Stratified squamous epithelium is a tissue composed of multiple layers of cells that line various body surfaces. The cells at the surface layer are flat (squamous), while deeper cells are typically cuboidal or columnar. This multi-layered structure is adapted for protection, acting as a robust physical barrier against mechanical stress and chemical abrasions. The lower, basal layer constantly divides, pushing older cells toward the surface. This renewal process allows superficial layers to be sloughed off and replaced, making the tissue well-suited to areas that experience high wear and tear.
The Protective Structure: Keratinized vs. Non-Keratinized Forms
The structural composition of stratified squamous epithelium divides it into two primary types, determined by the presence or absence of keratin in the outermost cell layers. Keratinized epithelium contains a significant accumulation of this tough, fibrous protein, rendering the surface cells metabolically inactive and dead.
As cells in the keratinized form move toward the surface, they accumulate keratin and lose their nuclei and organelles, forming a protective, waterproof layer called the stratum corneum. This layer creates a highly effective barrier against dehydration and physical trauma. Conversely, the non-keratinized form lacks a substantial keratin layer, meaning the surface cells remain alive and retain their nuclei. This tissue must be kept moist by bodily secretions, as it is less resistant to drying out.
Locations Requiring Extreme Durability: Keratinized Epithelium
Keratinized stratified squamous epithelium is found in locations facing constant external exposure, requiring maximum durability and resistance to water loss. The most recognized example is the epidermis, the outermost layer of the skin covering the entire body. The thick, keratin-rich layer provides defense against physical trauma, microbial invasion, and environmental chemicals.
This barrier also prevents desiccation, or water loss, from underlying tissues, regulating the body’s hydration. The keratin layer’s thickness varies based on the amount of friction an area endures. For instance, the soles of the feet and palms of the hands have notably denser keratin layers to withstand constant pressure and rubbing. Certain areas within the oral cavity, such as the hard palate and the superior surface of the tongue, also exhibit keratinization to resist high chewing forces.
Locations Requiring Moist Protection: Non-Keratinized Epithelium
Non-keratinized stratified squamous epithelium lines internal surfaces needing protection from abrasion while remaining moist for functional purposes. These surfaces rely on lubricating secretions, such as mucus, to maintain integrity and prevent drying.
This tissue lines the entire oral cavity, including the cheeks and soft palate, providing a flexible lining necessary for speech and initial food processing. It also forms the lining of the esophagus, where its smooth, moist surface is essential for the easy passage of food during swallowing. The vagina and anal canal are also lined with this tissue, providing protection from friction and abrasive waste material, respectively, while remaining pliable. Finally, the cornea of the eye is covered by a non-keratinized layer, which protects delicate underlying structures while remaining transparent and wet for clear vision.