Histology: Epithelial Tissue Structure

Basement Membrane

• A thin extracellular layer separates the epithelium and connective tissue
• This layer is called the basement membrane

Epithelium Categories

• Covering and lining epithelium is classified based on location and function
• Simple epithelium is found in areas with minimal wear and tear, specialized for absorption or filtration
• Stratified epithelium is found in areas with high wear and tear
• Pseudostratified epithelium has a single layer of cells, but some cells don't reach the surface giving a multilayered appearance
• Pseudostratified cells secrete mucus or contain cilia to move mucus and foreign particles

Epithelium Cell Shapes

• Squamous cells are flat, scale-like, and attached like a mosaic
• Cuboidal cells are cube-shaped in cross-section, sometimes appearing as hexagons
• Columnar cells are tall and cylindrical, appearing as rectangles set on end
• Transitional cells have a combination of shapes, found where there is a high degree of distension or expansion
• Transitional cells can range from cuboidal to squamous in different layers, depending on how much they are pulled out of shape

Simple Epithelium

• Consists of a single layer of cells
• Types: squamous, cuboidal, columnar

Stratified Epithelium

• Consists of at least two cell layers
• Types: squamous, cuboidal, columnar, transitional

Pseudostratified Epithelium

• Has a single layer of cells but appears stratified

Simple Squamous Epithelium

• Single layer of flat, scale-like cells
• Adapted for diffusion, osmosis, and filtration
• Found in the air sacs of lungs, kidney filtering areas, inner surfaces of the ear

Stratified Epithelium

• More durable than simple epithelium, protecting underlying tissues from wear and tear

Stratified Squamous Epithelium

• Cells in superficial layers are flat, while deep layers vary in shape
• Basal cells continually multiply pushing surface cells outward
• Surface cells become dehydrated, shrink, and harden

Nonkeratinized Stratified Squamous Epithelium

• Found on wet surfaces exposed to wear and tear
• Examples: lining of the mouth, tongue, esophagus, vagina

Keratinized Stratified Squamous Epithelium

• Surface cells are modified into a tough layer containing keratin (a protein resistant to water and friction)
• Found in the outer layer of skin

Stratified Cuboidal Epithelium

• Relatively uncommon
• Found in ducts of sweat glands, fornix of the conjunctiva, cavernous urethra, pharynx, epiglottis
• Primarily for protection

Stratified Columnar Epithelium

• Basal layer consists of shortened, irregularly polyhedral cells
• Superficial cells are columnar
• Functions in protection and secretion
• Found in parts of the male urethra, larger excretory ducts, anal mucous membrane

Transitional Epithelium

• Similar to nonkeratinized stratified squamous epithelium
• Outer layer cells are large and rounded, allowing stretching without breaking
• Lines the bladder and urethra
• When stretched, cells become squamous-like

Simple Cuboidal Epithelium

• Cells have a central nucleus
• Functions in secretion and absorption
• Found in: surface of ovaries, lens capsule, pigmented epithelium of the retina, kidney tubules, smaller glands

Simple Columnar Epithelium

• Nuclei located near the base of the cells
• Can be ciliated or nonciliated
• Nonciliated type lines the digestive tract, excretory ducts, and gallbladder
• Ciliated type lines upper respiratory tract, uterine tubes, uterus, paranasal sinuses, central canal of the spinal cord

Modified Columnar Cells

• Luminal surfaces can be modified based on location and function
• In the digestive tract, cells protect underlying tissues and aid in food-related activities
• Microvilli: increase surface area for absorption of nutrients and fluids in the small intestine
• Goblet cells: secrete mucus as a lubricant in the digestive tract

This quiz covers the structure and classification of epithelial tissues, including the basement membrane and various cell shapes. Test your understanding of different epithelial types and their functions in the human body.