Medical School · Year 1 · Histology · includes a quiz and discussion video

Lecture 2: Epithelial Tissue

Unit 1.2: Histology and Basic Tissues

Learning Objectives

By the end of this lecture, students will be able to:

1. Classify epithelial tissues by number of cell layers and cell shape
2. Describe specialized surface modifications of epithelial cells (microvilli, cilia, stereocilia)
3. Identify the types of cell junctions and their functions
4. Correlate epithelial structure with function in different organ systems
5. Describe the basement membrane and its components
6. Recognize common epithelial types in histological sections

Characteristics of Epithelial Tissue

Epithelial tissues form the linings and coverings of the body, creating continuous sheets of closely packed cells with minimal extracellular matrix between them. These tissues share several defining characteristics that distinguish them from other tissue types. Epithelia are avascular, meaning they lack blood vessels and must receive all nutrients through diffusion from underlying connective tissue. All epithelial cells rest upon a specialized extracellular matrix called the basement membrane, which separates them from the underlying tissue. Epithelial cells connect to their neighbors through various types of cell junctions, creating cohesive sheets that function as barriers. One of the most clinically relevant features of epithelia is their high regenerative capacity—these tissues constantly renew themselves through cell division, which allows them to recover from injury but also makes them susceptible to neoplastic transformation.

Epithelial tissues serve diverse functions throughout the body, always correlated with their specific structural characteristics. Protection represents a primary function, exemplified by the stratified squamous epithelium of the skin and oral cavity. Absorption occurs in the simple columnar epithelium of the intestinal lining, where surface modifications dramatically increase surface area. Secretion takes place in glandular epithelia, which we will explore in detail in the next lecture. Excretion of metabolic wastes occurs in kidney tubular epithelia. Sensory reception relies on specialized epithelia such as the olfactory epithelium of the nasal cavity. Transport of materials across epithelial surfaces, particularly through ciliary action, occurs in the respiratory tract.

A fundamental property of all epithelial cells is their polarity—the asymmetric organization of cell structure and function between different surfaces. The apical surface faces the lumen of an organ or the external environment and often displays specialized modifications suited to the epithelium's function. The basolateral surface includes both the lateral domain (contacting adjacent cells through junctions) and the basal domain (attached to the basement membrane). This polarity extends to the internal organization of the cell, with organelles distributed asymmetrically to support directional transport of molecules.

<image>Panel A: Apical surface of columnar epithelial cell displaying numerous microvilli projecting into pale yellow lumen with secretory vesicles clustered beneath apical membrane. Panel B: Lateral surface showing cell junctions connecting to adjacent cells with tight junctions near apex, adherens junctions below, and desmosomes scattered along the length. Panel C: Basal surface resting on basement membrane depicted as continuous purple line with underlying pink reticular layer and basally positioned nucleus. Panel D: Asymmetric organelle distribution with Golgi apparatus oriented toward apical surface and arrows indicating directional flow of transcytosis and secretion.</image>

Classification of Epithelia

Epithelial tissues are classified according to two criteria: the number of cell layers and the shape of the cells at the free surface. This classification system provides both descriptive terminology and functional insight.

Regarding cell layers, simple epithelia consist of a single layer of cells, with every cell touching the basement membrane. This arrangement facilitates diffusion and transport but provides relatively little protection. Stratified epithelia contain multiple cell layers, with only the basal layer contacting the basement membrane. These epithelia protect against mechanical, chemical, and microbial insults. Pseudostratified epithelium represents a unique category—though consisting of only a single layer of cells (all touching the basement membrane), it appears stratified because cells vary in height, and not all cells reach the luminal surface. Nuclei positioned at different levels reinforce this multilayered appearance.

Cell shape classification describes the morphology of cells at the free (apical) surface in simple epithelia, or at the surface layer in stratified epithelia. Squamous cells are flat and scale-like, with width exceeding height—their nuclei create characteristic bulges in very thin cells. Cuboidal cells appear roughly square in cross-section, with width approximately equal to height. Columnar cells are tall and column-shaped, with height exceeding width; their nuclei typically occupy basal positions. Transitional epithelium (urothelium) represents a special case where cell shape changes with organ distension.

<image>Panel A: Simple epithelia showing simple squamous with thin flat cells and bulging nuclei, simple cuboidal with cube-shaped cells and central round nuclei, and simple columnar with tall cells and basal oval nuclei. Panel B: Pseudostratified columnar epithelium showing cells of varying heights with nuclei at different levels, cilia at surface, and interspersed goblet cells with key anatomical locations listed. Panel C: Stratified squamous epithelium showing multiple layers with basal cuboidal cells transitioning to surface flat cells with magenta basement membrane beneath. Panel D: Transitional epithelium showing dome-shaped umbrella cells at surface overlying smaller rounded cells with specialized features highlighted in blue.</image>

Types of Epithelium

Simple Epithelia

Simple squamous epithelium consists of a single layer of flat cells that resemble fried eggs when viewed from above—central nuclei create bulges in the otherwise thin cytoplasm. This epithelium provides a minimal barrier that facilitates passive diffusion and filtration. It lines surfaces where rapid exchange must occur: the alveoli of the lungs (enabling gas exchange), the glomerular capsule of the kidney (allowing filtration), and all blood and lymphatic vessels. When lining blood vessels, simple squamous epithelium is called endothelium; when lining body cavities (pleural, peritoneal, pericardial), it is called mesothelium.

Simple cuboidal epithelium features cells that appear square in cross-section with centrally located, round nuclei. This epithelium lines structures where secretion and absorption occur in relatively equal measure. Kidney tubules utilize simple cuboidal epithelium for reabsorption of filtered substances. Thyroid follicles employ this epithelium for hormone synthesis and release. The ovary surface (germinal epithelium) and small excretory ducts of many glands also feature simple cuboidal lining.

Simple columnar epithelium contains tall cells with oval nuclei positioned near the basal portion of each cell, creating a distinctive row of nuclei at uniform height. This epithelium excels at absorption and secretion, functions often enhanced by surface modifications. The stomach lining consists of simple columnar cells specialized for mucus secretion and protection against acid. The intestinal epithelium features simple columnar cells with prominent microvilli (forming a brush border) interspersed with mucus-secreting goblet cells. The gallbladder employs simple columnar epithelium for concentration of bile through water absorption.

<image>Panel A: Simple squamous epithelium at 400x showing lung alveolar wall with extremely thin cells where only flattened dark oval nuclei are visible bulging into airspaces with arrows indicating barely visible cell outlines. Panel B: Simple cuboidal epithelium at 400x displaying kidney tubule cross-sections with cube-shaped cells surrounding circular lumens and central round nuclei with visible chromatin pattern. Panel C: Simple columnar epithelium at 400x showing intestinal villus with tall cells exhibiting basal nuclei aligned in single row and interspersed clear goblet cells. Panel D: Striated brush border visible as pink band at apical surface with H&E staining showing pink cytoplasm, purple nuclei, and pale pink basement membrane region with 20 micrometer scale bars.</image>

Complex Epithelia

Pseudostratified columnar epithelium appears multilayered but is actually simple epithelium with cells of varying heights. All cells contact the basement membrane, but shorter cells fail to reach the luminal surface. Nuclei at different levels create the pseudostratified appearance. The respiratory tract, from the nasal cavity through the bronchi, features pseudostratified ciliated columnar epithelium with goblet cells—often called respiratory epithelium. Cilia beat in coordinated waves to propel mucus (produced by goblet cells) upward toward the pharynx, clearing inhaled particles. The epididymis and vas deferens contain pseudostratified columnar epithelium with stereocilia rather than cilia.

Stratified squamous epithelium provides maximum protection against abrasion, representing the most durable epithelial covering. Multiple cell layers ensure that surface damage affects only the most superficial cells, while basal cells continuously divide to replace shed cells. This epithelium exists in two forms. Keratinized stratified squamous epithelium, found in the epidermis of the skin, features surface cells filled with keratin protein and lacking nuclei—these dead cells form a tough, water-resistant barrier. Non-keratinized stratified squamous epithelium retains nuclei in surface cells and remains moist; it lines the oral cavity, esophagus, vagina, and cornea.

Stratified cuboidal and stratified columnar epithelia are relatively rare, typically consisting of only two cell layers. They line larger excretory ducts of salivary glands and sweat glands, where they provide more protection than simple epithelia while maintaining some secretory capacity.

Transitional epithelium (urothelium) represents a specialized stratified epithelium designed to withstand the changing volume of urine-containing organs. It lines the urinary tract from the renal calyces through the bladder to the proximal urethra. The surface cells, called umbrella cells or dome cells, have a distinctive rounded appearance and may be binucleate. When the bladder is empty (relaxed state), the epithelium appears thick (6-8 cell layers) with rounded surface cells. When the bladder fills (stretched state), the epithelium thins to 2-3 layers and surface cells flatten. The apical membrane of umbrella cells contains specialized plaques of uroplakin proteins that provide an impermeable barrier, preventing urine from diffusing back into tissues.

<image>Panel A: Relaxed transitional epithelium with thick appearance of 6-8 cell layers showing large dome-shaped umbrella cells with pink cytoplasm and some binucleate cells overlying smaller rounded cells. Panel B: Electron microscopy inset showing specialized apical membrane with uroplakin plaques appearing as rigid-looking membrane segments and clearly visible basement membrane. Panel C: Stretched transitional epithelium appearing thin with only 2-3 cell layers where surface cells are flattened and elongated horizontally with arrows indicating stretch direction. Panel D: Anatomical context diagram showing bladder outline in empty versus full states with magenta basement membrane and pale pink cytoplasm with purple nuclei.</image>

Surface Modifications

Epithelial cells display remarkable structural adaptations at their apical surfaces that enhance specific functions. These modifications involve cytoskeletal elements and dramatically increase the efficiency of epithelial tissues.

Microvilli are finger-like projections of the apical cytoplasm that increase surface area 20-30 fold. Each microvillus contains a core of actin filaments bundled by fimbrin and villin proteins, with the actin anchored to the terminal web beneath the apical membrane. Individual microvilli measure approximately 1 μm in length and 0.1 μm in diameter—too small to resolve with standard light microscopy. However, when present in large numbers (as in intestinal absorptive cells), they create a visible striated border or brush border. Microvilli are most prominent in epithelia specialized for absorption: the small intestinal lining and kidney proximal tubule cells. The plasma membrane covering microvilli contains transport proteins and enzymes appropriate to the absorptive function.

Cilia are longer motile projections (5-10 μm) that propel fluids and particles across epithelial surfaces. The internal structure of a cilium, called the axoneme, consists of microtubules arranged in a characteristic 9+2 pattern: nine peripheral doublets surrounding two central singlets. Dynein motor proteins on the peripheral doublets generate the bending motion that produces ciliary beating. Cilia beat in coordinated metachronal waves that create unidirectional fluid flow. Ciliated epithelia line the respiratory tract (moving mucus toward the pharynx), the fallopian tubes (moving the ovum toward the uterus), and the ependyma of brain ventricles (circulating cerebrospinal fluid). Each ciliated cell may possess 200-300 cilia.

Stereocilia, despite their name, are not cilia but rather extremely long microvilli (up to 120 μm). They contain actin filament cores rather than microtubules and are non-motile. Stereocilia increase surface area and often appear branched. They line the epididymis (facilitating sperm maturation through absorption of fluid) and form the mechanosensory hair bundles of the inner ear (where deflection opens ion channels for hearing and balance).

Flagella are structurally similar to cilia (9+2 microtubule arrangement) but much longer, and typically only one per cell. In humans, the only flagellated cells are spermatozoa, where the flagellum provides propulsion.

<image>Panel A: Microvilli electron micrograph showing closely packed projections with actin filament cores as electron-dense bundles extending into terminal web with cross-section inset showing hexagonal actin arrangement at 0.5 micrometer scale. Panel B: Cilia displaying longer projections with 9+2 microtubule arrangement visible in cross-section inset showing basal bodies anchoring to cytoplasm and dynein arms on peripheral doublets at 1 micrometer scale. Panel C: Stereocilia showing extremely long branching projections with actin cores as found in epididymis with staircase arrangement of increasing length at 2 micrometer scale. Panel D: Comparison schematic showing all three modifications to scale with microvilli shortest, cilia intermediate, and stereocilia longest with actin in red and microtubules in green indicating core composition.</image>

Cell Junctions

Epithelial cells connect to one another and to the underlying basement membrane through specialized junctions that provide adhesion, barrier function, and communication. The junctional complex at the apical end of lateral cell surfaces includes, from apical to basal: tight junctions, adherens junctions, and desmosomes. Gap junctions and hemidesmosomes occur at other locations.

Tight junctions (zonula occludens) form the most apical component of the junctional complex, creating a continuous seal that encircles each cell like a belt. The transmembrane proteins claudins and occludins from adjacent cells interact in the intercellular space, creating a series of sealing strands visible as anastomosing ridges in freeze-fracture electron microscopy. Tight junctions serve two critical functions: as a barrier, they restrict paracellular passage of molecules between cells, while as a fence, they maintain cell polarity by preventing lateral diffusion of membrane proteins between apical and basolateral domains. Tight junction "leakiness" varies considerably between epithelia—renal proximal tubules have relatively leaky tight junctions to permit some paracellular transport, while bladder epithelium has exceptionally tight junctions to prevent urine from leaking back into tissues.

Adherens junctions (zonula adherens) lie immediately below tight junctions and also encircle cells continuously. The transmembrane proteins are cadherins (specifically E-cadherin in epithelia), which require calcium ions for their adhesive function—hence their name. On the cytoplasmic side, cadherins link to the actin cytoskeleton through adapter proteins including catenins (α, β, and p120). Adherens junctions provide mechanical stability and transmit cytoskeletal forces between cells. They also participate in cell signaling; notably, β-catenin can translocate to the nucleus and affect gene transcription.

Desmosomes (macula adherens) appear as spot welds scattered along lateral cell membranes, providing strong mechanical adhesion. Desmosomal cadherins—desmogleins and desmocollins—form the transmembrane adhesive components. On the cytoplasmic side, these proteins connect through plakoglobin and plakophilin to desmoplakin, which links to intermediate filaments (keratin in epithelial cells, desmin in cardiac muscle). This connection to intermediate filaments distributes mechanical stress throughout the cell and tissue. Desmosomes are particularly abundant in tissues subject to mechanical stress, such as the epidermis and cardiac muscle.

Gap junctions provide direct communication between adjacent cells by creating channels that permit passage of ions and small molecules (<1 kDa). Six connexin proteins assemble to form a connexon (hemichannel), and connexons from adjacent cells align to create a complete channel. Gap junctions enable rapid electrical coupling (essential in cardiac muscle for coordinated contraction) and metabolic cooperation (sharing of nutrients and signaling molecules). They appear as plaques of closely spaced channels in freeze-fracture preparations.

Hemidesmosomes connect the basal surface of epithelial cells to the underlying basement membrane. Despite their name, they are structurally distinct from desmosomes, utilizing integrin transmembrane proteins (α6β4) rather than cadherins. Integrins bind to laminin in the basement membrane while linking intracellularly to intermediate filaments through plectin and BP230. Hemidesmosomes anchor the epithelium firmly to its substrate.

<image>Panel A: Tight junction at apex showing continuous belt with blue claudins and orange occludins as interlocking proteins spanning intercellular space with freeze-fracture inset showing anastomosing sealing strands. Panel B: Adherens junction with green E-cadherin molecules interacting in calcium-dependent manner connected through yellow alpha-catenin and beta-catenin to red actin filaments. Panel C: Desmosomes appearing as electron-dense plaques with purple desmogleins and desmocollins in center and brown intermediate filaments looping through cytoplasmic plaques plus gap junctions as channel clusters with hexameric connexon inset. Panel D: Hemidesmosomes at base connecting to magenta basement membrane through integrins with intermediate filaments inserting into cytoplasmic plaques and color legend identifying all molecular components.</image>

Basement Membrane

The basement membrane is a specialized sheet of extracellular matrix that underlies all epithelial tissues, providing structural support, filtration capabilities, and regulatory signals. It can be subdivided into two components with distinct origins and compositions.

The basal lamina, produced by epithelial cells themselves, contains two layers visible by electron microscopy. The lamina lucida (or lamina rara) lies immediately beneath the epithelial cells and appears electron-lucent (pale). The lamina densa lies deeper and appears electron-dense (dark). The major molecular components of the basal lamina include type IV collagen, which forms a network providing structural stability; laminin, a large glycoprotein that mediates cell attachment through integrin receptors; and heparan sulfate proteoglycans (particularly perlecan and agrin), which contribute to filtration properties and growth factor binding. Nidogen (entactin) cross-links laminin and collagen networks.

The reticular lamina lies beneath the basal lamina and is produced by underlying connective tissue, particularly fibroblasts. It consists primarily of type III collagen (reticular fibers), which can be visualized with silver stains. The reticular lamina is thicker in regions subjected to mechanical stress.

The basement membrane performs multiple functions. It provides structural support, anchoring epithelium to underlying tissue. It acts as a selective filtration barrier—notably in the kidney glomerulus, where the basement membrane filters blood plasma based on molecular size and charge. It establishes epithelial cell polarity by providing positional cues. It regulates cell behavior by sequestering growth factors and presenting them to epithelial cells during wound healing. It guides epithelial cell migration during tissue repair and development. The basement membrane also serves as a barrier to cancer invasion; penetration through the basement membrane marks the transition from carcinoma in situ to invasive carcinoma.

Visualizing the basement membrane requires special techniques since it stains poorly with routine H&E. The PAS (periodic acid-Schiff) stain colors it magenta due to its high glycoprotein content. Silver stains (reticulin stain) demonstrate the reticular lamina as black fibers. Electron microscopy clearly reveals the lamina lucida and lamina densa as distinct layers.

<image>Panel A: High-magnification electron micrograph view of epithelial-connective tissue junction showing basal cell membrane with hemidesmosomes and lamina lucida as pale electron-lucent zone approximately 40 nm thick. Panel B: Lamina densa appearing electron-dense at approximately 50 nm thick with reticular lamina beneath showing type III collagen fibrils in cross and longitudinal section at 100 nm scale. Panel C: Molecular organization inset showing blue type IV collagen chicken-wire network, red cross-shaped laminin molecules spanning to collagen network, and green perlecan with heparan sulfate chains. Panel D: Comparative light microscopy at 20 micrometer scale showing basement membrane barely visible with H&E, bright magenta line with PAS, and black reticular fibers with reticulin stain.</image>

Clinical Correlations

Several important diseases result from defects in epithelial structures, highlighting the functional significance of cellular junctions and basement membrane components.

Epidermolysis bullosa comprises a group of inherited disorders characterized by skin blistering following minor mechanical trauma. Different forms result from mutations affecting different components of the dermal-epidermal junction. Epidermolysis bullosa simplex involves mutations in keratin 5 or 14, causing cell fragility within the basal layer. Junctional epidermolysis bullosa results from defects in hemidesmosome components (such as laminin-332 or integrin α6β4), causing separation within the basement membrane zone. Dystrophic epidermolysis bullosa involves mutations in type VII collagen of anchoring fibrils, causing separation below the basement membrane.

Pemphigus encompasses autoimmune blistering diseases in which antibodies target desmosomal proteins. Pemphigus vulgaris, the most common form, involves antibodies against desmoglein 3, causing separation between keratinocytes (acantholysis) and intraepidermal blisters. Patients develop painful erosions of oral mucosa and flaccid skin blisters. The related disease pemphigus foliaceus involves antibodies against desmoglein 1, affecting more superficial epidermal layers. Pemphigoid diseases, by contrast, involve antibodies against hemidesmosomal components and cause subepidermal blistering.

Primary ciliary dyskinesia (PCD) results from defects in ciliary structure, most commonly absence or dysfunction of dynein arms. Without functional dynein motors, cilia cannot beat effectively. Patients develop chronic sinusitis and bronchiectasis due to impaired mucociliary clearance, along with male infertility (immotile sperm) and female subfertility (reduced tubal transport of ova). In approximately 50% of patients, defective cilia during embryonic development cause situs inversus (mirror-image organ arrangement)—the combination of situs inversus, bronchiectasis, and sinusitis constitutes Kartagener syndrome.

Metaplasia refers to the reversible conversion of one differentiated cell type to another, typically in response to chronic irritation or environmental change. The substitute cell type is usually better suited to the altered conditions. In the respiratory tract of smokers, the normal pseudostratified ciliated epithelium may undergo squamous metaplasia, transforming into stratified squamous epithelium. This provides better protection against irritants but eliminates mucociliary clearance. In Barrett esophagus, chronic acid reflux causes the normal stratified squamous epithelium of the distal esophagus to transform into intestinal-type columnar epithelium, which is more resistant to acid but carries increased risk of adenocarcinoma.

Summary

Epithelial tissues are classified by the number of cell layers (simple versus stratified) and by the shape of cells at the free surface (squamous, cuboidal, columnar). This classification reflects function: simple epithelia facilitate exchange processes while stratified epithelia provide protection. Pseudostratified and transitional epithelia represent specialized adaptations for the respiratory and urinary systems, respectively.

Surface modifications enhance epithelial function. Microvilli, with their actin cores, dramatically increase absorptive surface area in the intestine and kidney. Cilia, with their 9+2 microtubule arrangement and dynein motors, create coordinated beating that propels mucus and other materials across epithelial surfaces. Stereocilia, despite their misleading name, are long microvilli serving absorptive and mechanosensory functions.

Cell junctions create epithelial integrity and function. Tight junctions seal the paracellular pathway and maintain cell polarity. Adherens junctions and desmosomes provide mechanical adhesion, linking to actin and intermediate filaments respectively. Gap junctions enable direct cell-to-cell communication. Hemidesmosomes anchor epithelia to the basement membrane.

The basement membrane underlies all epithelia, providing structural support, acting as a filtration barrier, and regulating cell behavior through signaling. Its major components—type IV collagen, laminin, and heparan sulfate proteoglycans—are produced by both epithelial cells and underlying connective tissue.

Clinical disorders affecting epithelial structures demonstrate the importance of these components. Epidermolysis bullosa results from defective cell-matrix attachment, pemphigus from autoimmune targeting of desmosomes, primary ciliary dyskinesia from structural defects in cilia, and metaplasia from adaptive changes in epithelial type.

Key Terms

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