Types of Stratified Epithelium
In biology, "epithelium" refers to cells that line or cover a surface of our bodies, such as skin cells or the lining of inner body cavities, organs, tubes or ducts. Cells' shape and layered formation characterize the various types of epithelium in the body. Simple epithelium consists of a single layer of cells attached to a basement membrane. Stratified epithelium, found in various areas in the body, consists of two or more layers of cells.
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Stratified Squamous Epithelium
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Stratified squamous epithelium consists of multiple layers of flat, elongated cells and resides on surfaces that usually encounter friction, causing the outermost layer of cells to slough off. This type of stratified epithelium is either keratinized, wherein the outer layer of cells are dead, dry and hardened as with skin, or nonkeratinized, wherein the outer layer of cells line the lumen of moist surfaces of passageways leading to the exterior of the body, such as the esophagus, mouth, cornea, anus and vagina.
Stratified Cuboidal Epithelium
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Stratified cuboidal epithelium consists of at least two layers of cells shaped like a cube or square. Larger glandular ducts such as eccrine sweat gland ducts commonly contain this type of layered epithelium.
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Stratified Columnar Epithelium
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Multiple layers of cells in which the outermost layer of cells are taller than they are wide, in a rectangular shape, make up stratified columnar epithelial tissue. Large glandular ducts such as salivary or mammary glands contain columnar epithelium, in addition to part of the pharynx, larynx, urethra, anus and tubules of the male testis.
Pseudostratified Epithelium
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Pseudostratified epithelium derives its name from its resemblance to multiple layers of cells, but it actually consists of only one layer of various-sized cells with nuclei at different heights. Some cells are tall enough to reach the free surface, and some are not, but each cell connects to the basement membrane in a single layer. Locations that consist of pseudostratified epithelium include the respiratory tract such as the nasopharynx, trachea and bronchi, as well as the male reproductive system such as the epididymis, seminal vesicles and vas deferens. This type of epithelium may or may not be ciliated.
Transitional Epithelium
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Transitional epithelium is a stratified epithelium, but the cells' shapes change to accommodate the stretching of the tissue where these cells are found. This type of tissue lines the pelvis of the kidney, as well as most of the urinary tract including the bladder, urethra and ureter. The number of layers of cells changes depending on whether the bladder walls are stretched or relaxed. Surface cells often become bloated or inflated due to excess cytoplasm.
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