Comprehensive Study Notes on Histological Techniques, Laboratory Safety, and Pathological Anatomy

Organizational and Safety Standards in the Pathology Laboratory

The fundamental mission of an Anatomy and Pathology laboratory, as defined by RD 1277/2003, is to investigate the causes, development, and consequences of diseases. Operating within this environment requires strict adherence to legal and safety protocols, most notably Law 31/1995, which regulates the prevention of occupational risks. A cornerstone of laboratory safety is the absolute prohibition of mouth pipetting. Personnel must utilize Personal Protective Equipment (PPE) as the primary barrier against laboratory risks, while gas extraction hoods are specifically employed to aspirate dangerous vapors. Biological risks, such as contact with contaminated samples, are managed by treating such waste as biohazardous, often requiring sterilization before disposal. Chemical waste, conversely, must be separated according to its specific nature.

Professionally, a Technician in Pathological Anatomy and Cytodiagnosis is responsible for the registry, processing, and conservation of samples, as well as the preparation of reagents. However, the direct acquisition of the sample from the patient is not within their professional scope. To ensure the quality of these processes, laboratories utilize Standard Operating Procedures (SOPs, or PNT in Spanish), which are designed to improve traceability and reproducibility. Lab equipment is categorized into inventoriable material, which possesses a long useful life (e.g., centrifuges, microtomes, or biosafety hoods), and fungible/disposable material, such as gloves, which are used to minimize contamination. Specialized glassware is often made of borosilicate due to its high thermal and chemical resistance, while volumetric materials like the volumetric flask (matraz aforado) are used for precise measurement, unlike non-volumetric items like beakers or watch glasses.

Fundamental Tissue Processing: Fixation and Macroscopic Description

The primary objective of tissue fixation is the preservation of the tissue structure, preventing autolysis—defined as the autodigestion of cells by their own enzymes—and degradation by external agents. Once fixed, a sample undergoes a macroscopic description, which is an exhaustive process capturing specific physical data: weight, size, color, and consistency. In some specialized cases, such as the preservation of lipids within a sample, standard paraffin processing is avoided in favor of freezing techniques.

The Inclusion Process and Paraffin Block Preparation

The inclusion process is essential for providing the sample with the necessary hardness and consistency for sectioning. This process is divided into three distinct stages: dehydration, clearing (or lightening), and infiltration. Dehydration is typically achieved using a progressive series of alcohols, most commonly ethanol; this progression is vital to avoid structural alterations in the tissue. If this process must be interrupted, the sample should be kept in $70\,\%$ ethanol. Following dehydration, an intermediary clearing agent is used to bridge the gap between alcohol and paraffin. Xylene is the most prevalent clearing agent because it is miscible with both ethanol and paraffin. Finally, infiltration occurs using paraffin, typically at a temperature of approximately $60\,^\circ\text{C}$. In specialized microscopy, such as electron microscopy, epoxy resins are preferred over paraffin.

When creating the final paraffin block, the orientation of the sample in the mold is critical. The face of the sample placed in contact with the mold will be the first area to be cut. Inaccurate orientation can lead to non-representative sections of the tissue, rendering the microscopic visualization useless. Furthermore, the rate of cooling is significant; solidification that is too rapid can lead to fractures within the block. While paraffin is standard, gelatin can be used as an inclusion medium, offering the advantage of not requiring prior dehydration of the tissue.

Histological Sectioning and Microtomy

To allow light to pass through the tissue for optical microscopy, samples must be cut into extremely thin sections. The standard thickness for histological sections in optical microscopy ranges between $2\, \mu\text{m}$ and $6\, \mu\text{m}$. The primary instrument for this task is the microtome. In Pathological Anatomy, the rotation microtome (also known as the Minot microtome) is the most frequently used for paraffin-embedded tissues. For intraoperative biopsies where speed is essential, a cryostat is employed. The quality of the cut depends heavily on block orientation and the condition of the equipment; for instance, vibrations in the microtome or a damaged blade are the most common causes of striations (scratches) in the sections. Conversely, if a block is too warm, the resulting sections may curl or roll up.

Staining Principles and the Hematoxylin-Eosin Technique

Since most animal tissues are naturally colorless, staining is required for visualization. Staining can be direct, or indirect (requiring a mordant to facilitate the bond between the dye and the structure). The affinity of a dye is determined by the chemical nature of the cellular structures: basic dyes have an affinity for acidic structures like DNA, while acidic dyes target basic components. In the most common "set" staining technique in histology, Hematoxylin and Eosin (H&E), Hematoxylin acts as a basic dye that colors nuclei blue-violet, while Eosin is an acidic dye that colors the cytoplasm pink. In progressive staining, the intensity of the color is controlled by the length of time the sample is exposed to the dye.

Before staining can occur, paraffin-embedded sections must undergo deparaffinization, usually with xylene, to remove the inclusion medium. Subsequently, the tissue must be rehydrated through a series of decreasing concentrations of alcohol. Following the staining process, the sample must be dehydrated again because most mounting media are hydrophobic. The final mounting stage, where a coverslip is applied using a mounting medium, serves to conserve and protect the preparation while preventing the entry of air and water. Errors during mounting are the primary cause of air bubbles in the final slides, while deteriorated or poorly filtered dyes can result in unwanted precipitates.

Histochemistry and Identification of Specific Substances

Histochemistry aims to locate specific substances within tissues through chemical reactions. Key techniques include the PAS (Periodic Acid-Schiff) stain, which primarily detects polysaccharides by reacting with aldehyde groups. The Feulgen technique is specific for detecting DNA, while the Green Methyl-Pyronin stain is used to differentiate between DNA and RNA. For lipid detection, Oil-Red-O produces a red color. Metal deposits are identified using specific tests: Von Kossa for calcium, Prussian Blue (or Perls) for hemosiderin (ferric iron), and Masson-Fontana for melanin. Amyloidosis is specifically detected using Congo Red. In these chemical reactions, a positive control is essential to verify that the technique is functioning correctly, while a false negative occurs if a molecule is present in the sample but fails to be detected.

Advanced Immunohistochemistry and Molecular Assays

Immunohistochemistry (IHC) relies on the high specificity of antigen-antibody binding. This field distinguishes between monoclonal antibodies (derived from a single cell clone/hybridoma and recognizing a single epitope) and polyclonal antibodies (derived from different B-lymphocyte lines and recognizing multiple epitopes). Techniques can be direct or indirect; the indirect technique utilizes a labeled secondary antibody to detect the primary antibody bound to the target. Specific systems like the ABC (Avidin-Biotin Complex) utilize the high affinity between avidin and biotin, while the PAP method involves a Peroxidase-Antiperoxidase complex.

To ensure quality in IHC, antigenic unmasking is often performed using heat to expose epitopes hidden during fixation. Furthermore, blocking agents like hydrogen peroxide ($H_2O_2$) are used to eliminate endogenous enzymatic activity that might interfere with the results. Excessive background staining is often corrected by adjusting the dilution of the antibody. For visual detection, IHC uses either fluorochromes (like fluorescein, which emits a green light) in immunofluorescence, or chromogens in immunoenzymatic techniques. ELISA (Enzyme-Linked ImmunoSorbent Assay) is a related biochemical technique for detecting substances. Markers produced by tumor cells or induced by the organism in response to a tumor are known as tumor markers.

Specialized Staining Protocols for Connective and Nervous Tissues

There are three primary trichrome stains used to detect collagen in connective tissue: Masson, Van Gieson, and Gomori. The Masson Trichrome uses Weigert's hematoxylin for nuclei (dark/black), acid fuchsin for cytoplasm (red), and light green (or aniline blue) for collagen. The Van Gieson Trichrome is the simplest method, using acid fuchsin and picric acid, resulting in pink collagen and yellow muscle/cytoplasm. The Gomori Trichrome, though less defined, stains collagen blue and muscle/cytoplasm red.

Nervous tissue staining focuses on neurons, glia, and myelin. Argentic (silver) impregnation provides a structural view of the soma, dendrites, and axons, visible under both optical and electron microscopes, though it often only stains a small percentage of neurons. Non-silver techniques include the Nissl technique, which uses basic dyes like methylene blue or cresyl violet to stain Nissl bodies (rough endoplasmic reticulum rich in RNA) within the neuronal soma. For myelin, the Klüber-Barrera method is the standard, employing Luxol Fast Blue and a series of steps including lithium carbonate and cresyl violet. This results in blue-green myelin sheaths and violet neurons/Nissl bodies. Staining for glial cells is now primarily handled via immunohistochemical techniques.