Introduction to Pathology

• Pathology is the scientific study of disease.
• Pathology includes functional and structural changes in disease at all levels, including molecular and individual.

Subdivisions of Clinical Pathology

• Histopathology involves diagnosing diseases via tissue examination.
• Cytopathology involves diagnosing diseases via examination of isolated cells.
• Hematology involves studying blood disorders.
• Microbiology involves studying infectious diseases and their organisms.
• Immunology involves studying the body's specific defense mechanisms.
• Chemical pathology involves studying and diagnosing diseases using chemical changes in tissues and fluids.
• Genetics involves studying abnormal chromosomes and genes.
• Toxicology involves studying the effects of known/suspected poisons.
• Forensic pathology involves applying pathology for legal purposes, such as death investigations with suspicious circumstances.

Techniques in Pathology

• Gross pathology (morbid anatomy) is the study of disease with the naked eye, and is related to radiological changes.
• Autopsy is the dissection and examination of a corpse.
• Medico-legal autopsies determine cause of death and collect evidence (forensic medicine).
• Clinical autopsies are performed on patients who die in hospitals, serving clinical audit, education, and medical research purposes, especially for newly discovered diseases.
• Diagnostic discrepancies are found in about 30% of cases via autopsies.
• Light microscopy is the standard technique in histopathology.
• A sample needs to be thinly sectioned for light transmission.
• Tissue sections are stained to differentiate between various components.
• Standard staining is H&E (Hematoxylin & Eosin).
• Hematoxylin stains nuclei blue and is basophilic.
• Eosin stains cytoplasm pink, known as eosinophilic or acidophilic.
• Special stains detect specific elements in pathology fields.
• Giemsa is useful in identifying parasites/pathogenic bacteria like H pylori in the stomach.
• Giemsa 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 are used in microbiology.

Histochemistry

• Histochemistry uses chemical reactions to detect certain tissue components.
• An example is acid phosphatase in osteoclasts.
• Histochemistry is usually complex and not specific.
• Immunohistochemistry has replaced it currently.

Immunohistochemistry (IHC)

• This involves creating antibodies to react with specific substances in some cells, and is a vital tool in practice.
• A usage of IHC is diagnosis for poorly differentiated tumors or metastatic tumors and diagnosing invasion via loss of myoepithelial cells in breast cancer.
• Prognosis can be determined using HER2 in breast and tumors in the stomach.
• Treatment can be determined with ER/PR (Tamoxifen for breast cancer) and HER2 (Herceptin for breast cancer).
• Antigens can be membranous, cytoplasmic, or nuclear.

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): only in the epithelial cells of the prostate

Histopathology and Cytopathology

• Histopathology is the primary diagnosis mode and the gold standard involving a small piece of tissue and requires at least 2 days to prepare.
• Histopathology answers important disease questions about the tumors like histogenetical pattern, differentiation & grading, resection margins, or lymph nodes.
• Cytopathology uses dissociated cell collections from their surrounding tissue.
• Cytopathology is useful but less accurate than histopathology and is fast (1/2 hour).

Materials for Histopathology

• Biopsy is a removal of a tissue sample for microscope examination.
• Excisional biopsy is the surgical removal of entire abnormal area for diagnosis (and treatment), for small/superficial lesions.
• Incisional biopsy is the surgical removal of a portion of an abnormal area for diagnosis usually for large lesions needing more treatment.
• Endoscopy biopsy is done through endoscopy for lesions in the gastrointestinal, respiratory, urinary, and genital tracts.
• Fine Needle Biopsy (FNB) is when a small tissue amount is examined using a hollow needle, not always sufficient for diagnosis, such as in lymphoma.
• Fine Needle Aspiration (FNA) uses a needle to withdraw fluid for cytopathological examination.
• Smears are cells collected from a superficial surface, smeared on a slide, (pap smear (cervix), & blood smear).
• Curettage removes tissue with a curette from a cavity wall/another surface (skin and endometrium).

Preparation of Slides

• Paraffin Wax is the standard embedding medium for tissue processing.
• Paraffin Wax is cheap and safe.
• Paraffin Wax allows tissue blocks and slides to be saved for years.
• Paraffin Wax requires at least two days to prepare.
• Frozen Section is a rapid tissue section cooling the tissue to give an immediate report of the tissue sample.
• Frozen sections help diagnose lesion extent during surgery (30 minutes).
• It can be employed with immunohistochemistry.
• Frozen Sections lead to some problems: less accurate, poor morphology, lots of artifacts, and rapid deterioration rate.