Epithelial tissue is one of the four basic tissue types, forming coverings and linings throughout the body. When described as “stratified,” it means it is composed of multiple layers of cells, stacked one upon another. This multi-layered arrangement is a common feature across many surfaces and internal structures, highlighting its broad presence and significance.
Understanding Stratified Epithelial Tissue
Stratified epithelial tissue is characterized by its distinct multi-layered cellular arrangement, with two or more layers of densely packed cells. The deepest layer of cells is anchored to an underlying basement membrane, which provides an attachment site and separates it from connective tissue below.
This tissue is avascular, meaning it does not have a direct blood supply within its layers. Instead, nutrients and oxygen diffuse from blood vessels in the underlying connective tissue, reaching the epithelial cells.
Cells in the deeper, or basal, layers often exhibit a cuboidal or columnar shape, actively undergoing cell division to produce new cells. As new cells are formed, older cells are pushed towards the surface, where they eventually flatten and are shed, a process known as constant cell turnover. The specific shape of the cells in the most superficial layer determines the classification of the stratified epithelial tissue.
Primary Roles of Stratified Epithelial Tissue
The primary function of stratified epithelial tissue is protection against various external and internal stressors. Its multi-layered structure acts as a robust barrier, safeguarding underlying tissues from physical abrasion, mechanical stress, and chemical damage. For instance, the numerous layers effectively cushion against wear and tear in areas subjected to frequent friction.
This tissue also forms a protective barrier against pathogens. Furthermore, it plays a role in preventing excessive water loss from the body, particularly in exposed areas. While protection is its main role, some stratified epithelia may also have minor functions such as secretion, as observed in certain glandular ducts, or limited absorption.
Varieties of Stratified Epithelial Tissue
Stratified Squamous Epithelium
Stratified squamous epithelium is the most prevalent type, characterized by flattened, scale-like cells on its surface. This type can be further divided into keratinized and non-keratinized forms. Keratinized stratified squamous epithelium contains a tough, fibrous protein called keratin, which provides additional strength, durability, and waterproofing. Its surface cells often lack nuclei and are metabolically inactive. In contrast, non-keratinized stratified squamous epithelium lacks a significant keratin layer, and its surface cells retain their nuclei and metabolic functions, often being kept moist by mucus.
Stratified Cuboidal Epithelium
Stratified cuboidal epithelium is less common and consists of cube-shaped cells at its surface. Its function is primarily structural reinforcement.
Stratified Columnar Epithelium
Similarly rare is stratified columnar epithelium, which features column-shaped cells in its outermost layer. This type provides protection and can be involved in mucous secretion.
Transitional Epithelium
Transitional epithelium, also known as urothelium, is a unique type found only in the urinary system. Its superficial cells are typically dome-shaped or cuboidal when relaxed, but they have the ability to flatten and stretch when the organ they line becomes distended, accommodating changes in volume.
Locations in the Body
Stratified Squamous Epithelium Locations
The specific type of stratified epithelial tissue found in a particular location directly relates to the demands and functions of that area. Stratified squamous epithelium, due to its protective capabilities, is widely distributed. Keratinized stratified squamous epithelium forms the epidermis of the skin, offering protection against abrasion, physical damage, and water loss. Non-keratinized stratified squamous epithelium lines moist internal surfaces such as the mouth, esophagus, vagina, and anus, where it provides protection against friction and physical wear without requiring the waterproofing of keratin.
Stratified Cuboidal Epithelium Locations
Stratified cuboidal epithelium is found in the excretory ducts of various glands, including sweat glands and mammary glands. This arrangement offers a degree of protection and contributes to the structural integrity of these ducts.
Stratified Columnar Epithelium Locations
Stratified columnar epithelium is observed in specific areas such as parts of the male urethra and large ducts of some glands. It contributes to both protection and some secretion.
Transitional Epithelium Locations
Transitional epithelium is exclusively located within the urinary system, lining organs like the urinary bladder and ureters. Its unique ability to stretch and change shape allows these organs to expand and contract, accommodating fluctuations in urine volume without damage.