Cell Junctions & Polarity

Principles of Epithelial Polarity and Morphological Domains

• Concept of Epithelial Polarity:

  • Epithelial cells exhibit distinct structural, biochemical, and functional polarity.

  • Specific structural modifications, transport mechanisms, and protein complexes are segregated into discrete spatial regions of the cell membrane and cytoplasm.

  • The asymmetric organization reflects specialized functional responsibilities: protection, absorption, secretion, fluid movement, and selective transcellular permeability.

• The Three Structural Domains:

  • Apical Domain:

    • Directed toward the exterior surface of the body or the luminal cavity of internal organs, ducts, and tubes.

    • Houses specialized surface projections designed to alter surface area or propel luminal contents.

  • Lateral Domain:

    • In direct apposition and communication with adjacent neighboring epithelial cells.

    • Characterized by specialized intercellular adhesion structures and communication pathways that seal intercellular spaces and mediate intercellular signaling.

  • Basal Domain:

    • Rests securely on the basal lamina, anchoring the epithelial sheet to the underlying connective tissue stroma.

    • Contains structural adaptations for tissue adherence, mechanical tension resistance, and active ion transport.

• Summary of Domain-Specific Surface Specializations:

  • Apical Surface Specializations:

    • Microvilli

    • Stereocilia

    • Cilia

    • Flagella

  • Lateral Surface Specializations (Intercellular Junctions):

    • Tight junction (zonula occludens)

    • Adherens junction (zonula adherens)

    • Desmosome (macula adherens)

    • Gap junction (communicating junction, or nexus)

  • Basal Surface Specializations:

    • Basement membrane

    • Hemidesmosome

    • Basal and lateral membrane infoldings

Apical Domain Specializations

• Microvilli:

  • Light Microscopic Appearance: Under light microscopy ($\text{L/M}$), microvilli and their covering glycocalyx appear as a continuous, dense apical band termed the brush border or striated border.

  • Electron Microscopic Appearance ($\text{E.M.}$): Non-motile, microscopic finger-like extensions of the cytoplasm bounded externally by the plasma membrane.

  • Molecular and Cytoskeletal Architecture:

    • Each microvillus contains an internal core composed of a bundle of parallel actin microfilaments cross-linked to one another.

    • The basal ends of these actin filaments extend downward into the apical cytoplasm to intermingle with and anchor into the terminal web.

    • Terminal Web: A dense, horizontal meshwork of cytoskeletal filaments located directly beneath the microvillar base in the apical cytoplasm, conferring structural rigidity, shape, and mechanical support to the apex of the cell.

    • Glycocalyx (Cell Coat): A prominent filamentous coat composed of glycoproteins and glycolipids projecting from the plasma membrane of the microvillus, functioning in protection and cell recognition.

  • Anatomical Distribution:

    • Absorptive enterocytes of the small intestine.

    • Simple cuboidal epithelial cells lining the proximal convoluted renal tubules of the kidney.

  • Primary Function: Markedly expands the total luminal surface area of the plasma membrane to optimize cellular absorption of nutrients and solutes.

• Stereocilia:

  • Morphological Characteristics: Extremely long, slender, non-motile, irregular microvillar projections.

  • Internal Organization: Resemble standard microvilli by housing parallel arrays of actin microfilaments complexed with actin-binding proteins; share similar diameters and anchor basally into the apical terminal web. They differ from microvilli by being significantly longer and frequently exhibiting distal branching.

  • Anatomical Distribution:

    • Pseudostratified columnar epithelium lining the epididymis of the male reproductive tract.

    • Sensory hair cells located in the internal ear.

  • Primary Functions:

    • Dramatically increases epithelial surface area.

    • Facilitates high-capacity fluid absorption in the epididymis.

• Other Apical Specializations:

  • Cilia: Motile, hair-like apical projections containing microtubule axonemes that beat in coordinated waves to propel fluid, mucus, or cellular material along luminal surfaces.

  • Flagella: Elongated, solitary motile appendages structurally related to cilia that provide propulsion to cells (e.g., spermatozoa).

• Clinical Correlation: Celiac Disease:

  • Pathology and Synonyms: Also known as gluten-sensitive enteropathy or sprue; a chronic autoimmune disease of the small intestine.

  • Pathogenesis: Triggered by an inappropriate immune reaction directed against the wheat protein gluten during gastrointestinal digestion. The resulting immune response produces diffuse, chronic enteritis.

  • Histopathologic Hallmark: One of the earliest structural alterations is the blunting and eventual complete loss of microvilli (loss of the brush border) on the apical surface of absorptive enterocytes, along with villous atrophy.

  • Clinical Presentation: Severe reduction in intestinal absorptive surface area leads directly to malabsorption, nutritional deficiencies, and progressive pathological changes in the intestinal wall.

  • Reversibility: Malabsorptive symptoms and underlying intestinal epithelial architecture are fully reversible upon strict elimination of gluten from the patient's diet.

Lateral Domain Specializations and Intercellular Junctions

• Tight Junction (Zonula Occludens):

  • Morphology and Location:

    • Situated at the most apical boundary of the lateral cell surface.

    • Forms a continuous, belt-like perimeter encircling the apical margin of each cell (zonula meaning belt).

    • The outer leaflets of adjacent plasma membranes fuse completely, obliterating the extracellular space such that the intercellular distance is precisely $0\,\text{nm}$.

  • Molecular Architecture: Formed by the direct, homotypic contact and anastomosing strands of integral transmembrane proteins, predominantly claudins and occludins, spanning the opposing plasma membranes.

  • Primary Functions:

    • Forms a primary paracellular permeability barrier, preventing the unregulated leakage of fluid, electrolytes, and macromolecules between adjacent cells (e.g., across the gastrointestinal mucosa).

    • Establishes specialized biological barriers, such as the impermeable blood-brain barrier ($\text{BBB}$).

    • Maintains the polarity of the cell membrane by preventing lateral diffusion of integral membrane proteins between apical and basolateral domains.

    • Provides mechanical stability and tissue cohesion.

  • Structural Variant (Macula Occludens):

    • Presents as isolated, discontinuous, patch-like occluding contact zones rather than a continuous circumferential band.

    • Typically identified between endothelial cells of microvascular structures.

• Adherens Junction (Zonula Adherens):

  • Morphology and Location: Located immediately basal to the zonula occludens, encircling the apical lateral margin of adjacent cells in a belt