Histopathology is the branch of pathology that examines tissue specimens under the microscope to diagnose disease. It bridges gross anatomy, cell biology, and clinical medicine, providing definitive diagnoses for neoplasms, inflammatory conditions, infections, and systemic diseases.
The Histopathology Laboratory
A histopathology laboratory is organized into distinct workstations. The cut-up area receives fresh specimens, where pathologists or pathologists’ assistants describe gross features and select representative sections for processing. The processing laboratory handles fixation, dehydration, clearing, and paraffin infiltration using automated tissue processors. The embedding center orients tissue in paraffin blocks. The microtomy room produces thin sections, and the staining laboratory applies dyes to visualize cellular components. Finally, the reporting area is where pathologists examine slides under a microscope and issue diagnostic reports.
Types of Specimens
Biopsies are small tissue samples taken from living patients for diagnosis. Core needle biopsies (liver, kidney, prostate, breast) are cylindrical samples 1-2 mm in diameter and 10-20 mm long. Incisional biopsies sample a portion of a lesion; excisional biopsies remove the entire lesion. Endoscopic biopsies (gastrointestinal tract, bronchial mucosa) are tiny fragments that require careful handling.
Resections are larger surgical specimens — whole organs or substantial portions thereof — removed for treatment of established disease. Examples include colectomies for colon cancer, mastectomies for breast cancer, and lobectomies for lung cancer. These specimens require systematic dissection to assess margins, lymph node status, and tumor extent.
Cytology specimens include fine needle aspirates (FNAs), body fluids, and exfoliative samples (cervical Pap smears, urine). While processed differently from histology, cytology is complementary and often provides the first evidence of malignancy.
The Diagnostic Workflow
Before a specimen reaches the pathologist, it undergoes the sequence described in tissue processing and sectioning: fixation in formalin, gross examination and sectioning, automated processing through dehydration and paraffin infiltration, embedding in a paraffin block, sectioning at 3-5 µm on a rotary microtome, mounting on glass slides, and staining with hematoxylin and eosin (H&E).
The pathologist examines the stained slide, correlating microscopic findings with clinical history, laboratory data, and imaging results. When H&E alone is insufficient, ancillary techniques are employed. Immunohistochemistry uses antibodies to detect specific proteins — critical for tumor subtyping (cytokeratins for carcinomas, CD20 for B-cell lymphomas), infectious disease diagnosis, and predictive marker testing (ER, PR, HER2 in breast cancer; PD-L1 in lung cancer). Special stains highlight specific tissue components: periodic acid-Schiff (PAS) for glycogen and fungi, Masson’s trichrome for collagen, and Ziehl-Neelsen for acid-fast bacilli. Molecular techniques including PCR and in situ hybridization detect genetic alterations and infectious agents.
The Histopathology Report
The final report includes patient identifiers, specimen type and site, gross description, microscopic description, diagnosis (conclusion), and — where applicable — prognostic and predictive markers. Standardized synoptic reporting (checklist-based) is mandatory for cancer specimens in many jurisdictions, ensuring all critical elements (tumor type, grade, size, margin status, lymph node involvement, stage) are documented.
The turnaround time for routine histopathology is typically 2-5 working days from specimen receipt to final report. Frozen sections (intra-operative consultations) are completed within 15-20 minutes.
