Epithelium lecture

Basic Building Blocks of Life

  • Organs example: Skin

  • Tissues include:

    • Epithelium

    • Connective tissue

    • Muscle

    • Nervous tissue

  • Cells and extracellular matrix (ECM) form the tissues

  • Extracellular matrix components:

    • Fibers

    • Ground substance

    • ECM is non-cellular material that regulates cellular function

  • Intercellular (outside the cell) components include:

    • Fibers

    • Ground substance

    • Extracellular fluid

  • Basic tissues depend on ECM for organization and support

Four Basic Types of Tissues

  • Epithelium

  • Connective tissue

  • Muscular tissue

  • Nervous tissue

Epithelium: Overview and Key Facts

  • Found throughout the body

  • Lines organs and body cavities

  • Forms glands for secretion

  • Glandular tissue is derived from epithelium

  • Many cells in the body arise from embryonic epithelium

  • Example: hepatocytes (liver cells) are active, involved in metabolism, secretion, and detoxification

  • Main functions:

    • Absorption

    • Secretion

    • Protection

    • Exchange

  • MANY TUMORS ARISE FROM THE EPITHELIUM

Epithelium: Structure and Avascularity

  • Very cellular; little intercellular material

  • Avascular: relies on underlying connective tissue for support and nutrition

  • Cells are arranged in:

    • Sheets or layers (membranes)

    • Irregular masses/groups (glands)

  • Shape is closely tied to function

Epithelia: Layering and Shapes (Classification)

  • Simple: one layer

  • Stratified: more than one layer

  • Superficial cell shapes:

    • Squamous: flat

    • Cuboidal: cube-shaped

    • Columnar: tall, column-like

  • Other classifications:

    • Pseudostratified: not truly stratified (all cells contact basement membrane, but nuclei give a stratified appearance)

    • Transitional: urinary tract epithelium that can stretch

Histological Identification: Examples of Layered Epithelium

  • Simple squamous: single layer of flat cells

    • Endothelium == simple squamous lining of blood vessels

    • Mesothelium == simple squamous lining of visceral body cavities

  • Simple cuboidal: single layer of cube-shaped cells

    • Examples: kidney tubules, liver cells (hepatocytes), thyroid gland, many others

  • Simple columnar: single layer of tall, thin cells

    • Example: intestinal epithelium

  • Stratified squamous: multiple layers with flat superficial cells

    • Non-keratinized:

    • Nuclei present in all layers including apical surface

    • Typical locations: mouth, esophagus, vagina, anal canal

    • Keratinized:

    • Surface layers lack nuclei (keratin)

    • Typical locations: dry skin (integument), hard palate, areas of high wear

  • Stratified cuboidal and stratified columnar: found in duct systems (e.g., sweat gland ducts)

  • Pseudostratified columnar:

    • Single layer of tall cells that appear layered

    • All cells reach the basement membrane

    • Locations include the respiratory tract

  • Transitional:

    • Multiple layers

    • Apical cells are rounded but can appear flattened when stretched

    • Urinary system

Epithelia: Histological Identification (Recap)

  • Simple squamous: endothelium, mesothelium

  • Simple cuboidal

  • Simple columnar

  • Stratified squamous (non-keratinized vs keratinized)

  • Stratified cuboidal/columnar

  • Pseudostratified columnar

  • Transitional

Apical Surface Modifications of Epithelium

  • Microvilli

  • Cilia

  • Stereocilia

  • Flagella

Microvilli

  • Cytoplasmic projections from the apical surface

  • Light microscopy: brush border (striated border)

  • Transmission Electron Microscopy (TEM):

    • Finger-like projections

    • Composed of microfilaments covered by cell membrane

  • Function: increase surface area to enhance absorption (not motile)

  • Locations:

    • Mainly in intestinal tract and proximal renal tubules

    • Other sites as well

Cilia

  • Cylindrical structures on the surface of specialized epithelial cells

  • Contain microtubules

  • Function: motile; move mucus, debris, or fluid in a directed manner

  • Locations:

    • Respiratory epithelium

    • Parts of the reproductive tract

Stereocilia

  • Not true cilia; long microvilli

  • Composed of microfilaments; not motile

  • Function: increase surface area for absorption

  • Location: parts of the male reproductive tract

Flagella

  • Long cilia; motile

  • Function: propel the cell (e.g., sperm tail movement)

  • Location: spermatozoa (sperm tails)

Glandular Epithelium

  • Numerous gland types throughout the body; glandular epithelium consists of active secretory cells

  • Exocrine glands:

    • Secretion delivered into ducts that open onto external or internal surfaces

    • Example: Salivary glands

  • Endocrine glands:

    • Ductless glands

    • Secretions released into lymph or blood

    • Example: Thyroid gland produces thyroid hormone and calcitonin

  • Goblet cells: unicellular mucus-secreting glands found in respiratory tract and intestines

Glands and Pancreas (Glandular Epithelium, Visual Reference)

  • Exocrine glands secrete into ducts

  • Endocrine glands secrete directly into bloodstream

  • Pancreas serves both roles: exocrine (digestive enzymes into ducts) and endocrine (hormones from islets into blood)

Goblet Cells

  • Unicellular glands that secrete mucus

  • Located in respiratory tract and intestinal tract

Epithelia: Specialized Functions

  • Absorption: intestinal epithelium

  • Secretion: pancreatic secretions

  • Transport: endothelium in vessels; ocular and other specialized epithelia

  • Excretion: kidney epithelium

  • Protection: against mechanical damage and dehydration

  • Contraction: myoepithelium—specialized cells in glands to squeeze out secretions

Basal Lamina and Basement Membrane

  • Basement membrane consists of basal lamina ++ reticular lamina

  • Located in the extracellular matrix (ECM)

  • Composed of proteins, fibers, and carbohydrates

  • Provides structural support for epithelia

  • Regulates tissue shape, cell polarity, cell proliferation

  • Controls cell migration and other behaviors

PAS Staining and Basement Membrane Visualization

  • PAS (periodic acid–Schiff) stain highlights carbohydrates

  • Basement membrane of epithelium is PAS-positive

  • In a sample, the following may all be PAS-positive due to carbohydrate-rich structures:

    • Simple columnar epithelium

    • Goblet cells

    • Brush border

    • Connective tissue

Junctions and Adhesion in Epithelia

  • Epithelia are specialized for extensive cell–cell contacts via junctions

  • Major Junction Types:

    • Zonula occludens – Tight junction (belt-like): seals adjacent cells

    • Zonula adherens – Adhering belt: helps hold cells together and maintains polarity

    • Desmosome – Macula adherens: strong, spot-like attachments between cells

    • Gap junctions – Communication channels allowing small molecule exchange

How Epithelial Cells Attach to Underlying Tissue

  • Cells hold on to the underlying connective tissue via basement membrane and adhesion structures

  • Key components and terms include:

    • Cadherins (Ca2+2+-dependent cell–cell adhesion proteins) in adherens and desmosomes

    • Ig-like CAMs (cell adhesion molecules)

    • Integrins (cell–matrix adhesions) in hemidesmosomes and focal contacts

    • Focal contacts (focal adhesions) connect actin cytoskeleton to the ECM

    • Hemidesmosomes (via integrins) anchor cells to the basement membrane

    • Gap junctions provide intercellular communication

  • Non-junctional adhesion mechanisms include proteoglycans and other ECM components

Basal Membrane Details

  • Basal membrane is the interface between epithelial cells and underlying connective tissue

  • It comprises:

    • Basal lamina (secreted by epithelial cells)

    • Reticular lamina (secreted by underlying fibroblasts)

  • It provides anchorage, directional cues, and barrier functions

Basement Membrane Composition and Function (Summary)

  • Located in the extracellular matrix

  • Proteins, glycoproteins, proteoglycans, and collagen form the basal lamina and reticular lamina

  • Functions include structural support, regulation of tissue architecture, cell polarity, and cell migration

Functional Summary of Epithelia

  • Functions:

    • Absorption

    • Secretion

    • Transport

    • Excretion

    • Protection

    • Contraction (via myoepithelial cells)

  • Epithelium is highly dynamic, with turnover and remodeling in response to injury, signaling, and mechanical stress

Clinical and Real-World Relevance (Integrated Points)

  • Tumors often arise from epithelial tissues due to high turnover and exposure to environment

  • Basement membrane integrity is critical to prevent invasive migration of tumor cells; disruption can enable metastasis

  • PAS staining is a classic pathology tool to identify basement membranes and carbohydrate-rich structures like goblet cell mucins and brush borders

  • Mucociliary clearance in the respiratory tract depends on ciliated epithelia; dysfunction can lead to respiratory disease

  • Goblet cell mucus production is essential for lubrication and barrier defense in mucosal surfaces

  • Glands (exocrine vs endocrine) illustrate how epithelial cells support diverse physiological roles, from digestion (pancreas) to hormone regulation (thyroid)

Quick Reference: Key Terms

  • Endothelium: simple squamous epithelium lining blood vessels

  • Mesothelium: simple squamous lining body cavities

  • Basal lamina: foundational layer of the basement membrane secreted by epithelia

  • Reticular lamina: supporting layer secreted by underlying connective tissue

  • Zonula occludens: tight junction

  • Zonula adherens: adherens junction

  • Desmosome: spot desmosome

  • Gap junction: intercellular channel

  • Hemidesmosome: epithelial–basement membrane attachment via integrins

  • Focal contact: cell–ECM anchoring point via integrins

  • Cadherins: calcium-dependent cell–cell adhesion molecules

  • Integrins: receptor proteins mediating cell–ECM adhesion

  • PAS staining: detects carbohydrates, highlights basement membrane and glycoconjugates

Notes on Terminology and Structure

  • Simple == one cell layer; Stratified == multiple layers; Pseudostratified == appears layered but is a single layer; Transitional == multi-layered with variable cell shapes depending on stretch

  • Apical surface modifications are key to epithelial function and vary by tissue type and location

  • Glandular epithelium encompasses both unicellular (e.g., goblet cells) and multicellular glands (exocrine and endocrine)

  • The basement membrane acts as both a barrier and a scaffold guiding cell behavior; disruption can influence tumor invasion and tissue regeneration


Epithelial tissue is a fundamental building block of life, forming tissues with cells and extracellular matrix.

Overview of Epithelium

  • It is one of the four basic tissue types (epithelium, connective tissue, muscle, nervous tissue).

  • Found throughout the body, lining organs and cavities, and forming glands.

  • Functions include absorption, secretion, protection, and exchange.

  • Many tumors originate from epithelium due to its high turnover.

Structure and Classification

  • Epithelium is highly cellular with little intercellular material and is avascular, relying on underlying connective tissue for nutrition.

  • Cells are arranged in sheets or irregular masses.

  • Classified by layering (simple: one layer; stratified: multiple layers) and cell shape (squamous: flat; cuboidal: cube-shaped; columnar: tall).

  • Specialized classifications include pseudostratified (appears layered but is single) and transitional (stretchable).

  • Examples for histological identification include simple squamous (endothelium, mesothelium), simple cuboidal (kidney tubules), simple columnar (intestinal), stratified squamous (keratinized/non-keratinized skin/mouth), pseudostratified columnar (respiratory), and transitional (urinary).

Apical Surface Modifications

  • Microvilli: Non-motile projections that increase surface area for absorption (e.g., intestine, renal tubules).

  • Cilia: Motile, microtubule-containing structures that move fluids or debris (e.g., respiratory tract, reproductive tract).

  • Stereocilia: Long microvilli, non-motile, for absorption (e.g., male reproductive tract).

  • Flagella: Long, motile cilia that propel cells (e.g., sperm).

Glandular Epithelium

  • Consists of active secretory cells, forming exocrine (ducts, e.g., salivary glands) and endocrine (ductless, into blood/lymph, e.g., thyroid gland) glands.

  • Goblet cells are unicellular mucus-secreting glands found in the respiratory and intestinal tracts.

  • The pancreas is unique, serving both exocrine and endocrine roles.

Basal Lamina and Basement Membrane

  • The basement membrane provides structural support, regulates tissue shape, and controls cell behavior.

  • It consists of the basal lamina (secreted by epithelial cells) and the reticular lamina (secreted by underlying fibroblasts).

  • Rich in carbohydrates, making it PAS (periodic acid–Schiff) stain-positive.

Junctions and Adhesion

  • Epithelial cells have extensive cell-cell contacts via junctions:

    • Zonula occludens (Tight junction): Seals adjacent cells.

    • Zonula adherens (Adhering belt): Holds cells together, maintains polarity.

    • Desmosome (Macula adherens): Strong, spot-like attachments.

    • Gap junctions: Communication channels.

  • Cells attach to the underlying connective tissue through the basement membrane and adhesion structures involving cadherins (cell-cell), integrins (cell-matrix via hemidesmosomes and focal contacts), and Ig-like CAMs.

Functional Summary and Clinical Relevance

  • Epithelia perform diverse functions including absorption, secretion, transport, excretion, protection, and even contraction (myoepithelial cells).

  • They are highly dynamic tissues.

  • Clinically, their high turnover makes them prone to tumor formation; basement membrane integrity is crucial to prevent metastasis.

  • PAS staining is a key pathology tool, and functions like mucociliary clearance (cilia) and mucus production (goblet cells) are vital for health.