Epithelial Tissue Characteristics Quiz

Epithelial Tissue/Epithelium

• Epithelial tissue is composed of closely aggregated polyhedral cells that adhere strongly to one another and to a thin layer of extracellular matrix (ECM).
• The cells form cellular sheets that line the cavities of organs and cover body surfaces.
• They line all external and internal surfaces of the body and all substances that enter or leave an organ.

Main Characteristics of Tissue Types

• Epithelial: Aggregated polyhedral cells, small amount of extracellular matrix, lining of surfaces, body cavities, and glandular secretion.
• Connective: Several types of fixed and wandering cells, abundant extracellular matrix, support and protection of tissues/organs.
• Muscle: Elongated contractile cells, moderate amount of extracellular matrix, strong contraction, body movements.
• Nervous: Elongated cells with extremely fine processes, very small amount of extracellular matrix, transmission of nerve impulses.

Parenchyma and Stroma

• Parenchyma: Cells responsible for an organ's specialized functions.
• Stroma: Supporting role in the organ; always connective tissue except in the brain and spinal cord.

Epithelial Tissues

• Composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM.
• Forms cellular sheets lining cavities of organs and covering body surfaces.
• Lines all external and internal body surfaces and substances entering/leaving organs.

Principal Functions of Epithelial Tissues

• Covering, lining, and protecting surfaces (e.g., epidermis).
• Absorption (e.g., intestinal lining).
• Secretion (e.g., parenchymal cells of glands).
• Includes myoepithelial cells and specialized sensory cells.

Characteristic Features of Epithelial Cells

• Shapes and dimensions vary, ranging from tall columnar to cuboidal to low squamous cells.
• Epithelial cell nuclei vary in shape, elliptic (oval), spherical, or flattened, corresponding roughly to cell shape.
• Most epithelia are adjacent to connective tissue containing blood vessels.
• Lamina propria underlies the epithelia lining organs of digestive, respiratory, and urinary systems.

Characteristic Features of Epithelial Cells — Contact Area

• Contact area between tissues increased by small evaginations called papillae.
• Covering of skin or tongue.
• Epithelial cells demonstrate polarity: basal pole, apical pole, and lateral surfaces.

Basement Membranes

• Basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules.
• A semipermeable filter for substances reaching epithelial cells from below.
• Two parts on TEM: basal lamina and reticular lamina.

ECM Components of Basal Lamina

• Type IV collagen
• Laminin (glycoprotein), attach to integrins
• Nidogen and perlecan (CHON and proteoglycan, respectively).

ECM Components of Reticular Lamina

• Type III collagen
• Type VII collagen

Functions of Basement Membrane

• Filter
• Structural Support
• Organize proteins
• Maintain cell polarity
• Cell-to-cell interactions
• Scaffold allowing rapid epithelial repair and regeneration

Intercellular Adhesion & Other Junctions

• Epithelial cell junctions, major structural features and functions. (Table provided).

Tight Junction (Zonula Occludens)

• Forms a band completely encircling each cell.
• Most apical of the junctions.
• Band of branching strands in the membrane around each cell's apical end.
• Seals between membranes due to tight interactions between claudin and occludin proteins.
• Restricts movements of membrane lipids and proteins.

Adherent Junction (Zonula Adherens)

• Encircles epithelial cells, usually immediately below the tight junction.
• Firmly anchors cells to neighbors.
• Mediated by cadherins and catenins.
• Forms a "terminal web."

Desmosome (Macula Adherens)

• Resembles a single "spot-weld"; does not form a complete belt.
• Disc-shaped structures.
• Contains members of the cadherin family (desmogleins and desmocollins).
• Provides firm cellular adhesion and strength.

Hemidesmosome

• On the basal epithelial surface, cells attach to the basal lamina by anchoring junctions.
• Structurally resembles a half-desmosome.
• Integrins primarily bind to laminin molecules in the basal lamina.

Gap Junction (Nexus)

• Mediates intercellular communication.
• Transmembrane proteins (connexins) form hexameric complexes (connexons) with central pore (~1.5nm).
• Permits intercellular exchange of small molecules (< 1.5 nm) allowing coordinated cellular function in many tissues.

Specializations of the Apical Cell Surface

• Microvili: Seen on apical cell surfaces of absorption-specialized epithelia (e.g., brush border of small intestine).
• Increase surface area (20-30 fold).
• Covered with glycocalyx.
• Contain bundled actin filaments.

Stereocilia

• Less common, typically longer and less motile than microvilli.
• Found in absorptive epithelial cells lining the male reproductive system (e.g., epididymis).
• Increase surface area, facilitating absorption.
• Important for motion-detecting functions in inner ear sensory cells.

Cilia

• Long, highly motile apical structures containing internal arrays of microtubules.
• Primary cilium: not motile, involved in detecting light, odors, and fluid flow.
• Motile Cilia: typical cilia, abundant on cuboidal/columnar cells.
• Axoneme: 9 + 2 assembly of microtubules.
• Basal bodies: structure similar to centrioles, exhibit rapid beating patterns.
• Found in respiratory epithelium and sperm cells.

Types of Epithelia

• Covering/Lining Epithelia: Organized into one or more layers covering surfaces or lining cavities. Classified by layer number (simple or stratified) and cell shape (squamous, cuboidal, columnar). (Table provided).
• Secretory/Glandular Epithelia: Function mainly to produce and release macromolecules (proteins, lipids, carbohydrates). Classified by cell number (unicellular or multicellular) (e.g. goblet cells) or secretion method (exocrine, endocrine). (Table provided).

Review of Naming

• Step 1: Number of layers (simple, stratified, pseudostratified, transitional).
• Step 2: Cell shape (squamous, cuboidal, columnar).
• Optional: Special features (microvilli, cilia, stereocilia, goblet cells, keratinized, non-keratinized).

Renewal of Epithelial Cells

• Epithelial tissues are relatively labile, renewed continuously by mitotic activity and stem cell populations.
• Rate of renewal varies widely.
• In stratified tissues, stem cells and mitosis primarily occur in the basal layer in contact with the basal lamina.
• Epithelia are capable of rapid repair and replacement.

Endocrine Glands

• Will be discussed in another chapter.

Exocrine Glands

• Organized as a continuous system of many small secretory portions and ducts.
• Secretory units in both exocrine and endocrine glands are supported by a stroma of connective tissue
• Classified by morphology (simple/compound; tubular/acinar/tubuloacinar). This is based of the formation of ducts from the secretory portion. (Table provided).

Classification (Based on Fate of Secretion)

• Merocrine: Without destruction, (release protein via exocytosis) (e.g., sweat glands).
• Holocrine: Total destruction (e.g., sebaceous glands).
• Apocrine: Partial destruction (e.g., mammary glands).

Classification (Based on Type of Secretion)

• Serous: Synthesize non-glycosylated proteins, with rough ER and Golgi complex. Heavily stained (e.g., acini of pancreas, parotid salivary glands).
• Mucous: Contain glycosylated proteins (mucins), hydrophilic, lightly stained.
• Mixed/Seromucous: Mixture of digestive enzymes and watery mucus (e.g., submaxillary/mandibular glands).

Test your knowledge on the various characteristics and functions of epithelial tissues. This quiz covers essential topics such as cilia, macromolecule secretion, and junction formation. Perfect for students studying histology or related biological sciences.