Pathology is the medical specialty dedicated to the study of disease, focusing on its causes, development, and effects. Pathologists are physicians who interpret laboratory findings, providing the scientific basis for diagnosis and treatment planning. Anatomic Pathology (AP) is a specialized branch that focuses on the structure of the body, examining organs, tissues, and cells to diagnose conditions.
Defining Anatomic Pathology and Its Distinction from Clinical Pathology
Anatomic Pathology (AP) is the diagnosis of disease achieved through the macroscopic, microscopic, biochemical, immunologic, and molecular examination of organs and tissues removed from the body. The primary goal is to identify structural abnormalities that reveal the presence, nature, and extent of a disease process. This process involves the anatomic pathologist analyzing physical specimens to provide a definitive diagnosis.
This branch is distinct from Clinical Pathology (CP), also known as Laboratory Medicine, which focuses on the analysis of body fluids. Clinical pathologists examine blood, urine, spinal fluid, and other liquid samples to assess overall physiological function and identify chemical or microbial anomalies. While AP looks at the morphology, or structure, of solid tissue, CP is concerned with the chemical constituents, cell counts, and microbiological characteristics of fluids.
The two fields are deeply interconnected, but their methodologies differ significantly; AP relies heavily on visual examination under a microscope, while CP uses automated analyzers and chemical testing. Anatomic pathologists provide insight into structural changes caused by disease, such as the presence and type of a tumor. Clinical pathologists, conversely, diagnose conditions like diabetes or infection by measuring functional markers in the bloodstream.
The Types of Specimens Examined
Anatomic Pathology encompasses the analysis of three main categories of material, each requiring specialized handling and interpretation. Surgical Pathology is the most common category, involving the examination of solid tissue removed during biopsies and larger surgical resections. A biopsy may be a small core of tissue taken with a needle, while a resection is an entire organ or mass removed during an operation.
Cytopathology involves the study of individual cells or small clusters of cells, rather than intact tissue architecture. This includes procedures like Pap smears, where cells are scraped from a surface, or fine needle aspirates (FNA), where cells are drawn from a mass using a thin needle. The focus here is on subtle cellular changes, such as those indicating precancerous lesions or early malignancy.
Autopsy Pathology, or post-mortem examination, is performed to determine the exact cause of death or to study the progression of a known disease. While less frequent than surgical or cytopathology, autopsies remain an important tool for quality assurance, medical education, and understanding rare or complex diseases.
The Diagnostic Journey of a Tissue Sample
The first step upon arrival in the lab is “grossing,” which involves a macroscopic examination where the pathologist or a trained assistant visually inspects, measures, weighs, and meticulously describes the tissue. Critical areas of the specimen are then precisely selected and trimmed into small pieces, typically no more than three to four millimeters thick, to ensure proper chemical penetration.
These small pieces are immediately placed into a fixative, most commonly 10% neutral buffered formalin, to chemically stabilize the tissue and prevent cellular decay. Following fixation, the tissue undergoes automated “processing,” starting with dehydration, where water is progressively removed using increasing concentrations of alcohol. Next, a “clearing” agent, such as xylene, is introduced to remove the alcohol, making the tissue ready to accept the embedding medium.
The final stage of processing is infiltration with molten paraffin wax, which replaces the clearing agent and permeates the tissue structure. This saturated tissue is then placed into a mold and surrounded by more molten wax, which cools and solidifies to create a hard “paraffin block.” This solid block provides the necessary support for extremely thin slicing (microtomy), typically four to six micrometers thick.
These thin slices are floated onto glass slides and stained to make the cellular structures visible under a microscope. The standard method is the Hematoxylin and Eosin (H&E) stain, where Hematoxylin colors cell nuclei blue, and Eosin colors the cytoplasm and extracellular matrix pink. The stained slide is then presented to the pathologist for microscopic review, interpretation, and the generation of the final diagnostic report.
The Critical Role in Patient Management
The final Anatomic Pathology report serves as the authoritative foundation for nearly all subsequent patient treatment decisions. The pathologist’s diagnosis confirms or rules out disease, but also provides essential details necessary for clinical management. For a tumor, the report specifies whether the growth is benign or malignant, determines the precise type of cancer, and assigns a grade based on how aggressive the cells appear.
A particularly important element is the evaluation of surgical margins, which tells the surgeon if the entire tumor was removed, or if residual disease remains at the edge of the excision. Beyond the basic H&E diagnosis, pathologists use advanced techniques like immunohistochemistry and molecular testing to identify specific protein targets or genetic mutations within the tissue. These molecular markers are increasingly used to determine which targeted therapies or immunotherapies are most likely to be effective for an individual patient.
The accurate interpretation and timely delivery of this information directly impact a patient’s prognosis and the selection of the correct course of action. The anatomic pathologist functions as a physician consultant whose findings translate directly into the personalized treatment strategy for the patient.