General medical pathology
The modern practice of pathology is divided into a number of subdisciplines within the discrete but deeply interconnected aims of biological research and medical practice. Biomedical research into disease incorporates the work of a vast variety of life science specialists, whereas, in most parts of the world, to be licensed to practice pathology as medical specialty, one has to complete medical school and secure a license to practice medicine. Structurally, the study of disease is divided into many different fields that study or diagnose markers for disease using methods and technologies particular to specific scales, organs, and tissue types. The information in this section mostly concerns pathology as it regards common medical practice in these systems, but each of these specialties is also the subject of voluminous pathology research as regards the disease pathways of specific pathogens and disorders that affect the tissues of these discrete organs or structures. (See also Gross pathology).
Anatomical pathology (Commonwealth) or anatomic pathology (United States) is a medical specialty that is concerned with the diagnosis of disease based on the gross, microscopic, chemical, immunologic and molecular examination of organs, tissues, and whole bodies (as in a general examination or an autopsy). Anatomical pathology is itself divided into subfields, the main divisions being surgical pathology, cytopathology, and forensic pathology. Anatomical pathology is one of two main divisions of the medical practice of pathology, the other being clinical pathology, the diagnosis of disease through the laboratory analysis of bodily fluids and tissues. Sometimes, pathologists practice both anatomical and clinical pathology, a combination known as general pathology.
Cytopathology (sometimes referred to as "cytology") is a branch of pathology that studies and diagnoses diseases on the cellular level. It is usually used to aid in the diagnosis of cancer, but also helps in the diagnosis of certain infectious diseases and other inflammatory conditions as well as thyroid lesions, diseases involving sterile body cavities (peritoneal, pleural, and cerebrospinal), and a wide range of other body sites. Cytopathology is generally used on samples of free cells or tissue fragments (in contrast to histopathology, which studies whole tissues) and cytopathologic tests are sometimes called smear tests because the samples may be smeared across a glass microscope slide for subsequent staining and microscopic examination. However, cytology samples may be prepared in other ways, including cytocentrifugation.
Dermatopathology is a subspecialty of anatomic pathology that focuses on the skin and the rest of the integumentary system as an organ. It is unique, in that there are two paths a physician can take to obtain the specialization. All general pathologists and general dermatologists train in the pathology of the skin, so the term dermatopathologist denotes either of these who has reached a certainly level of accreditation and experience; in the USA, either a general pathologist or a dermatologist can undergo a 1 to 2 year fellowship in the field of dermatopathology. The completion of this fellowship allows one to take a subspecialty board examination, and becomes a board certified dermatopathologist. Dermatologists are able to recognize most skin diseases based on their appearances, anatomic distributions, and behavior. Sometimes, however, those criteria do not lead to a conclusive diagnosis, and a skin biopsy is taken to be examined under the microscope using usual histological tests. In some cases, additional specialized testing needs to be performed on biopsies, including immunofluorescence, immunohistochemistry, electron microscopy, flow cytometry, and molecular-pathologic analysis. One of the greatest challenges of dermatopathology is its scope. More than 1500 different disorders of the skin exist, including cutaneous eruptions ("rashes") and neoplasms. Therefore, dermatopathologists must maintain a broad base of knowledge in clinical dermatology, and be familiar with several other specialty areas in Medicine.
Pathologist performing a human dissection of the abdominal and thoracic organs in an autopsy room.
Forensic pathology focuses on determining the cause of death by post-mortem examination of a corpse or partial remains. An autopsy is typically performed by a coroner or medical examiner, often during criminal investigations; in this role, coroners and medical examiners are also frequently asked to confirm the identity of a corpse. The requirements for becoming a licensed practitioner of forensic pathology varies from country to country (and even within a given nation) but typically a minimal requirement is a medical doctorate with a specialty in general or anatomical pathology with subsequent study in forensic medicine. The methods forensic scientists use to determine death include examination of tissue specimens to identify the presence or absence of natural disease and other microscopic findings, interpretations of toxicology on body tissues and fluids to determine the chemical cause of overdoses, poisonings or other cases involving toxic agents, and examinations of physical trauma. Forensic pathology is a major component in the trans-disciplinary field of forensic science.
Histopathology refers to the microscopic examination of various forms of human tissue. Specifically, in clinical medicine, histopathology refers to the examination of a biopsy or surgical specimen by a pathologist, after the specimen has been processed and histological sections have been placed onto glass slides. This contrasts with the methods of cytopathology, which uses free cells or tissue fragments. Histopathological examination of tissues starts with surgery, biopsy, or autopsy. The tissue is removed from the body of an organism and then placed in a fixative that stabilizes the tissues to prevent decay. The most common fixative is formalin, although frozen section fixing is also common. To see the tissue under a microscope, the sections are stained with one or more pigments. The aim of staining is to reveal cellular components; counterstains are used to provide contrast. Histochemistry refers to the science of using chemical reactions between laboratory chemicals and components within tissue. The histological slides are then interpreted diagnostically and the resulting pathology report describes the histological findings and the opinion of the pathologist. In the case of cancer, this represents the tissue diagnosis required for most treatment protocols.
Neuropathology is the study of disease of nervous system tissue, usually in the form of either surgical biopsies or sometimes whole brains in the case of autopsy. Neuropathology is a subspecialty of anatomic pathology, neurology, and neurosurgery. In many English-speaking countries, neuropathology is considered a subfield of anatomical pathology. A physician who specializes in neuropathology, usually by completing a fellowship after a residency in anatomical or general pathology, is called a neuropathologist. In day-to-day clinical practice, a neuropathologist is a consultant for other physicians. If a disease of the nervous system is suspected, and the diagnosis cannot be made by less invasive methods, a biopsy of nervous tissue is taken from the brain or spinal cord to aid in diagnosis. Biopsy is usually requested after a mass is detected by medical imaging. With autopsies, the principal work of the neuropathologist is to help in the post-mortem diagnosis of various conditions that affect the central nervous system. Biopsies can also consist of the skin. Epidermal nerve fiber density testing (ENFD) is a more recently developed neuropathology test in which a punch skin biopsy is taken to identify small fiber neuropathies by analyzing the nerve fibers of the skin. This test is becoming available in select labs as well as many universities; it replaces the traditional nerve biopsy test as less invasive.
Pulmonary pathology is a subspecialty of anatomic (and especially surgical) pathology that deals with diagnosis and characterization of neoplastic and non-neoplastic diseases of the lungs and thoracic pleura. Diagnostic specimens are often obtained via bronchoscopic transbronchial biopsy, CT-guided percutaneous biopsy, or video-assisted thoracic surgery. These tests can be necessary to diagnose between infection, inflammation, or fibrotic conditions.
Renal pathology is a subspecialty of anatomic pathology that deals with the diagnosis and characterization of disease of the kidneys. In a medical setting, renal pathologists work closely with nephrologists and transplant surgeons, who typically obtain diagnostic specimens via percutaneous renal biopsy. The renal pathologist must synthesize findings from traditional microscope histology, electron microscopy, and immunofluorescence to obtain a definitive diagnosis. Medical renal diseases may affect the glomerulus, the tubules and interstitium, the vessels, or a combination of these compartments.
Surgical pathology is one of the primary areas of practice for most anatomical pathologists. Surgical pathology involves the gross and microscopic examination of surgical specimens, as well as biopsies submitted by surgeons and non-surgeons such as general internists, medical subspecialists, dermatologists, and interventional radiologists. Often an excised tissue sample is the best and most definitive evidence of disease (or lack thereof) in cases where tissue is surgically removed from a patient. These determinations are usually accomplished by a combination of gross (i.e., macroscopic) and histologic (i.e., microscopic) examination of the tissue, and may involve evaluations of molecular properties of the tissue by immunohistochemistry or other laboratory tests.
There are two major types of specimens submitted for surgical pathology analysis: biopsies and surgical resections. A biopsy is a small piece of tissue removed primarily for surgical pathology analysis, most often in order to render a definitive diagnosis. Types of biopsies include core biopsies, which are obtained through the use of large-bore needles, sometimes under the guidance of radiological techniques such as ultrasound, CT scan, or magnetic resonance imaging. Incisional biopsies are obtained through diagnostic surgical procedures that remove part of a suspicious lesion, whereas excisional biopsies remove the entire lesion, and are similar to therapeutic surgical resections. Excisional biopsies of skin lesions and gastrointestinal polyps are very common. The pathologist's interpretation of a biopsy is critical to establishing the diagnosis of a benign or malignant tumor, and can differentiate between different types and grades of cancer, as well as determining the activity of specific molecular pathways in the tumor. Surgical resection specimens are obtained by the therapeutic surgical removal of an entire diseased area or organ (and occasionally multiple organs). These procedures are often intended as definitive surgical treatment of a disease in which the diagnosis is already known or strongly suspected, but pathological analysis of these specimens remains important in confirming the previous diagnosis.
Clinical pathology is a medical specialty that is concerned with the diagnosis of disease based on the laboratory analysis of bodily fluids such as blood and urine, as well as tissues, using the tools of chemistry, clinical microbiology, hematology and molecular pathology. Clinical pathologists work in close collaboration with medical technologists, hospital administrations, and referring physicians. Clinical pathologists learn to administer a number of visual and microscopic tests and an especially large variety of tests of the biophysical properties of tissue samples involving Automated analysers and cultures. Sometimes the general term "laboratory medicine specialist" is used to refer to those working in clinical pathology, including medical doctors, Ph.D.s and doctors of pharmacology. Immunopathology, the study of an organism's immune response to infection, is sometimes considered to fall within the domain of clinical pathology.
Hematopathology is the study of diseases of blood cells (including constituents such as white blood cells, red blood cells, and platelets) and the tissues, and organs comprising
the hematopoietic system. The term hematopoietic system refers to tissues and organs that produce and/or primarily host hematopoietic cells and includes bone marrow, the lymph nodes, thymus, spleen, and other lymphoid tissues. In the United States, hematopathology is a board certified subspecialty (licensed under the American Board of Pathology) practiced by those physicians who have completed a general pathology residency (anatomic, clinical, or combined) and an additional year of fellowship training in hematology. The hematopathologist reviews biopsies of lymph nodes, bone marrows and other tissues involved by an infiltrate of cells of the hematopoietic system. In addition, the hematopathologist may be in charge of flow cytometric and/or molecular hematopathology studies.
Immunopathology is a branch of clinical pathology that deals with an organism’s immune response to a certain disease. When a foreign antigen enters the body, there is either an antigen specific or nonspecific response to it. These responses are the immune system fighting off the foreign antigens, whether they are deadly or not. Immunopathology could refer to how the foreign antigens cause the immune system to have a response or problems that can arise from an organism’s own immune response on itself. There are certain problems or faults in the immune system that can lead to more serious illness or disease. These diseases can come from one of the following problems. The first would be Hypersensitivity reactions, where there would be a stronger immune response than normal. There are four different types (type one, two, three and four), all with varying types and degrees of an immune response. The problems that arise from each type vary from small allergic reactions to more serious illnesses such as tuberculosis or arthritis. The second kind of complication in the immune system is Autoimmunity, where the immune system would attack itself rather than the antigen. Inflammation is a prime example of autoimmunity, as the immune cells used are self-reactive. A few examples of autoimmune diseases are Type 1 diabetes, Addison’s disease and Celiac disease. The third and final type of complication with the immune system is Immunodeficiency, where the immune system lacks the ability to fight off a certain disease. The immune system’s ability to combat it is either hindered or completely absent. The two types are Primary Immunodeficiency, where the immune system is either missing a key component or does not function properly, and Secondary Immunodeficiency, where disease is obtained from an outside source, like radiation or heat, and therefore cannot function properly. Diseases that can cause immunodeficiency include HIV, AIDS and leukemia.
Radiation pathology is study of the interaction between human tissues and radiation, as well as the problems and diseases that can arise from the use of radiation. When human tissue is exposed to radiation, it can be genetically altered and deformed; in turn, this could lead to a variety of illnesses that could be minor or deadly.
Molecular pathology is focused upon the study and diagnosis of disease through the examination of molecules within organs, tissues or bodily fluids. Molecular pathology is multidisciplinary by nature and shares some aspects of practice with both anatomic pathology and clinical pathology, molecular biology, biochemistry, proteomics and genetics. It is often applied in a context that is as much scientific as directly medical and encompasses the development of molecular and genetic approaches to the diagnosis and classification of human diseases, the design and validation of predictive biomarkers for treatment response and disease progression, and the susceptibility of individuals of different genetic constitution to particular disorders. The crossover between molecular pathology and epidemiology is represented by a related field "molecular pathological epidemiology".
 Molecular pathology is commonly used in diagnosis of cancer and infectious diseases. Molecular Pathology is primarily used to detect cancers such as melanoma, brainstem glioma, brain tumors as well as many other types of cancer and infectious diseases. Techniques are numerous but include quantitative polymerase chain reaction (qPCR), multiplex PCR, DNA microarray, in situ hybridization, DNA sequencing, antibody based immunofluorescence tissue assays, molecular profiling of pathogens, and analysis of bacterial genes for antimicrobial resistance. Techniques used are based on analyzing samples of DNA and RNA. Pathology is widely used for gene therapy and disease diagnosis.
Many conditions, such as this case of geographic tongue
, can be diagnosed partly on gross examination, but may be confirmed with tissue pathology.
Oral and maxillofacial pathology
Oral and Maxillofacial Pathology is one of nine dental specialties recognized by the American Dental Association, and is sometimes considered a specialty of both dentistry and pathology. Oral Pathologists must complete three years of post doctoral training in an accredited program and subsequently obtain diplomate status from the American Board of Oral and Maxillofacial Pathology. The specialty focuses on the diagnosis, clinical management and investigation of diseases that affect the oral cavity and surrounding maxillofacial structures including but not limited to odontogenic, infectious, epithelial, salivary gland, bone and soft tissue pathologies. It also significantly intersects with the field of dental pathology. Although concerned with a broad variety of diseases of the oral cavity, they have roles distinct from otorhinolaryngologists ("ear, nose, and throat" specialists), and speech pathologists, the latter of which helps diagnose many neurological or neuromuscular conditions relevant to speech phonology or swallowing. Owing to the availability of the oral cavity to non-invasive examination, many conditions in the study of oral disease can be diagnosed, or at least suspected, from gross examination, but biopsies, cell smears, and other tissue analysis remain important diagnostic tools in oral pathology.