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Epithelial Tissue
Types & Functions

Explore the cellular sheets that line every surface of your body: from skin to organ cavities, and learn how their structure dictates their function.

Anatomy & Physiology

What Is Epithelial Tissue?

Epithelial tissue (epithelium) is one of the four primary tissue types. It forms continuous sheets of cells that cover body surfaces, line hollow organs, and form glands. Every substance entering or leaving the body must cross at least one epithelial layer.

Core Characteristics

Cellularity
Composed almost entirely of cells with minimal extracellular matrix between them.
Polarity
Each cell has a distinct apical (free) surface and a basal surface attached to the basement membrane.
Avascular
No blood vessels penetrate epithelium: nutrients diffuse from capillaries in the underlying connective tissue.
Regeneration
High mitotic rate allows rapid replacement of damaged or worn cells from the basal layer.

Why Epithelium Matters

Epithelial tissue performs five essential functions that keep the body alive. Understanding these functions is the key to predicting which type of epithelium you will find at any given location.

1
Protection: Shields underlying tissues from abrasion, pathogens, and chemical damage.
2
Absorption: Selectively takes in nutrients and ions (e.g., intestinal lining).
3
Filtration: Allows passage of small molecules based on size and charge (e.g., kidney glomerulus).
4
Secretion: Produces and releases substances such as mucus, hormones, and enzymes.
5
Sensation: Contains sensory nerve endings for detecting stimuli (e.g., taste buds).

The Classification System

Every epithelial type is named using two criteria: the number of cell layers and the shape of the cells at the apical surface. Combine one term from each column to get the full name.

LAYERS Simple (1 layer) Stratified (2+ layers) Pseudostratified (1 layer) CELL SHAPE Squamous (flat) Cuboidal (cube-shaped) Columnar (tall) +

Simple Epithelia

Simple epithelia have a single layer of cells. Because every cell contacts the basement membrane, these tissues are thin and specialized for absorption, secretion, and filtration.

Simple Squamous Lung alveoli, blood vessels Simple Cuboidal Kidney tubules, gland ducts Simple Columnar Stomach, intestines Basement Membrane Pseudostratified Columnar Trachea, upper respiratory tract (ciliated, with goblet cells)

Stratified Epithelia

Stratified epithelia contain two or more cell layers. Their primary role is protection against abrasion. They are named by the shape of cells at the apical (top) surface.

Stratified Squamous
Most common stratified type. Found in skin (keratinized), mouth, esophagus, and vagina (non-keratinized). Built for abrasion resistance.
Stratified Cuboidal
Rare. Typically two layers. Found in ducts of larger glands such as sweat glands and mammary glands.
Stratified Columnar
Also rare. Found in the male urethra and parts of the pharynx. Limited distribution in the body.

Transitional Epithelium

Transitional epithelium (urothelium) is a special stratified type found exclusively in the urinary system: the bladder, ureters, and part of the urethra. Its unique ability is stretching.

Relaxed (empty bladder) Dome-shaped apical cells Stretched (full bladder) Flattened apical cells

Simple vs. Stratified

Simple Stratified
Layers One Two or more
Primary Function Absorption, secretion, filtration Protection against abrasion
Location Internal linings (lungs, intestines, kidneys) External or high-friction areas (skin, mouth)
Thickness Thin, fragile Thick, durable
Regeneration Speed Moderate High (basal layer divides rapidly)

How to Identify the Type

1
Count the layers. Can you see only one row of nuclei touching the basement membrane? If yes, it is simple. Multiple rows? Stratified. One row at different heights? Pseudostratified.
2
Look at cell shape (apical surface). Flat and scale-like: squamous. Roughly as tall as wide: cuboidal. Taller than wide: columnar.
3
Check for specializations. Cilia on the apical surface? Goblet cells secreting mucus? Keratin deposits? These narrow down the subtype.
4
Combine layer + shape. State the number of layers first, then cell shape. For stratified, name the shape of the apical layer only.

Glandular Epithelium

One or more epithelial cells that produce and secrete a product form a gland. Glands are classified by where they release their product.

Exocrine Glands
Secrete products onto body surfaces or into body cavities via ducts. Examples: sweat glands, salivary glands, pancreas (exocrine portion).
Endocrine Glands
Ductless glands that secrete hormones directly into the blood. Examples: thyroid, adrenal glands, pituitary gland.

Example: Lung Alveoli

Air Space Capillary
\( \text{O}_2 \) diffusion
 Simple Squamous Epithelium Flat cells

Example: Small Intestine

BM Goblet cell Microvilli (brush border) Simple Columnar Epithelium with Goblet Cells

Example: Skin (Epidermis)

BM Dead Corneum Granular Spinous Basal Keratinized Stratified Squamous Epithelium

Common Mistakes

✕ Confusing Pseudostratified with Stratified
Pseudostratified has one layer where all cells touch the basement membrane, but nuclei sit at different heights. Stratified has multiple true layers.
✕ Naming Stratified by the Basal Layer
Stratified epithelium is always named by the shape of the apical (top) layer, not the basal cells, even though basal cells are often cuboidal or columnar.
✕ Forgetting Epithelium is Avascular
Students often assume epithelium has its own blood supply. It does not: nutrients reach epithelial cells by diffusion from capillaries in the underlying connective tissue.

Summary Table

Type Function Location
Simple Squamous Diffusion, filtration Alveoli, glomerulus, serosa
Simple Cuboidal Secretion, absorption Kidney tubules, gland ducts
Simple Columnar Absorption, secretion GI tract, gallbladder
Pseudostratified Columnar Secretion, propulsion (cilia) Trachea, bronchi
Stratified Squamous Protection (abrasion) Skin, mouth, esophagus
Transitional Stretch, protection Bladder, ureters

Question 1 of 5

Which type of epithelial tissue lines the lung alveoli to facilitate rapid gas exchange?

Question 2 of 5

Pseudostratified columnar epithelium appears multilayered, but in reality it has:

Question 3 of 5

A tissue sample shows multiple cell layers with flat cells at the apical surface and a keratinized outer layer. This tissue is most likely from the:

Question 4 of 5

Which characteristic is NOT true of epithelial tissue?

Question 5 of 5

Endocrine glands differ from exocrine glands because endocrine glands: