Introduction to Pathology

Objectives

• Pathology lecture aims to equip students with the ability to describe pathologists' activities, encompassing anatomic and clinical pathology subdivisions
• Students will learn the different methods employed in histopathology laboratories

What Is Pathology?

• It is the scientific study of disease
• Pathology examines functional and structural changes in disease, from the molecular level to effects on patients

Subdivisions of Clinical Pathology

• Histopathology involves diagnosing diseases by examining tissues
• Cytopathology diagnoses diseases through examination of isolated cells
• Hematology studies disorders related to cellular and coagulable blood components
• Microbiology focuses on infectious diseases and causative organisms
• Immunology studies the body's specific defense mechanisms
• Chemical pathology studies and diagnoses diseases via chemical changes in tissues and fluids
• Genetics studies abnormal chromosomes and genes
• Toxicology studies the effects of known or suspected poisons
• Forensic pathology utilizes pathology for legal purposes, such as investigating suspicious deaths

Techniques of Pathology

• Gross pathology, or morbid anatomy, involves studying disease with the naked eye
  • It is related to radiological changes
• An autopsy involves dissecting and examining a corpse
  • Medico-legal autopsies determine the cause of death and gather evidence for forensic medicine
  • Clinical autopsies on patients who died in the hospital assists with clinical audits, education, medical research, especially for newly discovered diseases.
  • Autopsies reveal diagnostic discrepancies in about 30% of cases

Light Microscopy

• Light microscopy is a standard technique used in histopathology
• Samples must be thinly sectioned to allow light transmission
• Tissue sections are stained to differentiate components
• Standard staining is H&E (Hematoxylin & Eosin) for nuclei and cytoplasm
  • Hematoxylin stains nuclei blue (basophilic)
  • Eosin stains cytoplasm pink (eosinophilic or acidophilic)

Special Staining

• Special staining detects specific elements or is used in specific pathology fields
• Giemsa is useful for identifying parasites or pathogenic bacteria, such as H. pylori in the stomach
  • It is widely used in blood films (peripheral blood, bone marrow aspirate)
• Papanicolaou stain is used for cervical smears
• Periodic Acid-Schiff (PAS) detects glycogen, basement membranes, and fungal infections
• Congo Red detects amyloid
• Gram Stain & Ziehl-Neelsen for microbiology

Histochemistry

• Histochemistry uses chemical reactions to detect tissue components
• An example is acid phosphatase in osteoclasts
• Histochemistry is usually complex and not specific
• It has been largely replaced by immunohistochemistry

Immunohistochemistry (IHC)

• Immunohistochemistry involves creating antibodies that react with specific substances in some cells
• It is a very important tool in current practice
• IHC Uses:
  • Diagnosis: typing poorly differentiated tumors or identifying metastatic tumor origin, and diagnosing invasion by detecting the loss of myoepithelial cells in breast cancer
  • Prognosis: assessing HER2 in breast and gastric cancers
  • Treatment: guiding treatment with ER/PR (Tamoxifen) for breast cancer and HER2 (Herceptin) for breast cancer.

Examples of Immunostains

• CD3: T-Lymphocytes
• CD20: B-Lymphocytes
• Cytokeratins: Intermediate filaments in epithelial cells
• Desmin: Intermediate filaments in muscular cells
• Ki-67: Cytoplasmic protein in dividing cells
• HER2: Receptor on breast cancerous cells
• Prostate-specific antigen (PSA): Specific to prostate epithelial cells

Histopathology & Cytopathology

• Histopathology is the primary mode of diagnosis, considered the gold standard
  • It uses a small piece of tissue and requires at least two days for preparation
  • Histopathology answers important questions about the disease, including tumor histogenetical patterns, differentiation, grading, resection margins, and lymph nodes
• Cytopathology uses a collection of cells dissociated from surrounding tissue
  • It is useful but less accurate than histopathology and is faster to prepare, requiring approximately 1/2 hour

Materials for Histopathology

• A biopsy involves removing a tissue sample for microscopic examination
• Types of Biopsies:
  • Excisional biopsy: surgical removal of the entire abnormal area for diagnosis and treatment, usually for small or superficial lesions
  • Incisional biopsy: surgical removal of only a portion of an abnormal area to diagnose, usually for large lesions needing further treatment
  • Endoscopy biopsy: performed through endoscopy for lesions in gastrointestinal, respiratory, urinary, and genital tracts
• Fine Needle Biopsy (FNB): Small amount of tissue taken for examination via hollow needle; may be insufficient for good diagnosis
• Fine Needle Aspiration (FNA): use a needle to withdrawal fluid for cytopathological examination.
• Smears: cells are collected from a superficial surface and smeared on a slide, such as pap smear (from uterine cervix), & blood smear
• Curettage: tissue removal with a curette from a cavity wall or another surface, like skin or endometrium

Preparation of Slides

• Paraffin Wax:
  • The standard embedding medium for tissue processing
  • It is cheap and safe.
  • Tissue blocks and slides can be saved for long durations (years)
  • Paraffin Wax slide preparation requires at least two days
• Frozen Section:
  • A rapid tissue section is created by cooling the tissue to provide an immediate tissue sample report
  • It helps diagnose lesions at the time of operation and can be used with Immunohistochemistry (30 minutes)
  • Can have several problems like less accuracy, poor morphology, lots of artifacts, and a rapid deterioration rate

References

• Rosai and Ackerman's Surgical Pathology (11th ed.) by John R. Goldblum, Laura W. Lamps, Jesse K. McKenney, Jeffrey L Myers

This pathology lecture introduces the activities of pathologists, covering anatomic and clinical pathology. Students will learn about histopathology laboratory methods. Pathology is the study of disease, examining functional and structural changes.