Epithelial tissue forms the body’s covering and internal lining, acting as a barrier between the internal environment and the outside world. Stratified squamous epithelium (SSE) is a major type of epithelial tissue characterized by multiple layers of cells. This structure provides resistance to physical forces and environmental stress. SSE is suited to protect surfaces subjected to constant wear and tear.
Defining the Structure of Stratified Squamous Epithelium
The structure of SSE is defined by its name, where “stratified” refers to the multiple layers of cells, and “squamous” describes the flattened shape of the cells at the exposed surface. This tissue is anchored to the underlying connective tissue by a basement membrane, but only the deepest layer of cells is in direct contact with it. These cells of the basal layer are often cuboidal or columnar in shape and function as stem cells, continuously dividing to produce new cells.
As new cells are generated, they are pushed upward toward the apical, or exposed, surface. During this ascent, the cells gradually change shape, becoming flatter and more scale-like (the characteristic squamous shape). This continuous replacement ensures that cells lost from the surface are quickly replenished. The multi-layered thickness provides the bulk needed to withstand abrasive forces.
Primary Function: Protection Against Abrasion and Damage
The function of stratified squamous epithelium is to offer mechanical protection for the underlying tissues. The multiple stacked layers provide a barrier, allowing the outermost cells to be sloughed off due to friction without exposing underlying structures. This protects against physical abrasion common in areas like the mouth or the skin.
The tissue also acts as a shield against chemical irritation and microbial invasion. Tightly packed cells, held together by cell-to-cell junctions like desmosomes, create a nearly impermeable surface. This structural integrity prevents pathogens and harmful substances from penetrating deeper layers.
Location and Functional Variation
Stratified squamous epithelium is classified into two types based on the presence or absence of a specific protein in the surface layers. Functional variation depends on the environment where the tissue is found.
Keratinized Epithelium
Keratinized stratified squamous epithelium contains the fibrous protein keratin in its superficial layers, making it resistant to water loss. This type forms the epidermis of the skin, including the surfaces of the palms and soles of the feet. The keratin-filled surface cells are dead, forming a durable, waterproof layer that protects against desiccation and physical trauma.
Non-Keratinized Epithelium
Non-keratinized stratified squamous epithelium lines internal surfaces that must remain moist, such as the mouth, esophagus, vagina, and cornea. These surface cells remain alive and retain their nuclei, relying on mucus or secretions for lubrication. While it protects against friction and abrasion, this form is less resistant to water loss and is more permeable than keratinized epithelium.