Classification of Epithelial Tissues: Stratified Epithelium

Stratified epithelium is a multi-layered tissue that covers exposed body surfaces and provides durability to withstand constant physical and chemical attacks. Cells at the basal layer continuously divide to replace old, damaged cells that wear off from the exposed surface, ensuring continuous protection and tissue renewal.

Keratinized epithelia have anucleated surface cells densely packed with keratin filaments, making them sturdy and water-resistant. They cover exposed surfaces like skin, palms, and foot soles. Non-keratinized epithelia retain nucleated surface cells with less keratin and line moist body surfaces such as the oral cavity, esophagus, and anus.

Stratified cuboidal epithelium is rare and typically consists of two layers of cuboidal cells lining the ducts of sweat and mammary glands. Stratified columnar epithelium is also uncommon, with superficial columnar cells that help protect portions of the pharynx, epiglottis, anus, and glandular ducts.

Transitional epithelium, or urothelium, is a specialized stratified lining found only in the urinary system, specifically in the ureter and urinary bladder. When the bladder is empty, apical cells are cuboidal with umbrella-shaped surfaces. As the bladder fills with urine, these cells flatten and transition to squamous shape, allowing the epithelium to stretch and accommodate volume changes.

The basal layer contains columnar or cuboidal cells that continuously divide to form new daughter cells. These new cells gradually flatten as they are pushed upward to replace damaged cells wearing off from the apical surface, maintaining the tissue's protective function and structural integrity.

Keratin is a fibrous protein that fills the dead cells of the apical layer in keratinized epithelia. It provides toughness and water-resistant properties, making these tissues ideal for exposed body surfaces like skin that experience constant friction and environmental exposure.

Stratified epithelium consists of multiple stacked cell layers, whereas simple epithelium is a single layer. Stratified epithelium is named after the shape of its most apical cells and provides greater durability for exposed surfaces. Understanding both types is essential to classification of epithelial tissues overview.