foreign cells or substances. It is the main defense against infectious diseases. When a person’s immune system is not functioning properly, the body is unable to fight off infections. Some individuals are born with immune deficiencies. Others may acquire the deficiency later in life through the use of immunosuppressive drugs to prevent rejection of an organ transplant or because of an infectious disease such as acquired immunodeficiency syndrome (AIDS). Regardless of the primary cause of the immune deficiency, the patient may die as a result of an infection that would be considered harmless in the general population. Another group of immune disorders includes the autoimmune diseases. In these disorders, the immune system begins to make antibodies against the body’s own tissues. For example, joint tissue is destroyed in rheumatoid arthritis, and nerve tissue is destroyed in multiple sclerosis. Allergies represent a third group of immune disorders. An allergic response is an overreaction to a substance that would ordinarily be considered harmless by the body. During this reaction, a chemical called histamine is released that causes such changes as rashes and upper respiratory symptoms. In more severe reactions, asthma or circulatory system collapse may occur.

Nutritional disorders include dietary deficiencies, excesses, and imbalances. Vitamins and minerals are needed to take part in certain chemical reactions in the body. In vitamin deficiencies, these reactions are blocked. For example, vitamin A is needed for the proper functioning of the nerve endings in the eye that are responsible for seeing black and white and in dim light; thus, a person with a vitamin A deficiency may develop night blindness. Proteins, carbohydrates, and fats must be ingested in sufficient amounts to supply energy and raw materials to build body tissue. Excessive consumption, however, is associated with such conditions as obesity, diabetes, and heart disease.

Trauma generally refers to injury done to the body by an external force, such as in an automobile accident. The damage to the body may be relatively minor but still have serious effects. Damage to a blood vessel may cause hemorrhaging and subsequent loss of blood. Injury to the brain or spinal cord may result in paralysis or loss of other body function. Physicians also use the word “trauma” to mean any occurrence that damages the body or organs. High fever, for example, may be said to cause trauma to the brain. Extreme emotions may also have effects on the body.

Toxins are poisons that may originate from the surroundings of an individual and be absorbed through the skin or inhaled. A person can be overcome by carbon monoxide from a defective furnace or automobile heater, for example. Toxins may also be ingested in water or food. Drug overdoses or accidental ingestion of household chemicals will also cause toxic reactions. Medical ecology is a field of medicine that concerns itself with the long-term toxic effects of chemicals released into the environment from plastics or other materials associated with modern life.

Cancers arise when the cell division process becomes abnormal. Ordinarily, cells in the body divide at a limited rate that is characteristic of a particular tissue. When cells divide rapidly in an uncontrolled manner and metastasize (spread) to other parts of the body, the condition is termed a malignancy or cancer. Cancers are grouped according to the type of tissue from which they develop. Carcinomas arise from epithelial tissue such as skin. Sarcomas arise from connective tissue such as bone or muscle. Leukemias result from abnormal and rapid reproduction of white blood cells in the bone marrow. Lymphomas are cancers of the lymphatic tissues, such as the lymph nodes or spleen. The severity of the cancer and the chances for recovery depend on the extent of the cancer when first diagnosed, the type of tissue or location involved, the speed at which the cells are dividing, and whether the cancer has spread to other areas of the body.

While it is convenient to place pathologies into separate groups, in reality their causes and effects overlap. An individual who is malnourished may have a weakened immune system and be vulnerable to infectious diseases. The immune system attacks not only foreign cells from outside the body but also abnormal cells arising within the body, such as cancer. Thus, patients with AIDS often develop cancer. Some individuals are at increased risk for cancer because of genetic factors called oncogenes. If these individuals smoke or eat unwisely, they may develop cancer, while others with the same inherited risk who adopt a prudent lifestyle do not. It is important for researchers and physicians to understand these complex pathological relationships so that they can diagnose and treat such conditions.

Diagnostic and Treatment Techniques

A pathologist must be familiar with the typical test values associated with body functions and with the microscopic appearance of healthy cells and tissues in order to be able to differentiate correctly a normal condition from a pathological one. This body of knowledge has involved the collection of tissues and the measurement of values gleaned from years of medical treatment and research. Although based on large populations, these values may be misleading. For example, the amounts of blood cholesterol were measured in Americans, and the average level of cholesterol was labeled “normal.” In reality, this value has been shown to be unhealthy and linked to heart disease.

Research pathology concentrates on the basic study of diseases or disorders. This type of study usually focuses on the cellular and biochemical aspects of a disease. Information is exchanged with other researchers in an effort to gain a complete understanding of the etiology of a disease and the mechanisms by which it damages the body. Laboratory experiments may be performed to determine if the condition can be stopped or at least slowed.

Other pathologists are more closely associated with patient treatment. A clinical pathologist is involved in the diagnosis of disease through study of body fluids, secretions, and excretions. Such a person must be knowledgeable in hematology (the study of blood), microbiology, and chemistry. A surgical pathologist is responsible for testing samples of cells and tissues excised during surgery. In such a procedure, called a biopsy, a small section of tissue is taken from the affected area and sent to the laboratory. The pathologist then examines the sample using a microscope and determines whether the tissue is normal or whether it indicates the presence of cancer or some other disease state.

At one time, it was necessary to perform what was known as exploratory surgery if laboratory tests and x-rays did not lead to a diagnosis. Less invasive, and safer, techniques have been developed to eliminate that need. Fiber-optic methods involve the use of light shining through a flexible tube containing glass fibers. The flexible tube can be inserted through a body opening or through a small incision, allowing the physician to see internal cavities and determine if any visible abnormalities are present or to perform a biopsy. In some cases, the condition can be corrected without further surgery.

Many other tools and techniques are available to clinical and surgical pathologists. Some abnormalities can be observed directly, such as a rash or external tumor. Microscopes are used to detect changes that are too minute to be seen by the naked eye. Another tool used to diagnose disease or disorders is the medical x-ray, which can determine the presence of a broken bone, kidney stones, or dense tumors. Soft tissues are not so easily visualized using normal x-ray techniques. Ingestion of an opaque substance such as barium may help to delineate the outline of a structure such as the esophagus or intestine. Other medical imaging procedures, such as ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT) scans, allow even more detailed visualization of body structures.

An autopsy pathologist examines the body after death. In a hospital setting, the purpose of an autopsy is usually to confirm or determine the natural cause of death. Even when the cause is known, an autopsy may be requested, since information obtained in this way may lead to better understanding of the pathological processes that resulted in death and suggest possible ways of preventing deaths in the future.

A forensic pathologist, also called a coroner or medical examiner, is an autopsy pathologist who works closely with the police and criminal justice officials. This relationship is especially important if there are suspicious circumstances surrounding the death or the discovery of the body, in order to determine whether the death has resulted from an intentional poisoning or a violent act. In some cases, a coroner must also attempt to identify the body. Such abnormalities in the body as scars, healed fractures, and dental cavities can aid in this effort. Samples of cells may be used for DNA fingerprinting, a comparison of genetic material from the deceased to that of someone believed to be a close relative, in an attempt to verify identification. If it is determined that a murder has occurred, the pathologist contributes to the investigation by determining such facts as the cause and time of death. Special care must be taken during any autopsy since the information may be used as legal evidence.

The correct diagnosis and identification of the etiology of a disease are essential to providing insight into possible treatments. Pathologists also play a key role in determining the mode of transmission. The following historical examples illustrate this application of pathology.

In the 1930s, it was discovered that the urine of some developmentally disabled children had a peculiar odor. Analysis of the urine showed the presence of an abnormal chemical that damaged the nervous system, resulting in mental deficiencies. The chemical was found to result from a failure of the body to make an enzyme necessary to break down the amino acid phenylalanine, which in turn was the result of a genetic defect, termed phenylketonuria (PKU). Simple blood and urine tests can determine the presence of this defect at birth. While this disease cannot be cured, those with PKU can be placed on a special diet low in phenylalanine, and so avoid the buildup of the amino acid and its resulting damage. In this genetic disease and others, once the precise mechanism of damage is found, efforts can be made to identify the presence of the gene and then to lessen its effects.

In July 1976, more than five thousand members of the American Legion attended a convention in Philadelphia, Pennsylvania. Within two weeks of returning home, nearly two hundred of them became ill, and twenty-nine of these died. Laboratory tests and autopsies identified the process that caused death as severe pneumonia accompanied by high fever but could not determine its origin. Although a pathogen was suspected, none could be found through microscopic examination of tissues or culture studies. Attention turned to the hotel and its air conditioning system in an attempt to determine if some toxin had spread through the air ducts, but no such substance was found. Several months later, a pathologist at the Centers for Disease Control in Atlanta, Georgia, was examining lung tissue sections taken from chick embryos and discovered the bacteria that had caused the illness, now known as Legionnaires' disease. Further tests showed them to be sensitive to the antibiotic erythromycin. When subsequent cases occurred, prompt diagnosis of the disease allowed the correct treatment to be given. The presence of these bacteria in water tanks associated with large air conditioning systems has led to preventive measures.

In 1981, the first cases of AIDS were identified. While the mode of transmission was discovered fairly quickly, it was not until 1986 that the human immunodeficiency virus (HIV) was identified. The pathogenesis of this disease begins with the infection of white blood cells, called helper T cells, that are an essential link between the identification of an invading pathogen and the production of antibodies by other cells called B cells. At first, the body begins to make antibodies against the virus as it would any other infectious disease. Then, over a period of several years, an increasing number of T cells are infected and destroyed. Eventually, the body loses the ability to make the antibodies necessary to fight all infectious diseases and to destroy cancer cells. After further study of the virus, researchers were able to discover a chemical, azidothymidine, or AZT (later called zidovudine), that could interfere with a key enzyme needed by the virus to reproduce. Although it could not cure the disease, the drug slowed its effects. AIDS research efforts are not aimed solely at attempts to kill the virus: by studying its pathogenesis, the means may be found to counteract the effects of the disease on the body’s immune system.

Perspective and Prospects

In the early days of medicine, knowledge of pathology was limited to what could be observed directly through the human senses. Treatments were empirical, a matter of trying different drugs or procedures until one was found that worked. Most basic knowledge of human anatomy and physiology was lacking. Often, human autopsies were not permitted because of religious and cultural practices.

A culture’s beliefs about disease influence medical practice. From the time of the ancient Greeks until the rise of modern medicine, various theories were accepted. Some believed that disease had supernatural origins. The term influenza, for example, came from the belief that the disease was caused by the influence of the stars. Others believed that the body’s functions were dependent on fluids in the body, called humors. Bleeding was used to release the bad humors that were causing the disease.

By the seventeenth century, dissection of cadavers was practiced to identify completely the normal and abnormal gross anatomy of the human body. The microscope was developed and used to study human tissues. In the late nineteenth century, it was shown that microorganisms could cause disease, which in turn led to specific tactics aimed at prevention and treatment. With knowledge of the infectious process and the development of anesthesia, surgery became more widespread. This trend, in turn, increased the knowledge of disease processes in tissues and organs.

In the twentieth century, more sophisticated techniques were developed to focus on processes at the cellular level. The electron microscope allowed researchers to visualize structures within cells. Other research showed that series of chemical reactions, called metabolic pathways, are necessary for proper cell function. The comparison of these pathways in normal cells with those in abnormal cells enabled researchers to understand the pathogenic effect of toxins and many genetic diseases. The use of computers in medical analysis has increased the precision of laboratory tests and has permitted the detection of abnormal chemicals in smaller amounts than were possible before.

During the effort to map the human genome—that is, to identify and locate all the genes found on the forty-six human chromosomes—one of the techniques employed was to compare the chromosomes of individuals known to have genetic diseases with those who do not, and so identify the abnormal gene. Identification of oncogenes (cancer-causing genes) or genes linked to such diseases as diabetes mellitus and heart disease may alert individuals at high risk in time for them to get regular diagnostic tests or to change their lifestyles. Knowledge gained by taking the study of disease to the genetic level will ultimately lead to more effective treatments and prevention for these and other pathological conditions.

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