Human Histology: Tissues and Microscopy

• These study notes cover human histology, including basic tissues, microscopy, and cellular components.
• They were made for medical laboratory science students.

Histology and Tissue Components

• Histology is the study of tissues, their arrangement, and their function in the body.
• Tissues are composed of cells and the extracellular matrix (ECM).
• Cells can have various functions like covering, absorption, secretion, and lining cavities.
• The ECM contains fibers (collagen, elastic, reticular), ground substance (GAG, proteoglycans, glycoproteins), and fluids.
• Tissues differ in cell type, ECM proportion, and tissue organization.

Histotechniques and Tissue Processing

• Histotechniques involve tissue acquisition and processing for microscopic study.
• Tissue acquisition methods include cytological swabs, biopsies, and surgical resections.
• Tissue processing generally involves:
  • Fixation to preserve structure.
  • Dehydration to remove water (using alcohol solutions).
  • Clearing to remove alcohol and make tissue miscible with paraffin (using xylene).
  • Infiltration with melted paraffin.
  • Embedding in a paraffin block.
  • Trimming the block for sectioning, which includes cutting in the microtome.
  • Sectioning (slicing) in the microtome is also a stage.
  • Staining (e.g., H&E) to visualize structures.
  • Mounting on slides.
  • Labelling.

Staining Methods

• Hematoxylin & Eosin (H&E) is a common dye combination.
• Hematoxylin (basic dye) stains acidic structures (DNA, RNA) blue (basophilic).
• Eosin (acidic dye) stains basic structures (cytoplasmic proteins) pink (acidophilic).
• Histochemistry uses natural chemical interactions between tissues and chemicals.
• Trichrome stains are used to visualize collagen.
• Periodic Acid Schiff (PAS) stains carbohydrates magenta.

Immunohistochemistry and Immunofluorescence

• Immunohistochemistry uses specific antigen-antibody interactions.
• A primary antibody binds to the target protein (antigen).
• A secondary antibody, with a chromogen, binds to the primary antibody.
• Immunofluorescence is similar but uses a fluorescent-tagged secondary antibody.
• Fluorescence is visualized under UV light.
• Direct methods tag the primary antibody directly.
• Indirect methods use an unlabeled primary antibody and a labeled secondary antibody.

Microscopy

• Microscopy allows observation of prepared tissue slides at various magnifications.
• Light microscopy uses natural light to illuminate the tissue.
• Light microscopy is used with histochemistry, immunochemistry, and immunofluorescence.
• All the types of light microscopy are based on the interaction of light with tissue components

Types of Light Microscopy include:

• Brightfield.
• Dark Field.
• Phase contrast.
• Polarizing.
• Fluorescence.
• Confocal.

Light Microscope Parts include:

• Eyepieces (ocular lens).
• Objective lenses.
• Head.
• Arm (frame).
• Stage controls.
• Coarse/fine adjustment knobs.
• Diopter adjustment.
• Nose piece.
• Stage clip.
• Mechanical stage.
• Aperture.
• Brightness adjustment.
• Condenser.
• Illumination.
• Light switch.
• Base.

Microscope Lenses

• The condenser collects and focuses light.
• Objective lenses enlarge and project the image. –Common magnifications: 4x, 10x, 40x.
• Eyepieces (oculars) further magnify the image (typically 10x).
• Total magnification is the product of objective and eyepiece magnification.

Other Microscopy Techniques

• Phase contrast microscopy uses refractive properties; good for live cells.
• Fluorescence microscopy requires fluorescent dyes and a UV light source.
• Confocal microscopy uses a laser and pinhole aperture for optical sectioning and 3D reconstruction.
• Electron microscopy uses electron beams for higher magnification and resolution.

Electron Microscopy Types include:

• Scanning Electron Microscopy (SEM): Captures surface contours.
• Transmission Electron Microscopy (TEM): Electrons pass through tissue, casting different shades depending on electron density.

Interpreting Images

• Darker areas in TEM indicate electron-dense, concentrated tissue.
• Lighter areas in TEM indicate less dense tissue.
• Artifacts are a natural part of histology.
• Histological images are 2D representations of 3D structures.
• Virtual microscopy uses software to simulate a microscope on a digital screen.

Basic Tissue Types and Epithelial Tissue

• The four basic tissue types are:
  • Epithelial.
  • Connective.
  • Muscle.
  • Nervous.
• Epithelial tissue consists of aggregated polyhedral cells with a small amount of ECM.

Epithelial tissue has the following main functions:

• Lining surfaces/cavities, and glandular secretion.
• Cells of epithelial tissue has the following properties:
  • Variable shapes/dimensions.
  • Nuclei reflect cell shape.
  • Avascular.
  • High mitotic index.
  • Polarity (apical, basal).
  • Lateral surfaces with folds to increase area.

Epithelial Tissue Types

• Simple: One cell layer.
• Stratified: Multiple layers.
• Pseudostratified: Appears layered but all cells contact the basement membrane.
• Cell shapes: squamous, cuboidal, columnar.
• Specializations: ciliated, keratinized.

Epithelial Cell Junctions include:

• Tight junctions (zonula occludens): Seal cells together to control passage of molecules.
• Adherens junctions (zonula adherens): Link cytoskeleton, strengthen tight junctions.
• Desmosomes (macula adherens): Strong intermediate filament coupling.
• Hemidesmosomes: Anchor cytoskeleton to basal lamina.
• Gap junctions (nexus): Allow direct transfer of small molecules/ions.

Apical Cell Surface Specializations include:

• Microvilli: Increase surface area.
• Cilia: Move substances along the surface.
• Stereocilia: For absorption or sensory function.

Basement Membrane is what the basal surface of all epithelia rests on:

• Provides support.
• Acts like a filter.
• Components:
  • Basal lamina.
  • Reticular lamina.
• It is capable of epithelial repair and regeneration.

Explore human histology, covering tissues and microscopy. Learn about tissue components, cells, and ECM. Understand histotechniques and tissue processing for medical laboratory science students.