Epithelial tissue forms continuous sheets of cells that serve as the body’s protective linings and coverings, acting as a selective barrier between the body’s internal and external environments. Stratified Squamous Epithelium (SSE) is one of the most mechanically robust types of this tissue, characterized by multiple layers of cells. This multi-layered structure is adapted to resist physical stress, ensuring that if surface cells are damaged or sloughed off, the deeper layers remain intact to maintain the barrier function.
Defining Stratified Squamous Epithelium
The term “Epithelium” refers to any tissue that lines a cavity or covers a surface. “Stratified” indicates that the tissue is composed of more than one cell layer. “Squamous” describes the shape of the cells at the very top, or apical, surface. These superficial cells are flattened and scale-like, which is the defining shape used for classification. While the top cells are squamous, the cells in the deepest layers, near the base, are typically cuboidal or columnar in shape.
This tissue exhibits strong polarity, having distinct apical and basal surfaces. The deepest layer of cells, the basal layer, is the only layer directly anchored to the basement membrane, which separates the epithelium from the underlying connective tissue. Like all epithelial tissues, SSE is avascular, lacking its own blood vessels. Nutrients must diffuse from the capillaries in the underlying connective tissue, a process that limits the metabolic activity of cells far from the source.
The Distinct Cell Layers
The architecture of Stratified Squamous Epithelium is defined by its specific layers, or strata, which represent various stages of cellular maturation as cells migrate toward the surface. The deepest layer is the Stratum Basale, also known as the germinative layer, where cells are cuboidal or columnar and actively undergo mitosis. This continuous cell division produces new cells that push older cells upward, ensuring the tissue is constantly renewed.
Immediately above the basal layer lies the Stratum Spinosum, or “prickle cell layer,” which is typically the thickest stratum. Cells here are polyhedral and held tightly together by numerous desmosomes, strong cell-to-cell junctions that resist tension. When prepared for viewing, these desmosomes make the cells appear spiny, giving the layer its name.
As cells continue upward, they flatten and enter the Stratum Granulosum, where keratinization begins. Cells start to accumulate keratohyalin granules, precursors to the tough, fibrous protein keratin. The nuclei and organelles in these cells begin to break down, initiating cell death.
In thick skin, such as the soles or palms, an additional clear layer, the Stratum Lucidum, is present between the Stratum Granulosum and the top layer. This translucent layer consists of flattened, dead cells filled with a proteinaceous material, contributing to the tissue’s thickness. The final and most superficial layer is the Stratum Corneum, consisting of multiple layers of dead, flattened cells (squames). These cells have lost their nuclei and are completely filled with keratin, forming a highly effective protective sheet that is continuously shed.
Key Functional Roles and Locations
The primary function of Stratified Squamous Epithelium is to provide robust protection against physical damage, chemical exposure, and microbial invasion. The functional properties of the tissue depend on whether it is keratinized or non-keratinized, a distinction based on the composition of the superficial layer.
Keratinized Stratified Squamous Epithelium forms the epidermis, the outer layer of the skin, including the palms and soles. This type is characterized by a thick Stratum Corneum where dead cells are heavily packed with keratin, a tough, water-resistant protein. Keratinization provides a strong, dry, and impermeable barrier that prevents water loss and resists the damaging effects of heat and chemicals.
Non-keratinized Stratified Squamous Epithelium lines moist internal surfaces, such as the mouth, esophagus, vagina, and cornea. While it provides protection against abrasion, its superficial cells remain alive and retain their nuclei, lacking the dense keratin layer found in skin. This lack of keratin means the surface must be kept moist by bodily secretions and is slightly more porous than keratinized skin. This type is adapted to environments where friction occurs under wet conditions.