Epithelial Cells Overview

• Sheets of cells lining body surfaces and internal cavities.
• Functions:
• Regulate entry and exit of substances, similar to plasma membrane.
• Respond to external stimuli and relay messages internally.
• Protect internal structures.
• Involved in secretion, sensation (photoreceptors in eyes, hair cells in auditory system), and nutrient absorption in intestines.

Types of Epithelial Cells

• Shapes and Structures:
• Simple: single layer.
• Stratified: multiple layers.
• Columnar: tall and thin.
• Cuboidal: short and fat.
• Squamous: flattened cells.
• Polarity:
• Apical side (top) faces a surface (air/water) and is involved in absorption/secretion.
• Basal side (bottom) connects to basal lamina made of collagen and laminin, which supports epithelial tissue.
• Lateral surfaces: connect cells side by side.

Cell Types in Intestinal Epithelium

• Brush Border Cells:
• Absorptive, have microvilli to increase surface area for nutrient absorption.
• Release absorbed nutrients into basal side.
• Goblet Cells:
• Secretory, secretes mucus from apical side, giving a goblet shape due to distension of cell membrane by secretory vesicles.

Cell Junctions in Epithelium

• Types:

1. Tight Junctions:
   • Prevent leakage between cells; composed of occludin and claudin.
   • Maintain the polarity of cell membranes in absorptive cells.
   • Can be visualized with dye experiments; dye cannot pass between cells due to tight junctions.
2. Adherens Junctions:
   • Link cytoskeletons of adjacent cells via cadherin (requires calcium for homophilic binding).
   • Connect to actin filaments; can change shape due to actin contraction.
   • Form a belt just below the apical surface.
3. Desmosomes:
   • Provides mechanical strength by linking cells via cadherin to intermediate filaments (keratin).
   • High tensile strength, particularly in skin epithelium.
4. Hemidesmosomes:
   • Link epithelial cells to the basal lamina; uses integrin to bind to laminin in the basal lamina.
   • Provide structural support, connecting to intermediate filaments.
5. Gap Junctions:
   • Allow direct communication between adjacent cells via connexins that form channels.
   • Essential for electrical signals in cardiac cells for coordinated contractions; can adapt to changes in signaling molecules (e.g., dopamine in retinal cells).

Summary of Junction Functions

• Tight Junctions: Barrier, prevent leaks.
• Adherens Junctions: Mechanical support, allow for shape change.
• Desmosomes: Resilience under stress, enabling tissue integrity.
• Gap Junctions: Communication, coordination of physiological responses.
• Hemidesmosomes: Anchor to the basal lamina for stability.

Plasmodesmata in Plants

• Different from animal junctions.
• Channels lined with plasma membrane allowing small molecules/proteins to pass, facilitating transcription coordination during plant development.