Introduction to Pathology

Alumkal

Introduction

  • Pathology is one of the most important and core branches of medical science.

  • It is the study of disease, including:

    • How diseases develop

    • How they affect the body

    • How they can be diagnosed

  • Pathology helps in diagnosis through:

    • Laboratory testing

    • Examination of tissues, cells, blood, and body fluids

  • Pathology acts as a bridge between:

    • Basic medical sciences (Anatomy, Physiology, Biochemistry)

    • Clinical medicine

  • In modern healthcare, almost every diagnosis is confirmed:

    • Directly or indirectly

    • With the help of pathology investigations and reports

History of Pathology

The history of pathology is long and fascinating. It has evolved from simple observation of the body to highly advanced molecular and genetic diagnostic techniques.

1 Ancient Period

  • In ancient times, diseases were believed to be caused by evil spirits, curses, or supernatural forces.

  • Egyptian and Greek civilizations started early medical documentation.

  • Hippocrates (460–370 BC) is considered the Father of Medicine.

    • He suggested that diseases occur due to imbalance in body fluids (humoral theory: blood, phlegm, black bile, yellow bile).

2 Roman Period

  • Galen (129–216 AD) expanded the humoral theory.

  • He contributed to anatomy and physiology through animal dissections.

  • However, the understanding of disease remained limited.

3 Middle Ages

  • During this period, medical progress slowed due to restrictions on human dissection.

  • Diseases like plague spread rapidly.

  • Still, hospitals and medical learning began developing gradually.

4 Renaissance Period (14th–17th Century)

This period marked a major turning point.

  • Human dissections were allowed and encouraged.

  • Andreas Vesalius (1514–1564) corrected many anatomical mistakes.

  • This helped in correlating disease with structural changes in the body.

5 Development of Autopsy-Based Pathology

  • Giovanni Morgagni (1682–1771) is known as the Father of Anatomical Pathology.

  • He correlated clinical symptoms with post-mortem (autopsy) findings.

  • This created the foundation of modern pathology.

6 Cellular Pathology (19th Century)

  • Rudolf Virchow (1821–1902) is called the Father of Modern Pathology.

  • He introduced the concept:
    “Omnis cellula e cellula”
    (All cells arise from pre-existing cells)

  • He proved that disease begins at the cellular level, not only in organs.

7 Rise of Microbiology and Infectious Disease Pathology

  • Louis Pasteur and Robert Koch proved that microorganisms cause diseases.

  • Koch’s postulates helped establish infectious disease pathology.

8 Modern Era (20th Century to Present)

Pathology has expanded massively with:

  • Histopathology

  • Clinical pathology

  • Immunohistochemistry

  • Molecular pathology

  • Genetic testing

  • Digital pathology and AI

Today, pathology is not only used for diagnosis but also for:

  • Prognosis

  • Treatment monitoring

  • Personalized medicine

 

Basic Definitions in Pathology

Understanding the basic terms is essential for every medical and paramedical student.

1 Pathology

Pathology is the scientific study of disease, including:

  • Cause (etiology)

  • Mechanism (pathogenesis)

  • Structural changes (morphology)

  • Functional changes and clinical effects

2 Disease

A disease is a condition in which normal body structure or function is disturbed.

3 Lesion

A lesion is any abnormal change in tissue or organ caused by disease.

4 Diagnosis

Diagnosis is the identification of a disease based on:

  • Symptoms

  • Signs

  • Laboratory findings

  • Histopathological examination

5 Etiology

Etiology means the cause of disease.
Example:

  • Bacteria → tuberculosis

  • Smoking → lung cancer

  • Vitamin deficiency → scurvy

6 Pathogenesis

Pathogenesis is the mechanism and sequence of events that lead to disease development.

7 Morphology

Morphology refers to the structural changes in tissues or cells due to disease.

8 Prognosis

Prognosis means the expected outcome of the disease.

Familiarisation

Pathology has its own set of frequently used terms. Knowing them improves clinical and lab understanding.

1 Inflammation

Inflammation is the body’s response to injury or infection.

Types:

  • Acute inflammation: sudden, short duration (e.g., abscess)

  • Chronic inflammation: long duration (e.g., tuberculosis)

2 Necrosis

Necrosis is irreversible cell death due to injury.

Common types:

  • Coagulative necrosis (myocardial infarction)

  • Liquefactive necrosis (brain infarct)

  • Caseous necrosis (TB)

3 Apoptosis

Apoptosis is programmed cell death (controlled and normal).

4 Degeneration

Degeneration means reversible cell injury where cells show abnormal changes but may recover.

Example:

  • Fatty change in liver

5 Hyperplasia

Increase in number of cells.

Example:

  • Benign prostatic hyperplasia

6 Hypertrophy

Increase in size of cells.

Example:

  • Left ventricular hypertrophy in hypertension

7 Metaplasia

Reversible change where one adult cell type is replaced by another.

Example:

  • Squamous metaplasia in smokers

8 Dysplasia

Disordered growth of cells, considered a premalignant condition.

Example:

  • Cervical dysplasia

9 Neoplasia

New abnormal growth of tissue (tumor).

Types:

  • Benign tumor

  • Malignant tumor (cancer)

10 Biopsy

Removal of tissue from living body for microscopic examination.

11 Autopsy

Examination of dead body to determine cause of death and study disease.

Branches of Pathology

Pathology is broadly divided into:

Anatomical Pathology

Deals with:

  • Tissues

  • Organs

  • Cells

Includes:

  • Histopathology

  • Cytopathology

  • Autopsy pathology

Clinical Pathology (Laboratory Medicine)

Deals with:

  • Blood

  • Urine

  • Body fluids

  • Biochemical tests

Includes:

  • Hematology

  • Clinical biochemistry

  • Microbiology

  • Immunology

 

Techniques Used in Pathology

Modern pathology depends on multiple techniques for accurate diagnosis.

Gross Examination

  • The first step in tissue examination.

  • The specimen is observed with naked eyes.

  • Features noted:

    • Size

    • Shape

    • Color

    • Consistency

    • Cut surface appearance

Example:

  • Tumor mass in breast biopsy

Fixation

Fixation preserves tissue and prevents decomposition.

Common fixative:

  • 10% Neutral Buffered Formalin

Purpose:

  • Maintains tissue architecture

  • Prevents autolysis and putrefaction

Tissue Processing

After fixation, tissue is processed in:

  1. Dehydration (using alcohol)

  2. Clearing (xylene)

  3. Paraffin wax embedding

This allows thin section cutting.

Microtomy

  • Cutting thin tissue sections (3–5 microns) using microtome.

  • Sections are placed on glass slides.

Staining

Staining highlights cells and tissues under microscope.

(A) Routine Stain

Hematoxylin and Eosin (H&E) stain

  • Hematoxylin stains nucleus (blue/purple)

  • Eosin stains cytoplasm (pink)

(B) Special Stains

Used when H&E is not enough.

Examples:

  • PAS stain → fungi, glycogen

  • Ziehl-Neelsen stain → acid-fast bacilli (TB)

  • Gram stain → bacteria

  • Congo red → amyloid

Cytology Techniques

Cytology studies individual cells.

Types:

  • Pap smear (cervical cancer screening)

  • FNAC (fine needle aspiration cytology)

FNAC is commonly used for:

  • Thyroid nodules

  • Breast lumps

  • Lymph nodes

Hematology Techniques

Used for blood-related diagnosis.

Includes:

  • Complete Blood Count (CBC)

  • Peripheral smear examination

  • ESR

  • Coagulation tests (PT, APTT)

Microbiological Techniques

Used to identify infectious agents.

Includes:

  • Culture and sensitivity testing

  • Microscopy

  • Serological tests

  • Molecular tests (PCR)

    close up shot of microscope at the blood laboratory and red blood cells under microscope take with art lighting and blue filter

Immunohistochemistry (IHC)

IHC uses antibodies to detect specific proteins in tissues.

Applications:

  • Tumor typing

  • Identifying tumor origin

  • Prognostic markers

Example:

  • ER/PR in breast cancer

  • HER2 status

Molecular Pathology

Modern diagnostic technique that detects genetic changes.

Includes:

  • PCR

  • RT-PCR

  • DNA sequencing

  • FISH (Fluorescence in situ hybridization)

Applications:

  • Detecting mutations in cancers

  • Viral load testing

  • Genetic diseases

Electron Microscopy

Used for very high magnification.

Applications:

  • Kidney diseases (glomerular pathology)

  • Some viral infections

  • Muscle disorders

Flow Cytometry

A powerful technique for analyzing cell markers.

Used mainly for:

  • Leukemia and lymphoma diagnosis

  • Immunophenotyping

 

Importance of Pathology in Modern Healthcare

Pathology is essential because it helps in:

  • Early diagnosis of diseases

  • Confirmation of clinical diagnosis

  • Cancer detection and staging

  • Monitoring treatment response

  • Identifying infectious organisms

  • Guiding personalized therapy