Introduction to Histopathology and Cytology

Core Concepts of Histopathology and Cytology

Histopathology is a specialized branch of pathology that focuses on the study of diseases as they manifest in tissue sections. The term itself is derived from three Greek words: "histos," meaning tissue, "pathos," meaning disease, and "logos," representing the study of a subject. Consequently, histopathology is defined as the scientific study of diseased tissues. To facilitate this study, tissues must be prepared through complex processes to ensure they are at an appropriate thickness for microscopic examination, allowing distinct structures within the tissue to be properly differentiated. This field is inherently linked to the biological hierarchy, which moves from the individual cell to tissues (such as cuboidal epithelium), then to organs (like the kidney), organ systems (such as the urinary system), and ultimately the whole organism.

Cytology, also known as cytopathology, is the branch of science dedicated to studying the structure, function, and pathology of individual cells. This discipline is typically utilized for screening and diagnosing diseases including cancer, infections, and various inflammatory conditions. It involves the microscopic examination of samples obtained from body fluids, such as blood, urine, or Cerebrospinal Fluid (CSF), or from cell scrapings, such as a Pap smear. The primary objective in cytology is to differentiate between benign and malignant cellular structures.

Comparative Analysis of Histology and Cytology

There are significant differences between histology and cytology regarding their scope, methodology, and cost. Histology involves the study of the microscopic structure and architecture of tissues. It covers a wide study area and involves a highly complex slide preparation process, making it the more expensive of the two disciplines. In contrast, cytology focuses on the structure and function of plant and animal cells. It has a narrower study area than histology and utilizes a relatively simple slide preparation process. Because it only studies the cellular areas rather than the entire tissue architecture, cytology is considered less expensive.

Methods of Histological Specimen Preparation

Histological specimens can be prepared in several formats, including whole mounts, sections, and cell blocks. Whole mounts involve preparations of an entire organism, such as a fungus or a parasite. These preparations must be extremely thin, generally no more than $0.2\,mm$ to $0.5\,mm$ in thickness. Sections represent the vast majority of preparations in histology. In this method, tissue is initially cut into pieces approximately $3\,mm$ to $5\,mm$ thick before being processed. Following processing, a microtome is used to cut very thin sections, typically measuring $5\,\mu m$ (microns) in thickness. Microtomes are specialized instruments equipped with automatic mechanisms for precision cutting. For successful sectioning, the tissue must be sufficiently hardened to prevent it from being crushed during the process. Once cut, these sections are stained and permanently mounted on slides.

Smears are another preparation type, commonly created from blood, bone marrow, or fluids such as pleural or ascitic fluid. These must be immediately fixed in alcohol to preserve cellular structures before staining. Smears can also be produced by crushing soft tissue between two slides or by creating an impression smear, where a clean slide is pressed against the moist surface of a tissue. This process imprints cells onto the slide for cytological examination.

Sources and Types of Histological Specimens

Specimens in the histopathology lab typically originate from either biopsies or autopsies. A biopsy involves removing a small piece of a lesion or tumor from living tissue to establish a diagnosis or cause of disease before the final removal of the mass. Common biopsy methods include excisional biopsy, incisional biopsy, and needle biopsy. Excisional biopsy is used for tumors of the skin, breast, upper respiratory tract, and upper gastrointestinal tract (GIT), where the entire tumor and a margin of normal surrounding tissue are removed. Incisional biopsy is utilized when a mass is too large for complete removal; in these cases, a representative small portion is taken. Needle biopsy, or Fine Needle Aspiration (FNA), is reserved for suspicious growths that are deep or difficult to access, such as those in the breast, lungs, kidney, or liver. Conversely, an autopsy involves obtaining tissue from a deceased body to study the cause of death and advance medical knowledge.

Role and Responsibilities of the Laboratory Technician

The technician in a histopathology laboratory carries a heavy burden of responsibility to ensure diagnostic accuracy. Their duties include the preservation of specimens and the meticulous labeling, logging, and identification of every sample. They are responsible for preparing the specimens to facilitate both gross (macroscopic) and microscopic examination. Furthermore, technicians must maintain comprehensive records of all laboratory activities.

The Histopathology Laboratory Environment and Equipment

The histopathology laboratory is a clinical unit that performs activities associated with diagnosis during and after surgery. This is especially critical during surgery when an emergency diagnosis is required to determine the extent of the surgical intervention. This process is most common when a malignant tumor is suspected; if confirmed, radical surgery is performed immediately to avoid subjecting the patient to a second traumatizing procedure. The lab utilizes a various range of equipment, including microtomes for section cutting, hot air ovens for impregnation, and tissue processing machines. Water baths are used to heat samples, while wax dispensers are used for embedding sections. For cutting frozen sections, a cryostat is required. Other essential items include centrifuges for fluid separation, microscopes for visualization, and safety cabinets for handling materials contaminated with pathogens. The lab also utilizes various "wares," a general term for glass and plastic materials like tissue boxes, molds, bottles, and cylinders. Essential chemicals and reagents include xylene and alcohol, alongside dyes such as haematoxylin, eosin, and alcian blue.

Safety Precautions and Professional Standards

Safety is paramount in the histopathology lab, where many machines operate $24 \times 7$ days a week. Technicians must check all electrical connections daily before leaving the laboratory. Because many chemicals, such as wax, xylene, alcohol, and acetone, are highly inflammable, care must be taken to prevent fire hazards, and fire extinguishers must be readily available at all times. Inflammable substances should be stored in a separate, dedicated location. Protective gear, specifically gloves, must be worn during staining and other procedures because many laboratory chemicals are carcinogenic or otherwise harmful to the skin.

Laboratory Workflow: From Reception to Filing

The workflow in a histopathology laboratory follows a strict twelve-step sequence: 1. Reception, 2. Registration, 3. Fixation, 4. Grossing, 5. Dehydration, 6. Clearing, 7. Impregnation, 8. Embedding, 9. Section cutting, 10. Staining, 11. Reporting, and 12. Filing. The process begins with transportation in leak-proof containers to prevent formalin spills. Specimens must be accompanied by adequate fixative, proper labeling, a biohazard bag, and a correctly filled requisition form containing relevant clinical details. This delivery must be prompt to ensure the integrity of the tissue.

Upon arrival, the process of specimen accessioning begins. This involves receiving, identifying, logging, and labeling the patient sample. Each specimen is assigned a unique identification number (ID) for tracking within a Laboratory Information Management System (LIMS). This tracking is vital for accurate testing, pathology reporting, and billing. Registration involves entering the patient's name, age, sex, OPD/MRD number, ward and bed number, the name of the clinician, and the details of the organ biopsied. Each specimen receives an accession number followed by the year (e.g., $1234/23$), which stays with the specimen through processing, reporting, and filing.

The Grossing Process and Detailed Examination

Grossing involves the macroscopic description of the specimen and the selection of representative parts to be placed into small plastic cassettes for processing. This description serves as a permanent record of what was received from the operating room. A grossing workbench requires trays, blades, scissors, forceps, rulers, gauze, cotton swabs, and gloves. Specimens are arranged serially by their accession numbers. The general format for a grossing report includes the number of pieces (or a summary statement), the size in centimeters ($cm$), the appearance, consistency, color, and contour, followed by the identification of the tissue.

Instruments used specifically for grossing include large scissors, scalpels, hacksaws for bone, and dissecting scissors. As an example, a hysterectomy specimen involves examining the uterus, both fallopian tubes, and ovaries. The grossing of such a specimen focuses on the cervix, the anterior and posterior halves of the uterine corpus, the parametrium, serosa, myometrium, and endometrium. Measurements are taken for specific metrics, such as the depth of maximal myometrial invasion ($x$) and the thickness of the uninvolved endometrial wall ($y$).