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• Histology comes from the Greek word “histo” meaning “tissue” or “web”
• Histology is the study of the tissues of the body and how these tissues are arranged to constitute organs
• Involves all aspects of tissue biology, focusing on how cell structure and arrangement optimize functions specific to each organ
• Tissues are made up of 2 interacting components: cells and extracellular matrix
• Extracellular matrix is highly organized molecules that form complex structures
• Functions of the extracellular matrix include furnishing mechanical support for the cells, transporting nutrients to the cells, and carrying away catabolites and secretory products
• Histopathology deals with the study of the basic disease process, correlating the etiology of disease with the course development of anatomic and clinical changes brought about by disease
• Histopathologic techniques involve the preparation of tissues for microscopic examination
• The aim of histopathological technique is to produce microscopic preparations of tissues usually stained, that represent as closely as possible their structures
• Different procedures have been adopted for the preparation of tissues for microscopic investigation
• Exfoliative cytology is a branch of science that deals with the study of cells that have been desquamated from epithelial surfaces originated from exfoliative cells coming from the lining epithelium and mucosa of different organs of the body
• Cytopathology is the study of abnormal cells as in cancer cells
• Cytotechnique is the study of the methods of preparing the cells for microscopic examination
• Tissue processing involves processing tissues from the body taken for diagnosis of disease processes in the histology laboratory to produce microscopic slides viewed under the microscope by pathologists
• Histotechniques are the techniques for processing tissues, whether biopsies, larger specimens removed at surgery, or tissues from autopsy
• Histotechnologists are involved in these techniques
• Fixation is done to prevent autolysis and bacterial attack, fix the tissues so they will not change their volume and shape during processing, and prepare tissue for clear staining of sections
• Dehydration is done to remove fixative and water from the tissue and replace them with dehydrating fluid
• Dehydration usually starts in 60% - 70% ethanol, progressing through 90% - 95% ethanol, then absolute ethanol before proceeding to the clearing stage
• Clearing is the process of replacing the dehydrating fluid with a fluid that is totally miscible with both the dehydrating fluid and the embedding medium
• Some clearing agents include Xylene, Toluene, Chloroform, and Benzene
• Infiltration involves the melted wax penetrating between the cells of the tissues to harden them before embedding
• The infiltrating/embedding medium is almost always Paraffin
• Embedding is the process by which tissues are surrounded by a medium such as agar, gelatin, or wax to provide external support during sectioning
• Trimming involves trimming the resulting paraffin block to expose the tissue for sectioning on a microtome
• Section cutting is the process of sectioning biological specimens into very thin segments for microscopic examination using a microtome
• Staining makes the various tissue components conspicuous and permits distinctions between them
• H & E stains are universally used for routine histological examination of tissue sections
• Hematoxylin stains acidic molecules shades of blue, while Eosin stains basic materials shades of red, pink, and orange