Tues 26th: on Epithelial Tissues, Membranes, and Glandular Epithelium

Introduction to Epithelial Tissue

• Definition: Epithelial tissue is a basic human tissue composed of tightly joined cells that form two primary structures:

  • Sheets (Membranes): These cover or line the internal and external body surfaces.

  • Secretory Units: These form the functional parts of glands.

• General Characteristics:

  • Cell Aggregation: Cells are closely aggregated and joined by junctional complexes.

  • Polarity: Epithelial cells typically exhibit structural and functional polarity.

  • Layers: They are organized in one or more layers.

  • Adhesion: The basal layer adheres to the basal lamina via hemidesmosomes.

  • Extracellular Matrix (ECM): There is very little ECM and almost no free intercellular substances.

  • Avascularity: Epithelial tissues do not contain blood vessels; they obtain nutrients through diffusion from underlying connective tissues.

  • Self-Renewal: These tissues exhibit significant mitotic activity for continuous renewal.

    • In stratified epithelia, stem cells and progenitor cells are located in the basal layer to facilitate this renewal.

Cell Polarity and Regions

• Basal Pole: The region of the cell attached to the basement membrane. It is anchored specifically by hemidesmosomes.

• Apical Pole: The region usually facing a space (lumen). It often contains apical specializations (e.g., cilia, microvilli).

• Lateral Pole: The side of the cell containing abundant cellular junctions. The cell membrane in this region is often folded (e.g., in cholangiocytes, hepatocytes, and pancreatic acini).

Structural Classification and Specific Functions of Epithelial Cells

• Squamous Cells:

  • Structure: Flattened cells.

  • Functions: Protection, transcytosis, and secretion.

• Cuboidal Cells:

  • Structure: Cells with approximately equal dimensions (height and width).

  • Functions: Protection, secretion, and absorption. They are often involved in active transport.

    • Spherical cells are classified as cuboidal when their width and height are similar, allowing them to efficiently fulfill their roles in glands and ducts.

    • Why active transport:

      • Active transport mechanisms are essential for maintaining cellular homeostasis, enabling cells to move ions and molecules against their concentration gradient.

      • It allows for the uptake of nutrients, such as glucose and amino acids, which are crucial for the metabolic functions of glandular epithelial cells.

• Columnar Cells:

  • Structure: Cells that are taller than they are wide.

  • Functions: Protection and primarily absorption.

    • Electrolytes and amino acids are often absorbed through the cell membranes of columnar epithelial cells, facilitating vital processes in the body. Additionally, these cells may contain microvilli to increase surface area, enhancing their ability to absorb nutrients efficiently.

    • Pepsin as a source of hydration has gained popularity, and while it provides immediate energy, it does not contribute to nutrient absorption in the same way that columnar epithelial cells facilitate the uptake of essential substances.

Intercellular Adhesions a nd Junctional Complexes

• Tight Junction (Zonula Occludens): An occluding junction that forms seals between adjacent cells to prevent leakage.

• Adherens Junction (Zonula Adherens): An anchoring junction providing strong cell-to-cell adhesion.

• Desmosome (Macula Adherens): An anchoring junction appearing as "spots" around cells to provide mechanical strength.

• Hemidesmosome: An anchoring junction that binds the basal pole of the cell to the basal lamina.

• Gap Junction: Channels that allow for intercellular communication and the passage of small molecules.

• Terminal Bar (Light Microscopy):

  • Located at the apical-lateral side of cells.

  • Historically referred to as "intercellular cement."

  • Under TEM, it is known as the Junctional Complex, consisting of:

    1. Zonula occludens: A band of tight junctions.

    2. Zonula adherens: Adherens junctions.

    3. Macula adherens: Desmosomes.

Apical Specializations

• Microvilli:

  • Function: Increase surface area for absorption.

  • Locations: Intestinal epithelium (covered by glycocalyx and digestive enzymes), Kidney Proximal Convoluted Tubules (PCT), and Distal Convoluted Tubules (DCT).

• Stereocilia:

  • Function: Increase surface area for absorption and detect motion.

  • Locations: Epididymis and hair cells of the inner ear.

• Cilia:

  • Motile Cilia: Move extracellular material (e.g., in the respiratory epithelium).

  • Primary Cilium: Non-motile; found in non-epithelial cells, enriched with receptors and signal transduction complexes for sensory functions.

  • Structure of Cilia:

    • Axoneme: Core assembly of $9 \times 2$ (nine peripheral microtubule doublets and two central microtubules).

    • Basal Body: Composed of nine triplets of microtubules (arrangement similar to centrioles). Anchored to the cytoskeleton by rootlets of dynamic tubulin protofilaments.

    • Microitubules of the axoneme are continuous with those in the basal body.

The Basement Membrane

• Definition: A protein-polysaccharide acellular membrane located between the basal layer of epithelium and the underlying connective tissue. It is PAS positive due to its carbohydrate content.

• TEM Components:

  1. Basal Lamina (BL): A thin network of fibrils produced by epithelial cells.

     • Type IV Collagen: Forms a 2D network.

     • Laminin: Glycoproteins that attach to integrins and project into the collagen network.

     • Nidogen and Perlecan: Cross-link laminin and help determine the porosity of the membrane.

     • Most superficial cell, the lamina is the part of the basal membrae that is in the contact with the basal layer of the epithelial

  2. Reticular Lamina: A diffuse meshwork produced by connective tissue cells.

     • Composed of Type III Collagen.

       • glycoproteins

     • Bound to the basal lamina by Type VII Collagen.

     • Is the deepest portion of the basement memebrane , providing structural support and anchoring epithelial tissues to the underlying connective tissue.

• Functions:

  • Provides structural support and attaches epithelia to connective tissue.

  • Organizes integrins and proteins in the plasma membrane.

  • Maintains cell polarity.

  • Localizes endocytosis and signal transduction.

  • Mediates cell-to-cell interactions.

  • Acts as a scaffold for rapid epithelial repair and regeneration.

  • Remember:

    • “The basement membrane consists of two parts produced by different cell types: the basal lamina, synthesized mainly by epithelial cells, and the reticular lamina, produced by the underlying connective tissue cells (mainly fibroblasts).”

    • “Epithelial cells produce the components of the basal lamina, including laminin, type IV collagen, and other glycoproteins.”

    • “If epithelial cells are damaged or removed, the basal lamina can remain intact and serve as a scaffold that supports regeneration and replacement of the epithelial cells.”

General Functions of Epithelial Membranes

• Lining/Covering: Covers body surfaces (exceptions include articular cartilage, tooth enamel, and the anterior surface of the iris).

• Protection: e.g., Skin.

• Absorption: e.g., Intestines.

• Transport: e.g., Ciliated respiratory tract moving mucus.

• Excretion: e.g., Kidney tubules.

• Gas Exchange: e.g., Lung alveoli.

• Gliding: Facilitated by mesothelium in the peritoneum, pleura, and pericardium.

Characteristics of Epithelial membranes

• Closely aggregated cells joint by junctional complexes o Many are polarized

• ✓Organized in one or more layers ✓Basal layer adhered to basal lamina by Hemidesmososmes

• ✓Have structural and functional polarity

  • ❖Important mitotic activity (self-renewal) o Basal layer in stratified epithelia:

    • ▪ Stem cells ▪ Progenitor cells

  • ✓Little ECM (almost no free intercellular substances)

• ✓Avascular (nutrients obtained by diffusion

Classification of Epithelial Membranes

Criteria for classification of epithelial membranes

1. Shape of cells

    ✓Squamous (flat cells)

    ✓Cuboidal (cell with equal dimensions)

    ✓Columnar (cells taller than wider)

2. Number of layers of cells

    ✓Simple (one layer of cells)

    ✓Stratified (more than one layer of cells)

3. Special epithelia

    ✓Transitional epithelium or Urothelium

    ✓Pseudostratified epithelium

1. Simple Epithelia (One layer of cells)

• Simple Squamous:

  • Structure: Flat cells that regulate the passage of substances; often exhibit transcytosis.

  • Examples: Lining of vessels (Endothelium), serous lining of cavities (Mesothelium: pericardium, pleura, peritoneum), Loop of Henle, intestinal serosa, and inner surface of the cornea.

  • Function: Facilitates viscera movement, active transport (pinocytosis), and secretion of biologically active molecules.

• Simple Cuboidal:

  • Structure: Cells with equal height and width; cytoplasm is rich in mitochondria for active transport.

  • Examples: Covering the ovary, thyroid follicles, kidney collecting tubules.

  • Function: Covering, secretion, and active transport.

• Simple Columnar:

  • Structure: Taller than wide; often have apical cilia or microvilli. Feature terminal bars at the apical end.

  • Examples: Lining of the intestine, gallbladder, oviduct.

  • Function: Protection, lubrication, absorption, and secretion.

2. Stratified Epithelia (Two or more layers of cells)

• Stratified Squamous Keratinized:

  • Structure: Superficial layers consist of dead, anucleate, flattened cells packed with keratin. Deeper layers are cuboidal.

  • Example: Epidermis of the skin.

  • Function: Protection, prevention of water loss/dehydration, and barrier against microorganism (MO) invasion.

• Stratified Squamous Non-keratinized:

  • Structure: Superficial cells are squamous but retain their nuclei and some metabolic activity. No keratin.

  • Examples: Mouth, esophagus, larynx, vagina, anal canal, outer lining of the cornea.

  • Function: Protection and secretion in humid internal cavities; prevents water loss.

• Stratified Cuboidal:

  • Example: Excretory ducts of sweat glands, developing ovarian follicles.

  • Function: Protection and secretion.

• Stratified Columnar:

  • Example: Conjunctiva and anorectal junction.

  • Function: Protection.

3. Special Categories

• Pseudostratified Columnar Epithelium:

  • Structure: All cells attach to the basement membrane, but nuclei are at different levels, giving the appearance of multiple layers. Not all cells reach the apical surface. Most cells reaching the surface are ciliated.

  • Example: Lining of the trachea, bronchi, and nasal cavity.

  • Function: Protection, secretion, and cilia-mediated transport of particles trapped in mucus.

• Transitional Epithelium (Urothelium):

  • Structure: Specialized for distension. The superficial layer contains large, dome-like cells known as umbrella cells.

  • Example: Bladder, ureters, renal calyces, proximal urethra.

  • Function: Protection and distensibility.

Glandular and Secretory Epithelia

• Secretory Cells: Specialized for the synthesis, storage, and secretion of various products:

  • Proteins: e.g., Pancreas.

  • Lipids: e.g., Sebaceous glands.

  • Complexes of Proteins and Carbohydrates: e.g., Salivary glands.

  • Water and Electrolytes: Filtered from blood (e.g., sweat glands).

• Unicellular Glands: Scattered secretory cells (e.g., Goblet cells in the intestinal and respiratory epithelium which secrete mucin).

• Development: Glands develop from covering epithelia via proliferation and downgrowth into the subjacent connective tissue.

  • Exocrine Glands: Retain connection to the surface epithelium via a duct.

  • Endocrine Glands: Lose their connection to the surface; their ducts disappear. Secretions are released into nearby capillaries.

• Solid Organ Histology:

  • Parenchyma: The epithelial portion, usually divided into lobules. Includes the secretory portion (acinus) and conducting portion (duct).

  • Stroma: The connective tissue framework, including the capsule, septa (partitions), and supporting tissue.

Classification of Exocrine Glands

By Duct Structure

• Simple: Unbranched duct.

• Compound: Branched duct.

By Secretory Portion Shape

• Tubular: Tube-like (can be straight, branched, or coiled).

• Acinar/Alveolar: Flask-like or sac-like (can be single or branched).

• Tubuloalveolar/Tubuloacinar: A combination where a tubular portion ends in a sac.

Specific Examples

• Simple Tubular: Intestinal glands (Crypts of Lieberkühn).

• Simple Branched Tubular: Uterine glands, stomach (pyloric) glands.

• Simple Coiled Tubular: Sweat glands.

• Simple Acinar: Urethral mucous glands.

• Simple Branched Acinar: Sebaceous glands.

• Compound Tubular: Submucosal mucous glands of the duodenum (Brunner’s glands).

• Compound Acinar: Exocrine pancreas.

• Compound Tubuloacinar: Salivary glands.

Modes of Secretion

• Merocrine Secretion: The most common type. Products (usually proteins) are released via exocytosis from apical secretory granules. The cell remains intact.

  • Serous Cells: Produce non-glycosylated proteins (enzymes). Feature well-developed RER and Golgi. Cytoplasm shows apical acidophilia and basal basophilia. (e.g., Pancreatic acini).

  • Mucous Cells: Produce heavily glycosylated proteins (mucins). Stain pale and are PAS positive. Feature wider ducts. (e.g., Goblet cells).

• Holocrine Secretion: The entire cell disintegrates to release its contents as it completes terminal differentiation. (e.g., Sebaceous glands of hair follicles).

• Apocrine Secretion: Involves the loss of membrane-enclosed apical cytoplasm, typically containing lipid droplets. (e.g., Mammary glands, which also use merocrine secretion).

Specialized Glandular Components

• Myoepithelial Cells:

  • Located at the basal end of secretory cells, between the cells and the basal lamina.

  • Joined to epithelial cells by desmosomes and gap junctions.

  • Rich in actin filaments and myousins; their contraction propels secretory products into the duct system.

• Endocrine Glands:

  • Lack myoepithelial cells and ducts.

  • Staining depends on the product: RER for proteins, SER for lipids/steroids.

  • Release proteins via exocytosis; lipophilic steroids are released by diffusion.

Renewal of Epithelial Cells

• Epithelial tissue is labile, meaning cells are continuously renewed by mitosis.

• Rates of Renewal:

  • Fast: Intestinal epithelium (renewed weekly).

  • Slow: Glandular tissues.

• Stem Cell Locations:

  • Stratified epithelia: Basal layers.

  • Complex epithelia: Specific niches away from differentiating cells.