# Stratified Epithelium: Types, Locations, and Function Stratified epithelium is an epithelial tissue built from two or more layers of cells in which only the deepest (basal) layer rests on the basement membrane and retains the ability to divide. The tissue is named by the shape of the cells at its free (apical) surface, not by the cells buried beneath them. That single rule explains most of what follows. A stratified squamous epithelium can sit on a base of cuboidal or columnar cells and still be called squamous, because the name describes the top, where the tissue meets the outside world. For veterinary students, this is the first useful fact about the tissue and the one most often missed on examinations. Stratified epithelium matters because it is the body's abrasion-resistant barrier. Skin, the oral cavity, the esophagus, the vagina, and the urinary bladder all take mechanical, chemical, or osmotic punishment that a single layer of cells could not survive. Where the body expects friction, it builds depth. Where it expects diffusion, it builds thinness. Recognizing which strategy a tissue has adopted tells you what that organ does before you read a single physiology chapter. ## What Makes an Epithelium Stratified Epithelia are classified by two features: the number of cell layers and the shape of the apical cells. A simple epithelium has one layer. A stratified epithelium has two or more. Pseudostratified epithelium looks layered because nuclei sit at different heights, but every cell touches the basement membrane, so it is a single layer. That distinction is not cosmetic. In pseudostratified tissue, all cells are anchored. In stratified tissue, only the bottom row is anchored, and everything above it is supported by its neighbors. The basal layer sits on the basement membrane and contains the dividing cells. Cells produced there are pushed upward, differentiate, and eventually reach the surface, where they are shed. The intermediate layers are transitional populations at various stages of maturation. The apical cells are post-mitotic, meaning they have permanently exited the cell cycle and can no longer divide. This arrangement is why stratified epithelia renew from the bottom rather than the top, and why an injury that destroys only the surface heals by migration and division of cells from below. The basement membrane itself is a thin sheet of extracellular matrix that anchors the epithelium to underlying connective tissue and acts as a selective barrier. In stratified epithelia, it also serves as the physical limit of the proliferative compartment. Cells that lose contact with it and move upward generally lose proliferative capacity as they differentiate. ## The Four Types of Stratified Epithelium ### Stratified Squamous Epithelium Stratified squamous epithelium is the most widespread stratified type in the body. It has many layers, and the cells at the surface are flattened, or squamous. Two forms exist, and the difference between them is one of the most practically important distinctions in veterinary histology. **Keratinized stratified squamous epithelium** has a surface layer of dead cells filled with keratin, a tough structural protein. The keratinized layer is water-resistant and highly abrasion-resistant. It forms the epidermis of the skin across essentially all domestic species. The thickness of this layer varies enormously. Thin skin, such as that on the abdomen of a dog or cat, has a relatively thin keratin layer. Thick skin, such as the digital pads of dogs and the planum nasale of cattle and sheep, has a much heavier keratinized surface. Species differences in skin thickness are real and clinically relevant. A dog's dorsal skin is measurably thicker than a cat's, and the epidermis of a horse's cannon region is thinner than the epidermis over the rump. These differences affect wound healing, drug absorption through the skin, and how much tissue a biopsy needle must traverse. **Non-keratinized stratified squamous epithelium** has living cells all the way to the surface. The apical cells are flattened but retain nuclei and are kept moist. This type lines the oral mucosa, the esophagus, the vagina, and the conjunctiva. It is protective but not waterproof. In the esophagus, the non-keratinized stratified squamous epithelium protects against the abrasion of swallowed food. In the vagina, it protects against friction and provides a barrier to infection. The transition between non-keratinized and keratinized epithelium is a gradient, not a switch. In regions like the lip, the mucosa of the hard palate, and the rumen of cattle, you can find intermediate states where the surface cells are partially keratinized. The degree of keratinization increases as you move from a moist internal surface toward a dry external one. This gradient is visible in the esophageal epithelium of the developing rabbit, where keratinized layers and mitotic divisions appear only near the end of gestation, and even then the surface is not uniformly cornified [1]. The same principle applies in adult animals: keratinization is a matter of degree, and the histologist's job is to describe where a given sample falls on that spectrum. ### Stratified Cuboidal Epithelium Stratified cuboidal epithelium is the least common of the four types. It consists of two or more layers of cells in which the apical cells are roughly cube-shaped, with a central round nucleus. It is found in the ducts of sweat glands and in some larger exocrine ducts. Its function is largely protective and secretory, providing a lining that can withstand the flow of glandular secretions without being worn away. This type appears in unexpected places during development and in disease. In the mouse vomeronasal organ, the lateral wall of the middle segment and the entire lumen of the rostral segment are lined by stratified cuboidal epithelium in the newborn animal, which later transforms into ciliated columnar epithelium except in the vomeronasal duct, where it becomes stratified squamous [2]. In thyroglossal duct cysts in humans, stratified cuboidal epithelium was detected only in cysts at the level of the hyoid bone, while stratified squamous and pseudostratified ciliated epithelium dominated other locations [3]. These examples are not directly about domestic species, but they illustrate a general principle: stratified cuboidal epithelium is often a transitional or regionally restricted lining, not a permanent feature of large surface areas. ### Stratified Columnar Epithelium Stratified columnar epithelium is rare in adult mammals. It has multiple layers, and the apical cells are taller than they are wide, with nuclei typically located toward the base of the cell. It is found in a few specific locations, including parts of the male urethra and the conjunctiva, and in the larger ducts of some glands. Its function is protective and, in some locations, secretory. The rarity of this type in adults is worth noting because it appears more often during development. The esophageal epithelium of the rabbit passes through a stratified ciliated columnar stage before becoming stratified squamous [1]. The larynx of the Suncus murinus, a small mammal, contains an intermediate epithelium in the transitional zone between stratified squamous and ciliated columnar epithelium, with gradations ranging from stratified squamous through stratified cuboidal to ciliated stratified low-columnar [4]. These findings reinforce the idea that stratified columnar epithelium is often a transitional form rather than a stable adult tissue. ### Transitional Epithelium (Urothelium) Transitional epithelium, also called urothelium, is unique to the urinary tract. It lines the renal pelvis, ureters, urinary bladder, and proximal urethra. Its distinguishing feature is its ability to stretch and recoil without losing barrier function. In the relaxed state, the apical cells are large and dome-shaped, often binucleated or multinucleated. As the bladder fills, the epithelium thins, and the apical cells flatten. This change in shape is reversible and is the reason the tissue was historically called transitional. The number of cell layers in urothelium varies by species and by distension. In a relaxed bladder, the epithelium may appear to have three to six layers. In a distended bladder, it may appear to have only two or three. This variation is normal and is one reason urothelium can be mistaken for a simple epithelium in a poorly distended sample. The apical cells are joined by tight junctions that maintain the urine-blood barrier, and the apical membrane is reinforced with plaques of uroplakin proteins that reduce permeability. Species differences in urothelial cell layers are well documented but not easily reduced to a single number. The general pattern is that larger species tend to have more layers in the relaxed state, but the range overlaps considerably. What matters for the student is the principle: urothelium is a stratified epithelium whose layer count is dynamic, and the apical cell shape is the most reliable identifying feature. ## Comparison Table of Stratified Types | Type | Layer count | Apical cell shape | Keratin status | Representative locations | Main function | |--|--|--|--|--|--| | Stratified squamous, keratinized | Many | Flattened, dead | Keratinized | Epidermis, digital pads, planum nasale | Abrasion and water barrier | | Stratified squamous, non-keratinized | Many | Flattened, living | Non-keratinized | Oral mucosa, esophagus, vagina, conjunctiva | Protection of moist surfaces | | Stratified cuboidal | Two or more | Cube-shaped | Non-keratinized | Sweat gland ducts, some exocrine ducts | Protection and secretion | | Stratified columnar | Two or more | Taller than wide | Non-keratinized | Male urethra, conjunctiva, some large ducts | Protection and secretion | | Transitional (urothelium) | Three to six, variable | Dome-shaped to flattened | Non-keratinized | Renal pelvis, ureter, bladder, proximal urethra | Stretchable barrier | ## Only the Basal Layer Divides The proliferative compartment of a stratified epithelium is confined to the basal layer. Cells in the intermediate and apical layers are post-mitotic. This is not a minor detail. It determines how the tissue renews, how it responds to injury, and why some epithelial cancers behave the way they do. In the mouse vomeronasal organ, mitotic figures were observed in the boundary between sensory and ciliated columnar epithelium and in the rostral segment during a specific postnatal window, but not in the differentiated apical layers [2]. In the developing rabbit esophagus, mitotic divisions were demonstrable at the 30th gestational day, coinciding with the appearance of a keratinized layer [1]. These observations fit the general rule: division happens at the base, differentiation happens above it. The practical consequence is that a stratified epithelium cannot repair itself by division of surface cells. When the surface is damaged, the repair comes from the basal layer, which produces new cells that migrate upward and differentiate. If the basement membrane is destroyed, the epithelium cannot regenerate properly, and healing proceeds by fibrosis instead. This is why deep wounds that remove the entire epidermis scar, while superficial abrasions that leave the basal layer intact heal without scarring. ## Locations and Functions in Domestic Species ### Skin and Epidermis The epidermis is a keratinized stratified squamous epithelium. Its thickness varies by species, body region, and age. In dogs, the epidermis is thin over the abdomen and thicker over the dorsum and digital pads. In cats, the epidermis is generally thinner than in dogs. In horses, the epidermis is thin over the cannon and thicker over the rump and neck. In cattle, the planum nasale has a thick keratinized epidermis, while the skin of the flank is thinner. These differences are not trivial. They affect how quickly a wound heals, how much protection the skin offers against ultraviolet radiation, and how drugs are absorbed through the skin. The epidermis is also a site of immune surveillance. Keratinocytes express thrombomodulin, a cell surface glycoprotein with anticoagulant activity. In human stratified squamous epithelium, thrombomodulin protein was detected in all samples tested, including oral mucosa, larynx, esophagus, uterine ectocervix, and vagina, with staining localized to the suprabasal layer [5]. This finding is not directly about domestic animals, but it illustrates that stratified squamous epithelia are not passive barriers. They participate in local hemostasis and inflammation. ### Oral Mucosa and Esophagus The oral mucosa and esophagus are lined by non-keratinized stratified squamous epithelium in most domestic species. The oral mucosa is moist, flexible, and resistant to abrasion from food. The esophagus is a muscular tube lined by non-keratinized stratified squamous epithelium that protects against the mechanical trauma of swallowed food. In the rabbit, the esophageal epithelium develops through a stratified ciliated columnar stage before becoming stratified squamous, and even at the end of gestation, some regions remain non-cornified [1]. In the rainbow trout, the esophageal epithelium shows a gradual transition from stratified cuboidal anteriorly to simple columnar distally, with mucous cells producing neutral, carboxylated, and sulphated mucosubstances [6]. This is a useful reminder that esophageal structure is not uniform across vertebrates, and that the stratified squamous pattern of mammals is one solution among several. ### Sweat Gland Ducts Sweat gland ducts are lined by stratified cuboidal epithelium. The duct carries sweat from the secretory coil to the skin surface, and the epithelium provides a protective lining that resists the flow of secretion. In species with abundant sweat glands, such as horses and cattle, these ducts are numerous and easily identified in histologic sections. ### Male Urethra and Conjunctiva The male urethra is lined by stratified columnar epithelium in its intermediate portion, with stratified squamous epithelium near the external opening. The conjunctiva, the membrane covering the inner surface of the eyelids and the sclera, is lined by stratified columnar epithelium with goblet cells. Both locations require a lining that can resist friction and, in the case of the conjunctiva, maintain a moist surface. ### Urinary Bladder and Urothelium The urinary bladder is lined by transitional epithelium. The same epithelium extends into the ureters and renal pelvis. Its function is to store urine without allowing its solutes to leak back into the blood. The apical cells are joined by tight junctions and reinforced by uroplakin plaques. When the bladder fills, the epithelium stretches and the apical cells flatten. When the bladder empties, the cells return to their dome shape. The urothelium is not immutable. In a heritable form of urothelial keratinizing squamous metaplasia, a truncating variant in the gene encoding retinoic acid receptor gamma (RARγ) was found to segregate with the phenotype in a single family, and mice heterozygous for the variant showed upregulation of cytokeratin-10 in the bladder and ureteric epithelium, consistent with keratinizing squamous metaplasia [7]. This is a human study, but it demonstrates that urothelium can be reprogrammed toward a squamous keratinizing phenotype when retinoic acid signaling is disrupted. The clinical lesson is that urothelial identity depends on active transcriptional maintenance, not just on anatomical position. ## How Stratified Epithelium Is Observed in Practice Histologic identification of stratified epithelium begins with low-magnification examination. The first question is whether the epithelium has one layer or more than one. If more than one, the next question is the shape of the apical cells. Flattened apical cells indicate squamous. Cube-shaped apical cells indicate cuboidal. Taller-than-wide apical cells indicate columnar. Dome-shaped apical cells with a variable layer count indicate transitional. The basal layer should be examined for mitotic figures. In a normal stratified epithelium, mitoses are confined to the basal and immediately suprabasal layers. Mitoses in the apical layers suggest dysplasia or neoplasia. The presence of keratin on the surface confirms keratinization, but the absence of a visible keratin layer does not rule out a keratinized epithelium, because keratinization is a gradient and may be focal. Special stains can help. Cytokeratin immunohistochemistry can distinguish squamous from columnar differentiation. In Rathke's cleft cysts with squamous metaplasia, CAM5.2 was positive in glandular epithelium and focally positive in squamous epithelium, while 34βE12 was positive in squamous epithelium and focally positive in glandular epithelium [8]. These patterns are not directly applicable to domestic species, but they illustrate the principle that different epithelial types express different cytokeratin profiles. In the urinary bladder, the layer count should be interpreted in the context of distension. A biopsy from a contracted bladder may show more layers than one from a distended bladder. The apical cell shape is the more reliable criterion. ## Clinical Relevance, Limitations and Common Mistakes The most common mistake is confusing stratified epithelium with pseudostratified epithelium. Pseudostratified epithelium is a single layer of cells in which nuclei sit at different heights, giving a false impression of stratification. Every cell in a pseudostratified epithelium touches the basement membrane. In a true stratified epithelium, only the basal layer touches the basement membrane. The distinction matters because pseudostratified epithelium is found in the respiratory tract and some ducts, while stratified epithelium is found in skin, oral mucosa, esophagus, and urinary tract. A misdiagnosis based on this confusion can lead to incorrect conclusions about tissue origin and function. A second common mistake is assuming that keratinization is an all-or-nothing property. It is not. The surface of the oral mucosa may be partially keratinized in some regions and non-keratinized in others. The rumen of cattle has a keratinized stratified squamous epithelium that is not the same as the epidermis, but it shares the property of surface keratinization. The degree of keratinization reflects the local mechanical and osmotic environment, and it can change with disease. A third mistake is treating the layer count of urothelium as fixed. It is not. The number of layers in the urinary bladder varies with distension and with species. A relaxed bladder may show five or six layers, while a distended bladder may show two or three. The apical cell shape is the more stable feature. A fourth mistake is assuming that all stratified epithelia renew at the same rate. Epidermis renews continuously, with a turnover time measured in weeks. Oral mucosa renews faster. Urothelium renews more slowly. These differences affect how quickly a tissue recovers from injury and how susceptible it is to cumulative damage. This article is educational and is not a substitute for veterinary diagnosis or treatment. Individual cases require a veterinarian. ## Quick Review 1. Stratified epithelium has two or more layers, and only the basal layer divides. 2. The type is named by the shape of the apical cells, not the basal cells. 3. Keratinized stratified squamous epithelium forms the epidermis and is water-resistant. 4. Non-keratinized stratified squamous epithelium lines the oral mucosa, esophagus, vagina, and conjunctiva. 5. Stratified cuboidal epithelium lines sweat gland ducts and some exocrine ducts. 6. Stratified columnar epithelium is rare in adults and appears in the male urethra and conjunctiva. 7. Transitional epithelium (urothelium) lines the urinary tract and changes shape with distension. 8. Pseudostratified epithelium is a single layer, not a stratified type. 9. Keratinization is a gradient, not a switch. 10. Species differences in skin thickness and urothelial layer count are normal. ## Frequently Asked Questions ### What is the difference between stratified and pseudostratified epithelium? Stratified epithelium has two or more layers, and only the basal layer rests on the basement membrane. Pseudostratified epithelium is a single layer in which all cells touch the basement membrane, even though their nuclei sit at different heights. The two are often confused because pseudostratified tissue looks layered in a two-dimensional section. ### Which stratified epithelium lines the urinary bladder? Transitional epithelium, also called urothelium, lines the urinary bladder, ureters, and renal pelvis. Its apical cells are dome-shaped when the bladder is relaxed and flatten when the bladder is distended. The layer count varies with distension and species. ### Is the epidermis keratinized or non-keratinized? The epidermis is keratinized stratified squamous epithelium. The surface cells are dead and filled with keratin, which makes the skin water-resistant and abrasion-resistant. The degree of keratinization varies by species and body region. ### Why does only the basal layer divide in stratified epithelium? The basal layer is the only layer in contact with the basement membrane, which provides the signals needed for [cell division](/blog/guides/cell-division). Cells that move upward lose contact with the basement membrane and exit the cell cycle. This arrangement ensures that renewal occurs from the base and that damaged surface cells are replaced from below. ### What is the function of stratified cuboidal epithelium? Stratified cuboidal epithelium protects the ducts of sweat glands and some exocrine glands. It provides a durable lining that resists the flow of secretions. It is less common than stratified squamous epithelium and is often found in restricted locations. ### Can stratified epithelium change from one type to another? Yes. Stratified epithelium can undergo metaplasia, in which one mature epithelial type is replaced by another. For example, urothelium can transition to keratinizing squamous epithelium when retinoic acid signaling is disrupted. Metaplasia is usually a response to chronic stress or a genetic defect, and it can have clinical consequences. ## Related Articles - [RNA Location: Types, Functions, and Cellular Distribution](/knowledge/molecular-biology/rna-location) - [Z-DNA Found in Biology: Location, Function, and Detection](/knowledge/molecular-biology/z-dna-found) - [Prokaryotic DNA: Location, Structure, and Function](/knowledge/diagnostics/molecular/prokaryotic-dna-location-structure-and-function) - [Plasmid in Bacteria: Structure, Types, and Functions](/knowledge/molecular-biology/plasmid-a-bacteria) - [Histone Acetyltransferase: Function, Types, and Mechanism](/knowledge/molecular-biology/histone-acetyltransferase) - [Plasmid Definition: Structure, Types, and Functions in Genetics](/knowledge/molecular-biology/plasmid-definition) - [Epithelial Cells: Types, Locations, and Functions](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/epithelial-cells-types-locations-and-functions) - [Keratinized Stratified Squamous Epithelium Guide](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/keratinized-stratified-squamous-epithelium-guide) - [Simple Cuboidal Epithelium: Function and Locations](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/simple-cuboidal-epithelium-function-and-locations) ## Sources 1. 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[Impaired retinoic acid receptor-γ signaling underlies a heritable form of urothelial keratinizing squamous metaplasia.](https://pubmed.ncbi.nlm.nih.gov/41830175/) 8. [Rathke's cleft cysts with significant squamous metaplasia--high risk of postoperative deterioration and close origins to craniopharyngioma.](https://pubmed.ncbi.nlm.nih.gov/23371400/)