Epithelial tissues are sheets of cells that cover body surfaces, line body cavities, and form glands. They are classified based on cell shape and arrangement.
Classification of Epithelial Tissues
Epithelial types are primarily categorized by two characteristics:
1. Cell Shape:
- Squamous: Thin, flat cells. Think of a fried egg. Found in areas requiring diffusion or filtration (e.g., alveoli of lungs, blood vessels).
- Cuboidal: Cube-shaped cells. Found in areas involved in secretion or absorption (e.g., kidney tubules, glands).
- Columnar: Tall, column-shaped cells. Often found where secretion or absorption is needed, and can have cilia (e.g., lining of digestive tract, fallopian tubes).
2. Cell Arrangement (Layers):
- Simple: A single layer of cells.
- Stratified: Multiple layers of cells.
- Pseudostratified: Appears stratified but is actually a single layer of cells with varying heights.
Combining these characteristics leads to various epithelial types:
- Simple squamous epithelium: Found in the alveoli of the lungs, facilitating gas exchange.
- Stratified squamous epithelium: Forms the epidermis of the skin, providing protection.
- Simple cuboidal epithelium: Lines kidney tubules, aiding in reabsorption.
- Stratified cuboidal epithelium: Rare, found in some ducts of glands.
- Simple columnar epithelium: Lines the digestive tract, involved in absorption and secretion.
- Stratified columnar epithelium: Found in the male urethra and salivary gland ducts.
- Pseudostratified columnar epithelium: Lines the respiratory tract, often ciliated to move mucus.
- Transitional epithelium: Lines the urinary bladder, capable of stretching. Also known as urothelium.
Additionally, some specialized types exist like keratinised stratified squamous epithelium, found in the outer layer of the skin, providing a tough, waterproof barrier. The type of epithelium present in a specific location reflects its function. For example, the thin, delicate simple squamous epithelium in the lungs allows for efficient gas exchange, while the thick, protective stratified squamous epithelium of the skin prevents water loss and protects against injury.
Specific examples from the provided references include the location of stratified columnar epithelium in the male urethra and salivary glands, pseudostratified columnar epithelium in the respiratory passage and gland ducts, and transitional epithelium in the urinary bladder, urethra, and ureter. These references also highlight the role of epithelium in various cancers, particularly ovarian cancer, where epithelial cells are the origin of the most common type.
