8.7: Classification of Epithelial Tissues: Stratified Epithelium
Stratified epithelium consists of several stacked layers of cells. They provide the durability to withstand constant physical and chemical attacks. Stratified epithelium is named after the shape of the most apical layer of cells. Stratified squamous epithelium is the most common type found in the human body. In this tissue, the apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The basal cells divide to form new daughter cells, which gradually become flattened while being pushed to replace the damaged cells wearing off from the top.
The top layer of some stratified squamous epithelia is called the keratinized epithelium and is covered with dead cells filled with keratin. Keratin provides toughness and a water-resistant property to this epithelium. This is why keratinized epithelia are found in exposed body surfaces like the skin.
In other cases, the top layer of flattened cells is not entirely filled with keratin and is called non-keratinized epithelium. This non-keratinized epithelia lines moist body surfaces, such as the esophagus, the vagina, parts of the anus, the floor of the oral cavity, and the dorsal part of the tongue.
Stratified cuboidal epithelium and stratified columnar epithelium are rare in the human body. Typically two layers of cuboidal cells are found lining the ducts of the sweat and mammary glands. In stratified columnar epithelium, superficial cells are columnar and help protect some portions of the pharynx, the epiglottis, the anus, and the glandular ducts.
A special kind of stratified epithelium is the transitional epithelium, or the urothelium, named for the shape change that occurs in the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically in the ureter and the urinary bladder. When the bladder is empty, this epithelium has cuboidal apical cells with convex, umbrella-shaped apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal-shaped to squamous-shaped. Thus, the urothelium appears thicker and more multi-layered when the bladder is empty but more stretched out and less stratified when it is full and distended.
Reference:
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues
