Histology

• Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin extracellular matrix
• Epithelial tissues form cellular sheets that line the cavities of organs and body surfaces, with functions including covering, lining, protection, absorption, and secretion
• Types of epithelia include covering or lining epithelia and secretory or glandular epithelia
• Classification of lining epithelia is based on the number of cell layers and the shape of the cells
• Shapes of cells in epithelia:

  • Squamous: flattened
  • Cuboidal: cube-shaped
  • Columnar: column-like
• Examples of epithelial types:

  • Simple squamous epithelium: lines vessels (endothelium) and serous lining of cavities like pericardium, pleura, peritoneum (mesothelium)
  • Simple cuboidal epithelium: found in renal collecting tubules and ducts
  • Simple columnar epithelium: present in renal collecting ducts and lining of the gallbladder
• Stratified epithelium types:

  • Stratified squamous epithelium (keratinized): found in the skin
  • Stratified squamous epithelium (nonkeratinized)
  • Stratified cuboidal epithelium: in sweat glands
  • Stratified columnar epithelium
• Pseudostratified columnar epithelium is found lining the human trachea
• Brown Adipose Tissue (BAT) contains cells with multiple lipid droplets interspersed among abundant mitochondria

  
• Connective tissue consists of cells like fibroblasts, mast cells, macrophages, plasma cells, leukocytes, and adipocytes
• Extracellular matrix (ECM) of connective tissue includes protein fibers and ground substance
• Types of protein fibers in ECM:

  • Collagen
  • Elastic
  • Reticular
• Ground substance of ECM includes glycosaminoglycan, proteoglycan, and glycoproteins
• Two general classes of connective tissue proper:

  • Loose (Areolar) Connective Tissue
  • Dense Connective Tissue
  • Regular Connective Tissue
  • Irregular Connective Tissue
• Specialized Connective Tissue types:

  • White Adipose Tissue: more common type specialized for fat storage
  • Brown Adipose Tissue: contains cells with multiple lipid droplets interspersed among abundant mitochondria
  • Reticular Connective Tissue
  • Adipose Tissue
  • Cartilage
  • Bone
  • Blood
• • Difference Interference Contrast Microscope (DIC): also known as Nomarski Interference Contrast Microscope, similar to phase contrast microscope, forms nearly 3D shadowed images under oblique illumination, works using 2 coherent beams of light from the same source and prisms
  • Fluorescence Microscope: uses fluorophores to generate fluorescence in biological samples, components include xenon arc or mercury vapor lamps, excitation filter, dichroic mirror, and emission filter
  • Confocal Microscope: similar to fluorescence microscope, uses fluorescent dyes, pinhole focuses light onto a specific spot in the sample and rejects out-of-focus glare
  • Electron Microscopes:
• Fundamental types of microscopes:

  • Light Microscopes:
  • Bright Field Microscope: most commonly used, illumination from tungsten light passed through the condenser
  • Dark Field Microscope: effective for observing live and unstained biological samples, uses an opaque disc under the condenser, specimen appears brightly lit against a dark background, used to identify Treponema pallidum
  • Phase Contrast Microscope: preferred for translucent and colorless specimens, annular ring at the substage condenser front focal plane, converts differences in specimen components into brightness or contrast differences in the image, first accurate method in manual counting of platelets
• • Transmission Electron Microscope (TEM): uses electrons transmitted through a specimen to produce a 2D image, specimen is often an ultrathin section of less than 100nm thick, essential systems include electron gun and condenser, image producing system, and image recording system
  • Scanning Electron Microscope (SEM): designed for studying surfaces of solid objects, narrow beams of electrons scattered or emitted from the specimen surface produce a 3D character of the surface, essential parts include source of electrons, electromagnetic lenses, electron detector, sample chamber, computer, and display
• Difference between light and electron microscopes:

  • Light microscopes use light for illumination and are suitable for live samples, while electron microscopes use electrons and are ideal for studying surfaces of solid objects
• Histology is the study of tissues, relying on microscopy to reveal cell, tissue, and organ structure
• Goals of Histology include understanding tissue structure not visible to the naked eye, the relationship between tissue structure and function, and providing a basis for treating diseased and injured tissues
• Tissues are groups of differentiated cells of similar structure and function, consisting of cells and extracellular matrix (ECM)
• Four basic types of tissues found in the human body are connective tissue, epithelial tissue, muscle tissue, and nervous tissue
• Methods of study in Histology include tissue processing, microscopy, and preparation of tissue slides for study
• Tissue processing involves steps like fixation, dehydration, clearing, infiltration, embedding, trimming, section cutting, staining, and mounting
• Fixation in tissue processing aims to avoid autolysis and preserve cell and tissue structure, using compounds like Phosphate Buffered Formalin, Formal Calcium, and Formal Saline
• Dehydration in tissue processing replaces water in the tissue with organic solvent, commonly using acetone or alcohols
• Clearing in tissue processing involves immersing tissue in a series of clearing agent-alcohol mixtures, with commonly used reagents like Amyl Acetate and Methyl Benzoate
• Infiltration in tissue processing involves immersing cleared tissue in a series of clearing-embedding medium mixtures at medium-high temperature
• Embedding in tissue processing positions infiltrated tissue in a mold filled with embedding medium, commonly using paraffin wax for light microscopy
• Section cutting in tissue processing involves slicing the hardened tissue block with a microtome into extremely thin sections
• Staining in tissue processing uses dyes to selectively color tissue components, with common stains like hematoxylin and eosin (H&E)
• Mounting in tissue processing involves placing a protective glass coverslip on the slide with clear adhesive before microscopic observation