Fundamentals of Physiology: Epithelial Cell Function Study Guide

General Characteristics of Epithelial Cells

  • Definition and Location:

    • Epithelial cells cover body surfaces and line body cavities.

    • They form the interface between the internal environment of the body and the external environment.

  • Primary Functions of Epithelial Cells:

    • Protection: They form a protective barrier against physical damage and pathogens.

    • Selective Barrier: They act as a gateway, controlling what enters and exits the body or specific organs.

    • Gland Formation: Epithelial cells are specialized to form glands and secrete products.

    • Regulation: They regulate the exchange of molecules between different compartments.

    • Absorption: They are specialized for taking up nutrients or fluids, such as in the epithelial lining of the small intestine.

  • Structural Components of Epithelial Tissue:

    • Apical Region: The surface facing the lumen or the exterior.

    • Basolateral Region: The surface attached to the basement membrane and adjacent cells.

    • Basal Lamina: A specialized layer of extracellular matrix that supports the epithelial cell layer.

    • Connective Tissue: Situated beneath the basal lamina, containing cells like fibroblasts.

    • Junctions: Specialized protein complexes that hold the cells together and facilitate communication.

  • Distinction Between Epithelial and Endothelial Cells:

    • Epithelial Cells:

    • Line both internal and external surfaces of the body.

    • Can be classified by shape: squamous, cuboidal, and columnar.

    • Can be organized into layers: simple (single layer) or stratified (multiple layers).

    • Endothelial Cells:

    • These are a specialized type of epithelial cells.

    • They specifically line the inner surfaces of blood vessels and lymph vessels.

    • simple squamous epithelium

Cell Polarity and the Domains of Epithelial Cells

  • Concept of Polarity:

    • Epithelial cells exhibit distinct polarity, meaning each surface (domain) has a specialized morphology and unique functions.

  • Apical Domain (Surface):

    • This domain faces the lumen or external environment and contains specialized structures:

    • Glycocalyx:

      • A carbohydrate coating made of glycoproteins and glycolipids.

      • In epithelial cells, it functions in cell-cell recognition, intercellular adhesion, and communication.

      • In endothelial cells, it regulates vascular permeability and interactions between blood and the endothelial cells.

    • Cilia:

      • Motile, fine hair-like cytoplasmic structures.

      • Length typically ranges between 1–10 μm1 \text{--} 10\,\mu\text{m}.

      • Function: Moving particles and fluid along the epithelial surface.

      • Locations: Trachea, large bronchi, and uterine tubes.

    • Microvilli:

      • Irregular projections of the cell membrane, approximately 1 μm1\,\mu\text{m} in length.

      • They are closely packed in cells with an absorptive function to increase surface area.

      • Locations: Small intestine and kidneys.

  • Lateral Domain:

    • This domain is responsible for cell-to-cell contact and adherence.

    • Tight Junctions (Zonula Occludens): Prevents the leakage of material between cells.

    • Adherens Junctions (Anchoring): Uses cadherins to bind cells together.

    • Desmosomes (Macula Adherens): Strong anchoring junctions using intermediate filaments.

    • Gap Junctions: Channels for communication and transport of small ions/molecules between cells.

  • Basal Domain:

    • This domain faces the underlying connective tissue.

    • Functions:

    1. Acts as an adhesion interface between epithelial cells and the extracellular matrix (ECM) using integrins and hemidesmosomes.

    2. Serves as a sieve or permeability barrier controlling entry and exit from cells.

    3. Controls cell organization and specialization.

Membrane Proteins and Disease Examples

  • Cystic Fibrosis (CF):

    • Nature of Disease: A life-shortening, autosomal recessive genetic disorder that affects multiple systems, primarily the lungs. multisystem disease, often difficult to treat

    • Symptoms: Difficulty breathing, coughing up thick sticky mucus blocking airways, frequent lung infections, and elevated sweat chloride levels.

    • Molecular Mechanism:

    • Caused by a mutation in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) protein.

    • CFTR is a transmembrane protein expressed on the apical region of epithelial cells lining the lungs, gut, and exocrine glands.

    • Normal CFTR Function:

    • Transports ions across the epithelial surface to create an osmotic gradient maintaining salt-water balance on many body surfaces.

    • This allows water to flow into the luminal space, keeping mucus hydrated.

    • goblet cells and submucosal glands produce mucous

    • mucous is effectively transported, facilitates optimal mucociliary clearance and airway defence.

    • Effect of CFTR Mutation:

    • Reduced channel number, function or both.

    • Water flow is inhibited, leading to dehydration of the airway surface liquid.

    • Result: Build-up of static, thick mucus on the outside of cells, static mucous.

    • less surface liquid

    • Decreased bicarbonate transport leads to an acidic pH.

  • E-Cadherin and Cancer:

    • TM proteins, major constitutions of adherens junction, important for cell-cell binding

    • cadherins mediate intercellular adhesion

    • in normal cells proliferation/growth ceases when cells come in contact with one another - contact inhibition

    • Malignant Transformation:

    • If E-cadherinE\text{-cadherin} is lost or mutated, cells lose contact inhibition.

    • cells undergo malignant transformation, uncontrolled cell proliferation, tumor formation

Specialized Functions in Various Systems

  • 1. Barrier Function (Skin):

    • Maintains a physical barrier against environmental insults, microbial invasion, chemicals, toxins, and allergens.

    • Facilitates communication between internal and external environments.

    • Example (Eczema): Damaged skin barrier leads to loss of moisture, inflammation, and penetration of allergens/bacteria.

  • 2. Gas Exchange (Lungs):

    • Occurs across the alveolar epithelium.

    • Type I Pneumocytes: Extremely thin cells lining 95%95\% of the alveoli; united by tight junctions to facilitate gas exchange.

    • Type II Pneumocytes: Produce a thin layer of surfactant that covers the alveolar surface to prevent collapse.

  • 3. Absorption (Gastrointestinal System):

    • Tall cells forming a single later (simple) with apical microvilli increase surface area for absorption.

    • Tight junctions ensure selective absorption by preventing material from passing between cells.

    • allows only selection of material to be absorbed

    • abnormal expression levels/activity of NA-K pump in diabetes, alzheimer’s disease, hypertension and tumours

    • Transport Mechanisms:

    • Apical membrane: Uses SGLT (Sodium-Glucose Linked Transporter) to bring glucose and Na+Na^+ into the cell.

    • Basolateral membrane: Uses GLUT-2 for glucose exit into the blood and the Na+-K+Na^+\text{-}K^+ pump to maintain ion gradients.

  • 4. Reabsorption (Kidneys):

    • water/solutes transported into bloodstream

    • Occurs in the Proximal Convoluted Tubule (PCT) where cells have extensive microvilli.

    • Substances Reabsorbed:

    • Average reabsorption: 100%100\% of glucose and amino acids; 65%65\% of salts and water.

    • Active transport is used for glucose and amino acids; passive transport is used for water and chloride ions.

    • Goal: To form concentrated urine while returning essential solutes to the bloodstream.

  • 5. Secretion (Pancreas):

    • Endocrine Function:

    • Islets of Langerhans contain Alpha cells and Beta cells.

    • Diabetes Mellitus Type 1: Occurs when Beta cells are destroyed, stopping insulin secretion.

    • Exocrine Function:

    • Pancreatic acinar cells secrete digestive enzymes, water, and a high concentration of NaHCO3NaHCO_3 into the duodenum.

    • Acinar Cell Injury: Caused by alcohol, smoking, or high-fat diets increase stress on acinar cells. loss of exocrine pancreatic function - reduces absorption of nutrients, malnutrition

    • Result: Malnutrition due to reduced nutrient absorption and chronic damage known as pancreatitis.