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EPITHELIAL TISSUE - Detailed & Comprehensive Notes

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1. INTRODUCTION & DEFINITION

• Close cell apposition with very little intercellular substance
• Location at a free surface (external or internal)
• Resting on a basal lamina
• Absence of internal blood vessels (avascular) - nutrients diffuse from underlying connective tissue

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2. EMBRYONIC ORIGIN

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3. FUNCTIONS OF EPITHELIAL TISSUE

1. Covering, lining, and protecting surfaces - e.g., epidermis protects against dehydration and microbial invasion
2. Absorption - e.g., intestinal lining absorbs digested nutrients
3. Secretion - e.g., parenchymal cells of exocrine and endocrine glands
4. Selective permeability/barrier - controls passage of substances between body compartments
5. Sensory reception - specialized epithelial cells form taste buds and olfactory epithelium
6. Contractile function - myoepithelial cells (found in salivary and mammary glands) can contract to help expel secretions
7. Ion and water transport - e.g., renal tubular epithelium regulates salt and water balance

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4. CHARACTERISTIC FEATURES OF EPITHELIAL CELLS

4.1 Cell Polarity

• Apical domain: faces external environment or internal lumen; bears specializations (microvilli, stereocilia, cilia)
• Lateral domain: communicates and adheres to adjacent cells via junctions
• Basal domain: attaches to the basement membrane; involved in anchoring and transport

4.2 Cell Shape and Nuclear Morphology

4.3 Avascularity

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5. BASEMENT MEMBRANE

Structure

• ~20-100 nm thick
• Contains: Type IV collagen (self-assembles into a two-dimensional mesh network), laminins (large glycoproteins that attach to transmembrane integrins in the basal cell membrane), nidogen (a short rod-like protein) and perlecan (a heparan sulfate proteoglycan) which cross-link laminins to the type IV collagen network
• Hemidesmosomes on the basal cell membrane anchor the epithelium to the basal lamina

• Contains type III collagen (reticular fibers)
• Anchored to the basal lamina by type VII collagen anchoring fibrils
• More diffuse and thicker than the basal lamina

Functions of the Basement Membrane

1. Structural support - mechanically supports epithelial cells
2. Attachment - anchors epithelium to connective tissue
3. Semipermeable filter - regulates which molecules pass between epithelium and connective tissue (its perlecan component determines pore size/porosity)
4. Cell polarity - organizes integrins and other plasma membrane proteins; maintains apical-basal polarity
5. Signal transduction scaffold - mediates cell-to-cell interactions and localizes signaling events
6. Regeneration scaffold - guides epithelial repair and regeneration after injury
7. Migration route - marks paths for cell migration during development and wound healing

Note: Similar "external laminae" with comparable composition surround muscle cells, peripheral nerves, and fat-storing cells - where they act as semipermeable barriers.

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6. INTERCELLULAR JUNCTIONS

6.1 Tight Junctions (Zonula Occludens)

• Most apically located junction in the junctional complex
• Form a belt completely encircling each cell
• Created by transmembrane proteins (claudins and occludins) that physically contact and seal off intercellular spaces
• Function: Create a barrier that prevents paracellular diffusion of ions and molecules between the apical and basolateral compartments - essential for maintaining epithelial polarity
• Also serve as a "fence" that restricts lateral diffusion of lipids and proteins between apical and basolateral membrane domains

6.2 Adherens Junctions (Zonula Adherens)

• Located just basal to tight junctions; form a belt around the cell
• Transmembrane proteins (cadherins) link to the actin cytoskeleton inside the cell via catenins
• Function: Strong mechanical adhesion between adjacent cells; also involved in signaling pathways that regulate cell proliferation and differentiation
• In light microscopy, the region of tight + adherens junctions appears as the "terminal bar"

6.3 Desmosomes (Macula Adherens)

• Disc-shaped or spot-like junctions; do not form continuous belts
• Transmembrane proteins (desmogleins, desmocollins - members of the cadherin family) are anchored intracellularly to intermediate filaments (keratin tonofilaments) via desmoplakin and plakophilin
• Function: Resist mechanical stress by distributing tensile forces across the cytoskeleton network; especially abundant in skin, cardiac muscle, and cervix
• Clinical relevance: Autoantibodies against desmogleins cause pemphigus vulgaris, a blistering skin disease

6.4 Gap Junctions (Nexus)

• Formed by transmembrane protein complexes called connexins that form hexameric channels (connexons); connexons from adjacent cells align to form a pore ~1.5 nm in diameter
• Function: Allow direct passage of small molecules and ions (up to ~1 kDa) between adjacent cells - enable electrical and metabolic coupling (important in cardiac epithelium and smooth muscle coordination)

6.5 Hemidesmosomes

• Located on the basal cell membrane
• Structurally similar to half-desmosomes; use integrin proteins (not cadherins) to anchor keratin intermediate filaments to the basal lamina
• Function: Attach the basal cell surface to the basement membrane, resisting delamination

The Junctional Complex

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7. SPECIALIZATIONS OF THE APICAL CELL SURFACE

7.1 Microvilli

• Short, finger-like cytoplasmic projections, ~1 μm tall and ~0.1 μm wide
• Supported internally by a core of actin microfilaments connected to the membrane by myosin I and ezrin; the actin filaments terminate in a dense horizontal meshwork called the terminal web
• When very densely packed and regular (as in intestinal absorptive cells and renal proximal tubule), they form a brush border or striated border
• Function: Dramatically increase apical surface area for absorption and secretion - a single intestinal cell may have ~3,000 microvilli, increasing surface area ~20-fold
• The glycocalyx coating microvilli contains digestive enzymes (e.g., lactase, sucrase) and other receptors

7.2 Stereocilia

• Long, irregular branching microvilli-like projections (despite their misleading name, they are NOT true cilia - they contain actin, not microtubules)
• Non-motile
• Found in: epididymis (maximize surface area for sperm maturation and fluid reabsorption) and hair cells of the inner ear (mechanosensory transduction)
• Much longer than ordinary microvilli

7.3 Cilia

• Long, highly motile projections, 5-10 μm long and ~0.2 μm wide (much longer and twice as wide as typical microvilli)
• Contain an internal axoneme - a core of microtubules in the classic 9 + 2 arrangement: nine outer peripheral microtubule doublets surrounding a central pair of singlet microtubules
• Each outer doublet has a complete A-tubule and a partial B-tubule
• Ciliary movement is powered by dynein ATPase (attached to the A-tubule of each doublet), which causes doublets to slide, bending the cilium in a coordinated power stroke and recovery stroke
• Kinesin and cytoplasmic dynein motors carry components in and out of cilia (intraflagellar transport, IFT)
• At the base of each cilium is a basal body (centriole-derived) anchoring the axoneme to the apical cytoplasm
• Found in: respiratory tract epithelium, oviduct, ependyma lining brain ventricles, some renal tubules
• Function: Move fluid and particles along epithelial surfaces (mucociliary clearance in the respiratory tract; propulsion of ovum in the oviduct)

• Clinical relevance: Defective motile cilia cause Primary Ciliary Dyskinesia (Kartagener syndrome) - bronchiectasis, situs inversus, and male infertility

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8. CLASSIFICATION OF EPITHELIUM

1. Number of cell layers (simple or stratified)
2. Shape of the surface cells (squamous, cuboidal, or columnar)

8.1 Simple Epithelia (One Cell Layer Thick)

A. Simple Squamous Epithelium

• Cells are wider than tall; flattened with flattened nuclei
• Locations: Alveoli of the lung, Bowman's capsule (parietal layer), loop of Henle, mesothelium lining pleural/peritoneal/pericardial cavities, endothelium lining blood and lymph vessels
• Functions: Allows passive diffusion across thin barrier; reduces friction for fluid and blood flow; forms filtration membranes

B. Simple Cuboidal Epithelium

• Cells approximately equal in height and width; spherical nucleus
• Locations: Kidney tubules (distal convoluted tubule), thyroid follicles, small collecting ducts, surface of ovary
• Functions: Secretion and absorption; forms barriers with moderate transport capacity

C. Simple Columnar Epithelium

• Cells taller than wide; oval nuclei basally positioned
• Locations: Lining of stomach, small and large intestine (absorptive), gallbladder, uterus, oviduct, many glands
• May be ciliated (oviduct, uterus, bronchioles) or non-ciliated (stomach, intestine)
• Functions: Absorption (intestine), secretion (stomach), transport of ova (oviduct)

8.2 Stratified Epithelia (Two or More Cell Layers)

A. Stratified Squamous Epithelium

• Multiple layers; surface cells are squamous/flat
• May be keratinized or non-keratinized

• As cells differentiate and migrate toward the surface, they become filled with keratin and other proteins, lose their nuclei and organelles, and form dead, scale-like squames
• Location: Epidermis of skin (thick in palms and soles)
• Function: Provides a waterproof, tough barrier; impedes water loss and microbial invasion; the outermost cells are sloughed and replaced continuously from basal stem cells

• Surface cells retain their nuclei; kept moist by secretions
• Locations: Oral cavity, esophagus, vagina, cornea, external anal canal
• Function: Protection against mechanical abrasion while remaining moist

B. Stratified Cuboidal Epithelium

• Two or more layers; surface cells cuboidal
• Locations: Ducts of sweat glands, sebaceous glands
• Function: Secretion and protection in large ducts

C. Stratified Columnar Epithelium

• Rare; surface cells columnar
• Locations: Parts of the male urethra, large excretory ducts of some glands, conjunctiva
• Function: Protection with secretory activity

8.3 Special Categories

A. Pseudostratified Epithelium

• Appears stratified at first glance because nuclei are at different heights
• However, all cells rest on the basement membrane - it is technically a simple epithelium
• Not all cells reach the free surface (short/basal cells do not reach it)
• Identification relies heavily on knowing its typical locations:
  • Pseudostratified ciliated columnar epithelium (respiratory epithelium): Lines the trachea and bronchi; cells include ciliated columnar cells, goblet cells, brush cells, and basal cells
  • Pseudostratified columnar with stereocilia: Epididymis and vas deferens
• Function: Mucociliary clearance (respiratory); sperm maturation and transport (reproductive)

B. Transitional Epithelium (Urothelium)

• Specialized stratified epithelium lining the lower urinary tract (from minor calyces of the kidney to the proximal urethra)
• Characterized by its ability to distend (stretch): surface "umbrella cells" (dome-shaped when relaxed, flat when stretched) change shape without rupturing the epithelial barrier
• Contains uroplakins - proteins specific to the plasma membrane of umbrella cells that make them almost impermeable to urine
• Function: Acts as a permeability barrier preventing urine from being reabsorbed; accommodates volume changes during filling and voiding

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9. SECRETORY EPITHELIA AND GLANDS

9.1 Types of Glands by Secretion Destination

9.2 Structural Classification of Exocrine Glands

• Acinar/alveolar: Rounded, sac-like secretory end-pieces (e.g., parotid gland, pancreas)
• Tubular: Cylindrical secretory units (e.g., intestinal crypts, sweat glands)
• Tubuloalveolar (seromucous): Both tubular and acinar units (e.g., submandibular gland)

• Simple: Unbranched duct (e.g., intestinal glands, sweat glands)
• Compound: Branched duct (e.g., salivary glands, pancreas, liver)

9.3 Mechanisms of Secretion

A. Merocrine (Eccrine) Secretion

• Secretory products are released by exocytosis - membrane-bound secretory vesicles fuse with the apical plasma membrane and discharge their contents without any loss of cell membrane or cytoplasm
• Most common secretory mechanism
• Examples: Pancreatic acinar cells (digestive enzymes), most salivary gland cells, intestinal goblet cells

B. Apocrine Secretion

• The apical portion of the cell (containing secretory vesicles) pinches off along with some surrounding plasma membrane and cytoplasm
• The cell survives and regenerates after secretion
• Examples: Apocrine sweat glands in the axilla/groin, mammary glands (lipid-rich milk fat)

C. Holocrine Secretion

• The entire cell disintegrates and becomes the secretory product
• Cells differentiate, accumulate secretory material, and lyse
• Examples: Sebaceous glands (sebum production)
• Clinical note: Excess holocrine secretion of sebum triggered by testosterone at puberty, combined with colonization by Cutibacterium acnes in blocked ducts, is the primary mechanism of acne vulgaris

9.4 Nature of Secretory Products

• Produce non-glycosylated proteins (mainly digestive enzymes and other proteins)
• Have abundant RER (basophilic base) and Golgi; apical zymogen granules stain intensely eosinophilic
• Examples: Pancreatic acini, parotid gland acini, serous demilunes of submandibular gland

• Produce heavily glycosylated proteins called mucins, which hydrate to form viscous mucus
• Secretory granules stain poorly with eosin (mucins washed out in routine processing) but are PAS-positive
• Examples: Goblet cells, sublingual gland, gastric surface mucous cells, cervical glands

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10. TRANSPORT ACROSS EPITHELIA

Absorption

• Movement from lumen → capillaries (apical → basolateral direction)
• Examples: Gallbladder (concentrates bile by absorbing water); intestinal epithelium (absorbs water and ions from digested material)

Secretion

• Movement from capillaries → lumen (basolateral → apical direction)
• Examples: Salivary and other exocrine glands; choroid plexus (secretes CSF)

Structural Adaptations for Transport

• Apical surface: Dense brush border (microvilli) for maximum surface area; allows free entry of Na⁺
• Basolateral membrane: Elaborately folded with many deep infoldings containing rows of mitochondria to supply ATP for Na⁺/K⁺-ATPase pumps that actively extrude Na⁺ into interstitial fluid
• Tight junctions are essential in all transporting epithelia to prevent back-leakage and maintain separate apical and basolateral compartments

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11. CELL POLARITY

• Tight junctions act as a "fence" preventing lateral diffusion of membrane lipids and proteins between apical and basolateral domains
• The basement membrane organizes the basal domain by signaling through integrins and laminin
• Polarity is critical for directional transport (absorptive or secretory function)

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12. RENEWAL AND REGENERATION OF EPITHELIAL CELLS

• Most epithelial cells have a limited lifespan and are continuously replaced by mitosis of stem/progenitor cells
• The rate of renewal varies by tissue:
  • Intestinal epithelium: complete renewal every 3-5 days
  • Epidermis: turnover every ~28-30 days
  • Some specialized epithelia (e.g., kidney tubules, olfactory) have slower renewal

Sources of New Epithelial Cells

• In stratified epithelia, the basal (germinal) layer contains stem cells that continuously divide; daughter cells migrate outward, differentiate, and are eventually shed from the surface
• In simple epithelia, stem cells are less well-defined morphologically but are scattered among the epithelial population or located at specific zones (e.g., crypts of Lieberkühn in the intestine)

Repair After Injury

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13. SUMMARY TABLE: TYPES OF COVERING/LINING EPITHELIA

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14. SUMMARY OF KEY POINTS

1. Epithelial tissue forms sheets of closely packed, polar cells resting on a basement membrane, with no internal blood supply.
2. Functions include protection, absorption, secretion, transport, and sensory reception.
3. The basement membrane is a bilaminar structure (basal lamina + reticular lamina) containing type IV collagen, laminins, nidogen, and perlecan; it anchors, supports, and guides regeneration.
4. Junctions (tight, adherens, desmosomal, gap, hemidesmosomal) maintain cohesion, polarity, and communication.
5. Apical specializations - microvilli (absorption), stereocilia (absorption/sensory), and cilia (transport) - adapt cells to their specific roles.
6. Classification is based on number of layers (simple/stratified) and surface cell shape (squamous/cuboidal/columnar).
7. Special types include pseudostratified epithelium (all cells contact basement membrane but not all reach surface) and transitional epithelium (urothelium, adapts to stretch).
8. Glands are exocrine (with ducts) or endocrine (ductless); exocrine secretion may be merocrine (exocytosis), apocrine (cell tip pinches off), or holocrine (whole cell disintegrates).
9. Epithelial renewal is continuous; basal stem cells replenish surface cells lost to normal wear or injury.

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