Introduction to Histology and Cell Structure

Histology Introduction

• Histology is the study of tissues
• The name "Histology" comes from Greek words "Histos" (tissue) and "logos" (study of)
• Histology studies the microscopic structure of cells and organs
• Histology is used to correlate structure to function, and understand pathologies of diseases
• Histology uses thin slices of tissue viewed under light or electron microscopes.
• This is sometimes enhanced by histological stains

Levels of Organization

• The levels of organization in the body are organized in a hierarchy
• Cell → Tissue → Organ → System

Cell Components

• Cytosol
• Organelles
• Inclusions
• Cytoskeleton
• Cell Membrane

Tissues

• Tissues are groups of similar, specialized cells designed to perform a particular function.
• Tissue types include: Epithelial, Connective, Muscle, Nervous

Microtechniques

• Microtechniques prepare tissue to study them microscopically
• Methods include paraffin method and freezing
• Basic techniques include fixation, dehydration, clearing, infiltration, embedding, cutting/sectioning, staining, and mounting.

Fixation

• Fixation preserves tissues to "life-like" state
• Prevents autolysis and putrefaction
• Simple fixatives: formaldehyde, alcohol, acetic acid, osmic acid, and picric acid
• Complex fixatives: Bouin's fluid, Zenker's fluid, and special mixtures
• 10% formaldehyde is the cheapest and easiest.

Dehydration

• Dehydration aims to remove water for paraffin impregnation
• Done by increasing alcohol concentrations
• Prevents tissue shrinkage

Clearing

• Clearing removes alcohol and allows paraffin to permeate the tissue.
• The tissue is immersed in xylene (xylol) which is miscible with both dehydration and embedding mediums
• The process is named for the resulting clear tissue appearance

Infiltration

• Infiltration (impregnation with paraffin) immerses the specimen in a medium for easy cutting
• Melted paraffin penetrates the tissue and replaces xylene
• Usually takes 15 minutes in the oven

Embedding

• Embedding in hard paraffin involves placing the tissue in a mould with melted paraffin, then allowing it to solidify
• Forms a hard paraffin block of wax that contains the tissue

Sectioning

• Sectioning cuts the paraffin block into thin slices (4-10 µm)
• This is done with a microtome to prepare for microscopic viewing
• Tissue sections are then transferred to glass slides

Staining

• Staining adds color to tissue structures, enabling differentiation
• Numerous different staining procedures exist
• Hematoxylin and eosin (H&E) is the most common stain
• H&E stains nuclei blue and cytoplasm pink

Hematoxylin and Eosin (H&E)

• H&E stains are commonly used in routine histological examination of tissue.
• Hematoxylin is used as a basic dye that stains acidic components of cells blue.
• Eosin is used as an acidic dye that stains basic components of cells reddish-pink
• Commonly used across tissue specimens

Other Staining Techniques

• Periodic acid-Schiff (PAS) stains polysaccharides (e.g., glycogen)
• Masson's Trichrome stains collagen
• Sudan black and Oil Red stains lipids

Mounting

• Mounting places a protective medium on the tissue specimen
• Refractive index of the mounting medium should be similar to the glass
• This is done to allow for microscopic viewing while protecting the slide

Microscopy

• Light Microscopy has a maximum magnification of x1000
• Electron Microscopy has a maximum magnification of x1,000,000

Embedding Medium

• Paraffin is the most common embedding medium

Section Thickness

• Typical section thickness for light microscopy is 4-10 μm

Additional Information

• Tissue Processing Steps for FFPE Samples
• Questions regarding the above topics and processes