Week 1

• Pathology is the study of the patterns, causes, mechanisms and effects of diseases.
• General Pathology involves the study of common changes in all tissues, such as inflammation, cancer, and ageing.
• Systemic Pathology involves the study of specific changes in organs, such as goiter, pneumonia, and breast cancer.
• Aetiology is the study of the factors implicated in the cause of disease, which can be intrinsic/genetic, acquired (infectious/nutritional/chemical/physical), or unknown – idiopathic.
• Pathogenesis is the sequence of events in the response of the cells and tissues to the aetiological agent.
• Morphological changes are structural alterations in cells or tissues due to the pathological process.
• Clinical significance is how the morphological changes and their distribution in different organs and tissues influence normal function and determine the clinical features (symptoms & signs), clinical course and prognosis of diseases.
• Cellular Pathology involves the study of tissue changes through the use of microscopy.
• Routine Tissue Preparation involves a multi-step process: surgical excision, fixation, grossing/cut-up, tissue processing, embedding, microtomy, and staining.
• A connective tissue stain can be used to highlight the delicate supporting meshwork of type III collagen in soft tissues such as liver, bone marrow, and the lymphatic system
• Fixation in Routine Tissue Preparation aims to preserve tissue architecture in a 'life-like' state, preventing 'autolysis' and micro-organism degradation.
• Fixatives have two broad mechanisms of action on proteins in cells and tissue: denaturation and cross-linking.
• Formaldehyde is the most commonly used fixative for routine histology.
• Glutaraldehyde is the most commonly used fixative for electron microscopy.
• Alcohols or Acetone are the most common fixatives used on cytological preparations.
• Cut-Up in Routine Tissue Preparation involves the examination and description of the specimen, recording macroscopic lesions and pathology, taking measurements and weights, noting margin status of surgically removed tumours, and dissecting larger specimens to produce representative pieces from appropriate areas.
• The macroscopic description is integral to the final histological report.
• Tissue processing in Routine Tissue Preparation involves three stages: dehydration, clearing, and infiltration with wax.
• The aim of tissue processing is the complete infiltration of tissue with a suitable medium (normally paraffin wax), which provides the tissue with the rigidity and support required, such that very thin sections can be cut for microscopic analysis.
• Paraffin wax is liquid at 60°C and can be infiltrated into tissue at this temperature - When cooled, it solidifies to a consistency that allows sections to be easily cut.
• Although tissue is now essentially water-free, it cannot be infiltrated by wax as wax and ethanol are largely immiscible.
• When solidified, the block with its attached cassette is removed from the mould and is ready for microtomy.
• Duration of processing is dependant on tissue type / size – commonly left to run overnight.
• Casettes containing tissue samples are placed in a chamber.
• The routine stain used universally as a starting point in providing essential structural information, is the hematoxylin and eosin (H&E).
• Ethanol is miscible with water, the water within the tissue is progressively replaced by alcohol.
• The intermediated solvent is termed the ‘clearing-agent’ and the process known as clearing.
• In order to reveal structural detail using brightfield microscopy the tissue requires staining.
• Dehydration of the specimen is commonly achieved by immersion in a series of alcohol (Ethanol) solutions.
• Tissue processing follows a multi-step process: Fixation, Grossing/’Cut-up’, Tissue processing, Embedding, Microtomy, Staining.
• Examples of stains used include Congo Red (Amyloid), Perls (Iron), Grocott (Fungal hyphae), ZN (Mycobacteria), Retic (Reticulin Fibres), EVG (Elastic fibres).
• The specimen is orientated in the mould because its placement will determine the “plane of section”.
• The orientation of the tissue within the mould determines what is ultimately seen down the microscope.
• Clearing as it gives the tissue a translucent or ‘clear’ optical clarity once tissue is completely impregnated due to the refractive index.
• This is particularly important for samples containing epithelial structures or lumens.
• Use of pressure / vacuum cycles and high temperature speeds up tissue impregnation of each reagent.
• Apart from a few natural pigments cells and tissue elements are colourless.
• Most commonly used clearing agent is Xylene.
• An intermediate solvent that is fully miscible with both ethanol and paraffin wax is required for infiltration of wax into the tissue.
• After tissue processing is complete & tissue is fully impregnated with wax it is embedded and encased with more molten wax using a mould.