Epithelial Tissue - Comprehensive Study Notes

Overview of Epithelia

• Epithelia are sheets of cells that are broadly contiguous; adjacent cells are held together by cell adhesion molecules with little intercellular material between them.

• They line internal and external surfaces, line body cavities and hollow organs, and form the major tissue in glands.

• Functions vary by location and include protection, secretion, and absorption.

• Epithelial tissue is mitotically active.

• Key takeaway: epithelia are the main cellular event, organized as sheets, and they rely on junctions and basement membrane for cohesion and function.

General Features of Epithelia

• Can be derived from all three germ layers. (
  $3$ germ layers)
• Cells show polarity (apical, basal, lateral domains).
• Little intercellular extracellular material between cells.
• Cells lie closely together and tend to form junctions.
• Rest on a basement membrane.
• Lack blood vessels (avascular).
• Line surfaces and cavities or form glands.
• Are mitotically active.

Derivation

• Epithelia can derive from all three germ layers: Ectoderm, Mesoderm, Endoderm.
• Examples listed in the lecture:
  • Ectoderm derivatives:
  • Epidermis of skin
  • Sweat glands and ducts
  • Endothelium lining blood vessels
  • Mesothelium lining body cavities
  • Lining of urinary and genital organs
  • Lining of gastrointestinal tract
  • Lining of respiratory tract
  • Liver
  • (Note: some items cross commonly taught lineages; the slide groups are as above.)
• Mesoderm derivatives: endothelium, mesothelium, etc., as listed above in the ectoderm group when presented.
• Endoderm derivatives: lining of GI tract, lining of respiratory tract, liver, etc.

Polarity of Epithelial Cells

• Epithelial cells exhibit distinct polarity with three domains:
  • Apical domain: faces lumen or external surface.
  • Lateral domain: interfaces with neighboring cells.
  • Basal domain: faces the basement membrane.
• In polarity, the nucleus is usually closer to the basal surface, and the Golgi apparatus is oriented toward the apical side (secretions delivered toward the apical surface).

Apical Domain and its Specializations

• Apical domain contains:
  • Microvilli: fingerlike cytoplasmic projections with an actin filament core; function to increase absorptive surface area (e.g., in the small intestine).
  • Stereocilia: unusually long, immotile microvilli; function as mechanoreceptors in hair cells and absorptive roles in the epididymis.
  • Cilia: motile structures that move fluid/particles along the epithelial surface (e.g., bronchial tree, oviducts); in sperm, flagellum enables forward movement; cilia have a 9+2 arrangement of microtubules: $9+2$ microtubule structure.
• Brush border (apical specialization): dense, refractile band along the apical surface especially in absorptive columnar cells (e.g., intestine).
  • Composed of densely packed microvilli and a glycocalyx; forms the brush border.
  • Function: increases surface area for absorption, provides protective layer, contains enzymes for terminal digestion of nutrients.
  • Cuticular border is a histological cue for absorptive epithelium (e.g., small intestine).

Basal Domain and Basement Membrane

• Basal domain is the part of the cell facing the basement membrane.

• Basement membrane is a thin, fibrous extracellular matrix separating epithelial cells from underlying connective tissue.

• Basal Lamina (a major component of the basement membrane) comprises two layers:

  • Lamina lucida
  • Lamina densa
  • Components: Collagen IV, heparan sulfate, fibronectin, laminin

• Reticular lamina (beneath the basal lamina) contains:

  • Collagen III (reticular fibers)
  • Collagen I
  • Proteoglycans

• Laminin is central to basal lamina organization:

  • The laminin molecule has three polypeptide chains intertwined for part of their length.
  • One end of laminin binds to an integrin on the cell membrane.
  • The ends near the cell form the lamina lucida; the other end binds to a collagen IV network forming the lamina densa.
  • Secreted by epithelial cells to form the basal lamina; the reticular lamina is secreted by fibroblasts beneath the basal lamina.
  • The lamina lucida is anchored to the basal lamina by hemidesmosomes, which are structurally similar to but chemically different from desmosomes.

• Hemidesmosomes: anchor epithelial cells to the basement membrane via integrins binding laminin; not true desmosomes.

• Basal lamina and basement membrane roles:

  • Anchor epithelial cells to underlying connective tissue.
  • Act as a mechanical barrier.
  • Influence differentiation of underlying endothelial cells.
  • Permit diffusion of nutrients and waste between epithelia and underlying tissues.

• Anatomical context:

  • Free nerve endings may penetrate the basement membrane to enter epithelia (pain endings).
  • Capillaries lie just beneath the epithelium but do not enter the epithelium.
  • Malignancy typically requires invasion through the basement membrane.

Lateral Domain and Cell Junctions

• The lateral domain provides cellular connections, forming the permeability barrier and enabling intercellular communication.

• Major types of cell junctions in epithelia:

  • Tight junctions (occluding junctions, zonula occludentes)
  • Desmosomes (anchoring junctions, including belt desmosomes and macula adherens)
  • Gap junctions (communicating junctions)

• Tight junctions (zonula occludentes):

  • Plasma membranes of neighboring cells are tightly sealed.
  • Proteins involved: occludins, claudins, junctional adhesion molecules.
  • Function: permeability barrier to water and solutes across the epithelium.

• Desmosomes (Anchoring junctions):

  • Attach the cytoskeleton of neighboring cells or to the extracellular matrix.
  • Components: intracellular anchor proteins, transmembrane adhesion proteins.
  • Do not prevent the flow of substances around cells (they anchor, not seal).
  • Types:
  • Zonula adherens (belt desmosomes): anchored by actin microfilaments; involve E-cadherin and Ca2+.
  • Macula adherens (desmosomes): anchored by cytokeratin intermediate filaments; provide strong adhesion.

• Gap junctions (communicating junctions):

  • Directly connect cytoplasm of adjacent cells, allowing passage of ions and small molecules up to about $1000\,\mathrm{Da}$ between cells.
  • Channels are formed by transmembrane connexins; an assembled channel is a connexon (6 connexin proteins).

• Practical examples:

  • Tight junctions in intestines prevent bacteria and toxins from entering the bloodstream.
  • Desmosomes in skin provide mechanical resilience to stretching and movement.
  • Gap junctions in cardiac muscle allow rapid spread of action potential for synchronized contraction.

Microvilli, Stereocilia, and Cilia

• Microvilli: actin-based core; increase absorptive surface area (e.g., intestinal epithelium).
• Stereocilia: unusually long, immotile microvilli; function as mechanoreceptors in hair cells and in epidiymal absorptive function.
• Cilia: motile extensions capable of moving fluids and particles along surfaces (e.g., bronchial tree, oviducts); in spermatozoa, the flagellum provides forward movement; structure features a central pair of microtubules with 9 peripheral microtubule doubles (9+2 arrangement).

Epithelia Fit into Functional Structures

• Epithelial tissues organize into functional units such as:
  • Lamellae (layers)
  • Acini (secretory sacs)
  • Tubules
  • Alveoli (air sacs)
  • Follicles (e.g., thyroid follicles)

Epidermis and Sweat Gland Example (Derivation/Function Context)

• In epidermis and skin-related epithelia, sweat glands show a distinct NaCl handling pattern:
  • Normal sweat: NaCl is absorbed in the duct, producing hypotonic sweat.
  • In cystic fibrosis (CF) or dysfunctional ducts: NaCl absorption is impaired, leading to relatively isotonic or hypertonic sweat.
  • The duct is involved in reabsorption and secretion processes that contribute to sweat tonicity.

Intermediate Filaments and Cytokeratins in Epithelia

• Cytokeratins are intermediate filaments found in epithelial cells; there are approximately 10 acidic and 10 basic cytokeratins that exist as acid–base pairs.
• Epithelial cells derived from different embryologic origins show different intermediate filament patterns:
  • Epithelia from endoderm or surface ectoderm typically contain cytokeratins and lack other intermediate filaments.
  • Epithelia derived from mesoderm usually contain cytokeratins as well.
  • Endothelial cells (vascular lining) contain vimentin instead of cytokeratins.
  • Wound healing may involve temporary expression of vimentin in epithelial cells.
  • In cancer, epithelial cells may express vimentin and sometimes lose cytokeratin expression.

Taxonomy and Glandular Structures

• Epithelia can form various glandular structures:
  • Glands may be simple (single duct) or compound (branched ducts).
  • Gland architecture can be described as tubular (tubules) or acinar (sac-like, secretory units).
  • Gland type and organization influence secretion patterns and regulatory mechanisms.

Summary of Key Terms to Recall

• Apical domain, Basal domain, Lateral domain
• Tight junctions (zonula occludentes)
• Zonula adherens (belt desmosome) and Macula adherens (desmosome)
• Gap junctions (connexons, connexins)
• Basal lamina, Lamina lucida, Lamina densa, Reticular lamina
• Laminin, Integrin, Collagen IV, Collagen I/III, Proteoglycans
• Hemidesmosomes
• Brush border and cuticular border
• Microvilli, Stereocilia, Cilia (9+2 structure)
• Cytokeratins vs Vimentin
• Common cell types: simple, stratified, squamous, cuboidal, columnar

Connections and Real-World Relevance

• Understanding epithelial polarity and junctions is essential for comprehending barrier functions (e.g., gut, skin) and tissue integrity.
• Basement membrane integrity is critical for preventing tumor invasion and for normal tissue organization.
• Junction types and their molecular components explain how tissues coordinate activity and maintain selective permeability.
• Abnormal intermediate filament expression relates to wound healing and cancer diagnostics.

Formulas and Notable Numbers

• Arrangement of ciliary microtubules: $9+2$ structure in motile cilia.
• Molecular weight threshold for gap junction communication: up to $1000\,\mathrm{Da}$ can pass through connexon channels.
• Three germ layers concept: $3$ germ layers dividing all epithelia origin.

References to Anatomy and Histology Contexts

• Epithelia derive from multiple germ layers and contribute to lining lumens and glands across organ systems.
• Basement membrane and basal lamina are essential interfaces with connective tissue and vasculature.
• Appositions such as tight junctions and adherens junctions provide both barrier and adhesive functions across epithelia.