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Epithelial Tissue

What Is Epithelium?

Fig. 4-1 from Junqueira's Basic Histology: Cuboidal epithelial cells (top row) rest on the basement membrane (red line), above connective tissue cells. Nuclear shape reflects cell shape - oval in cuboidal cells, flattened in squamous cells.

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General Characteristics

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Functions of Epithelial Tissue

1. Covering, lining, and protecting surfaces - e.g., the epidermis protects against dehydration and microbial invasion
2. Absorption - e.g., intestinal lining absorbs digested nutrients; gallbladder epithelium concentrates bile
3. Secretion - e.g., parenchymal cells of exocrine and endocrine glands
4. Transport - ion and water transport across kidney tubule epithelia maintains osmotic balance
5. Sensation - specialized sensory cells in taste buds and olfactory epithelium
6. Contraction - myoepithelial cells in salivary and mammary glands assist secretion
7. Gas exchange - thin simple squamous epithelium of alveoli facilitates O2/CO2 diffusion

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Classification of Epithelium

1. Number of cell layers - simple (one layer) or stratified (two or more layers)
2. Shape of surface cells - squamous, cuboidal, or columnar

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A. Simple Epithelia

1. Simple Squamous Epithelium

• Shape: Flat cells; width greater than height; flattened nuclei
• Locations:
  • Blood and lymphatic vessels (called endothelium)
  • Body cavities - pleura, peritoneum, pericardium (called mesothelium)
  • Bowman capsule of the kidney
  • Pulmonary alveoli (gas exchange)
• Functions: Exchange (gases, fluids), barrier, lubrication

2. Simple Cuboidal Epithelium

• Shape: Width, depth, and height approximately equal; spherical nuclei
• Locations:
  • Kidney tubules (proximal and distal convoluted tubules)
  • Thyroid follicles
  • Small collecting ducts of various glands
  • Surface of the ovary
• Functions: Absorption, secretion, excretion

3. Simple Columnar Epithelium

• Shape: Height appreciably exceeds width; oval nuclei near the base
• Subtypes:
  • Non-ciliated: Lines most of the GI tract (stomach to rectum); absorptive and secretory functions; goblet cells present
  • Ciliated: Lines parts of the uterine tube (fallopian tube) and the uterus; cilia move mucus or eggs
• Functions: Absorption (microvilli/brush border), secretion of mucus and enzymes

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B. Pseudostratified Epithelium

• Appearance: Looks stratified because nuclei are at different heights, but all cells actually contact the basement membrane - it is truly a simple epithelium
• Distribution: Respiratory tract (trachea, bronchi) - typically ciliated with goblet cells; male reproductive tract (epididymis) - with stereocilia
• Functions: Mucus secretion and mucociliary clearance in the airways; absorption in the reproductive tract

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C. Stratified Epithelia

1. Stratified Squamous Epithelium

Fig. 4-15 from Junqueira's Basic Histology: (b) Non-keratinized stratified squamous - esophageal lining; (c) cornea outer covering; (d) stratified cuboidal epithelium in sweat gland excretory ducts.

2. Stratified Cuboidal Epithelium

• Locations: Excretory ducts of sweat glands and salivary glands (typically 2 layers)
• Function: Additional secretory/absorptive capacity in ducts

3. Stratified Columnar Epithelium

• Locations: Conjunctiva of the eyelids; parts of the male urethra; large excretory ducts
• Function: Protection and mucus secretion

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D. Transitional Epithelium (Urothelium)

• A special category with unique characteristics
• Lines the lower urinary tract: from the renal minor calyces down to the proximal urethra (urinary bladder, ureters)
• Surface layer contains large dome-shaped "umbrella cells" that flatten when the bladder fills and round up when empty - this allows the epithelium to distend
• Protects underlying tissues from the hypertonic and cytotoxic effects of urine

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Summary Classification Table

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Apical Surface Specializations

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Glandular Epithelium

• Exocrine glands - secrete into ducts or directly onto a surface (e.g., sweat glands, salivary glands, pancreatic acini)
• Endocrine glands - secrete hormones directly into the bloodstream (e.g., thyroid, adrenal cortex, pituitary)

• Secretory mechanism: Merocrine (exocytosis), apocrine (loss of apical cytoplasm), holocrine (whole cell disintegrates)
• Duct structure: Simple (unbranched duct) vs. compound (branched duct)
• Secretory unit shape: Tubular, acinar/alveolar, or tubuloalveolar

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• Junqueira's Basic Histology: Text and Atlas, 17th ed., Chapter 4 (Epithelial Tissue), pp. 194-250
• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, Chapter 5 (Epithelial Tissue), pp. 314-325

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Epithelial Tissue - Illustrated Guide

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What Is Epithelial Tissue?

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Classification Overview

1. Number of layers: Simple (1 layer) or Stratified (2+ layers)
2. Cell shape at the surface: Squamous, Cuboidal, or Columnar

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A. SIMPLE EPITHELIA

1. Simple Squamous Epithelium

Key features: Cells are flat (width >> height), nuclei are flattened and bulge slightly into the lumen. Found in endothelium (blood vessels), mesothelium (body cavities), alveoli, and Bowman's capsule. Function: exchange, filtration, lubrication.

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2. Simple Cuboidal Epithelium

Key features: Width ≈ Height ≈ Depth; spherical nuclei centrally located. Found in kidney tubules, thyroid follicles, ovarian surface. Function: absorption, secretion, excretion.

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3. Simple Columnar Epithelium

Key features: Height > Width; oval nuclei near the base; apical microvilli (brush border) or cilia. Goblet cells (mucus-secreting) interspersed in GI tract. Function: absorption (intestine), secretion, movement of substances (ciliated type).

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B. PSEUDOSTRATIFIED COLUMNAR EPITHELIUM

Key features: Appears stratified but is actually ONE layer - all cells contact the basement membrane. Nuclei at different levels create the "pseudo" appearance. Found in trachea, bronchi, epididymis. Function: mucociliary clearance (traps and moves particles), absorption.

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C. STRATIFIED EPITHELIA

4. Stratified Squamous Epithelium (Keratinized)

Key features: Many cell layers; surface cells are dead, anucleate, packed with keratin. Basal cells are mitotically active cuboidal/columnar cells. Found in skin epidermis. Function: protection against abrasion, dehydration, and microbial invasion.

5. Stratified Squamous Epithelium (Non-Keratinized) + Stratified Cuboidal

Non-keratinized: Surface cells retain nuclei and remain metabolically active. Found in mouth, esophagus, vagina, cornea. Function: protection against abrasion in moist locations.

Stratified cuboidal (panel d): Two or more layers of cube-shaped cells. Found in sweat gland and salivary gland excretory ducts. Function: duct transport.

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D. TRANSITIONAL EPITHELIUM (Urothelium)

Key features: Unique distensible epithelium. Surface "umbrella cells" are large, dome-shaped and can flatten when the bladder fills and return to dome shape when empty. Lines entire lower urinary tract (ureters, urinary bladder, proximal urethra). Function: protection from hypertonic/cytotoxic urine; allows distension.

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Quick Visual Reference

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📚 Epithelial Tissue — Complete Study Guide

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SECTION 1: The Big Picture

• Cover all body surfaces
• Line all internal cavities and tubes
• Form all glands

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SECTION 2: Universal Features (Apply to ALL Epithelium)

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SECTION 3: Classification System

STEP 1 — How many layers?
├── ONE layer    → SIMPLE
└── TWO or more → STRATIFIED
    └── Looks multi but all touch basement membrane → PSEUDOSTRATIFIED (actually simple)

STEP 2 — Shape of SURFACE cells?
├── Flat (width > height)         → SQUAMOUS
├── Equal (width ≈ height)        → CUBOIDAL
└── Tall (height > width)         → COLUMNAR

STEP 3 (optional) — Apical modifications?
├── Keratinized / Non-keratinized (stratified squamous)
├── Ciliated / Non-ciliated (columnar)
└── Special: TRANSITIONAL (urothelium)

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SECTION 4: Master Comparison Table

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SECTION 5: Side-by-Side Comparison — Simple vs. Stratified

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SECTION 6: Apical Surface Specializations

Clinical Link - Cilia:

Kartagener Syndrome (Primary Ciliary Dyskinesia): Mutations in dynein arms of cilia → immotile cilia → chronic bronchitis (no mucociliary clearance), male infertility (immotile sperm), and situs inversus in ~50% of patients.

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SECTION 7: Glandular Epithelium

Exocrine vs. Endocrine

Secretory Mechanisms

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SECTION 8: Embryological Origins

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SECTION 9: Rapid-Fire Memory Tricks

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SECTION 10: Key Clinical Correlations

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SECTION 11: High-Yield Exam Summary

"Simple" types — 1 layer, specialized functions:

• Squamous: exchange (thinnest possible barrier)
• Cuboidal: secretion/absorption (equal dimensions = middle ground)
• Columnar: absorption/secretion (tallest = most organelles = hardest working)

"Stratified" types — multiple layers, protection:

• Squamous keratinized: DRY surfaces (skin)
• Squamous non-keratinized: WET surfaces (mouth, esophagus, vagina)
• Cuboidal: rare, ducts
• Columnar: rare, conjunctiva

Special types:

• Pseudostratified: Airways + epididymis (LOOKS stratified, IS NOT)
• Transitional: Urinary tract ONLY (stretchable + protective)

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SECTION 12: Self-Test Questions

A: Simple squamous epithelium; specifically called endothelium.

A: Pseudostratified columnar epithelium - all cells touch the basement membrane, but nuclei at different levels give a false stratified appearance.

A: Defective cilia (Kartagener syndrome/Primary Ciliary Dyskinesia) in the pseudostratified ciliated columnar epithelium of the respiratory tract.

A: Its surface umbrella cells can flatten and return to dome shape, allowing the epithelium to distend as the bladder fills. The cells also have specialized uroplakin membrane plaques to resist urine toxicity.

A: The esophagus is a moist internal surface where water loss is not a problem, so cells retain their nuclei and remain metabolically active. Skin is exposed to air and needs the keratin layer to prevent dehydration.

A: Merocrine (exocytosis) - salivary glands; Apocrine (apical cytoplasm lost) - mammary glands; Holocrine (entire cell disintegrates) - sebaceous glands.

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