Overview of Epithelial Cells

• Epithelial cells are sheets of cells lining the body surface and internal cavities.
• Functions include:
  • Regulating substance exchange (similar to cell plasma membranes).
  • Responding to external stimuli.
  • Protecting internal structures.
  • Secreting substances (e.g., mucus).
  • Serving as receptor sites for sensory stimuli (e.g., photoreceptors in eyes).
  • Nutrient absorption in the intestines.

Types of Epithelial Cells

• Epithelial cells vary in shape and layering:
  • Simple: Single layer of cells.
  • Stratified: Multiple layers.
  • Shapes:
  • Columnar: Tall and thin.
  • Cuboidal: Short and fat.
  • Squamous: Flattened cells.

Polarization of Epithelial Cells

• Epithelial sheets have distinct surfaces:
  • Apical Surface: Faces air or internal cavity (gut).
  • Basal Surface: Anchored to connective tissue (basal lamina).
  • Lateral Surface: Sides of cells.
• Polarization is crucial for function and nutrient transport.

Basal Lamina

• A thin, tough matrix of collagen and laminin supporting the epithelial layer.
• Integrins: Membrane proteins that bind epithelial cells to the basal lamina.

Specialized Epithelial Cells

• Brush Border Cells (Absorptive):

  • Feature microvilli to increase surface area for absorption.
  • Nutrients enter from the apical side; transported to the basal side.

• Goblet Cells (Secretory):

  • Shaped like goblets; secrete mucus from the apical surface.
  • Golgi apparatus and vesicles facilitate secretion to the apical side.

Cell Junctions

• Tight Junctions:

  • Prevent leakage of molecules between cells.
  • Formed by proteins occludin and claudin.
  • Maintain separation of membrane proteins in the apical and basal surfaces.

• Adherens Junctions:

  • Connect cells using cadherin proteins linked to actin filaments.
  • Facilitate shape changes and movement of epithelial sheets.

• Desmosomes:

  • Provide mechanical strength; link through cadherin and keratin filaments.
  • Help withstand stress and strain on epithelia, especially in skin.

• Hemidesmosomes:

  • Anchor cells to the basal lamina using integrins connecting to keratin.
  • Provide tensile strength against mechanical stresses.

• Gap Junctions:

  • Allow direct communication between neighboring cells (size of 2-4 nm).
  • Formed by connexons that create channels for ions and small molecules.
  • Essential for coordinated functions (e.g., electrical signaling in heart cells).

Functional Summary of Cell Junctions

• Tight Junctions:

  • Function: Barrier to leakage.
  • Proteins: Occludin and claudin.

• Adherens Junctions:

  • Function: Shape change via actin contraction; adhesion belt.
  • Proteins: Cadherin.

• Desmosomes:

  • Function: Distribute stress; tensile strength.
  • Proteins: Cadherin, keratin.

• Gap Junctions:

  • Function: Intercellular communication, signal coordination.
  • Proteins: Connexons.

• Hemidesmosomes:

  • Function: Cell adhesion to basal lamina.
  • Proteins: Integrin, keratin.

Comparison with Plant Cells

• Plant cells lack the aforementioned cell junctions due to rigid cell walls.
• Plasmodesmata: Similar to gap junctions, allow communication and transport of molecules between plant cells, involving smooth endoplasmic reticulum.