Histology: Basement Membrane and Gap Junctions

Overview of Tissues

• A tissue is a group of cells derived from a common progenitor that performs a coordinated function.
• Four main types of human tissues: epithelial, connective, muscular, and nervous.
• The scientific study of tissues is called histology.

General Functions of Human Tissues

• Epithelial Tissue: Covers and protects body surfaces, lines hollow organs, forms glands, and facilitates exchange with the environment.
• Connective Tissue: Protects, supports the body, connects organs, stores energy, and aids immunity.
• Muscular Tissue: Comprised of specialized cells that contract for force generation and heat production.
• Nervous Tissue: Detects stimuli and generates electrical signals (nerve impulses) to promote changes in muscle or gland actions.

Cell Junctions

• Cell junctions are regions of contact between adjacent cells.
• Main types of cell junctions include:
  • Tight Junctions: Form leakproof connections; common in the stomach lining and bladder.
  • Adherens Junctions: Resist pulling forces, using transmembrane proteins and plaques connected to microfilaments.
  • Desmosomes: Connect adjacent cells to resist contraction, connecting to intermediate filaments.
  • Hemidesmosomes: Anchor cells to the basement membrane, connecting intermediate filaments to laminins.
  • Gap Junctions: Protein channels (connexons) allowing signal sharing between cells.

Epithelial Tissue

• General Functions: Protects from injury, secretes fluids or enzymes, absorbs nutrients.
• Structure Descriptors: Defined by the number of layers (simple, stratified, pseudostratified) and cell shapes (squamous, cuboidal, columnar, transitional).
• Cell Polarization: Epithelial cells have distinct apical (exposed), basal (attached), and lateral (adjacent) surfaces.

Specific Types of Epithelial Tissue

• Simple Squamous Epithelium: Specialized for filtration and diffusion; includes endothelium and mesothelium.
• Simple Cuboidal Epithelium: Secretion and absorption, found in kidney tubules and thyroid gland.
• Non-ciliated Simple Columnar Epithelium: Secretes mucus (goblet cells) and absorbs nutrients; contains microvilli.
• Ciliated Simple Columnar Epithelium: Moves mucus for protection; found in bronchioles and oviducts.
• Stratified Squamous Epithelium: Protects against abrasion, can be keratinized or non-keratinized.
• Stratified Cuboidal and Columnar Epithelium: Rare, protects ducts or secretes tears; found in specific glands.
• Transitional Epithelium: Changes shape based on stretch; lines urinary organs.

Glandular Epithelium

• Comprises exocrine (short-distance secretion) and endocrine (hormonal secretion into blood) glands.
• Exocrine glands: Have ducts and include sweat and salivary glands; classified into unicellular (goblet cells) or multicellular.
• Mechanisms:
  • Merocrine: Exocytosis
  • Apocrine: Secretion from the apical face
  • Holocrine: Rupture of cells for secretion

Connective Tissues

• Typically not on body surfaces; consist of extracellular matrix (ECM) and cells which include fibroblasts, macrophages, plasma cells, and adipocytes.
• Extracellular Matrix (ECM): Can be liquid, solid, or gel-like; contains protein fibers (collagen, elastic, reticular) and ground substance.
• Fibroblasts: Most abundant, secrete ECM; collagen provides tensile strength while elastin offers elasticity.

Types of Connective Tissues

• Embryonic Connective:
  • Mesenchymal Tissue: Gives rise to various connective tissues.
  • Mucoid Tissue: Found in umbilical cord.
• Mature Connective:
  • Loose Connective Tissue: Includes areolar, adipose, and reticular tissues.
  • Dense Connective Tissue: Comprised of dense regular, irregular, and elastic types.
• Supporting Connective Tissue: Includes cartilage (hyaline, fibrocartilage, elastic) and bone.

Blood and Liquid Connective Tissues

• Blood: Contains plasma and blood cells; transports oxygen, nutrients, and waste.
• Lymph: Lower protein content, varies by location, predominantly lymphocytes or dietary fats.

Anatomical Membranes

• Consist of epithelial plus connective tissues, including mucous, serous, cutaneous, and synovial membranes.
• Mucous Membranes: Line cavities open to the exterior, protect against moisture loss and invaders.
• Serous Membranes: Line internal cavities, secreting lubricating serous fluid.
• Cutaneous Membranes: Form the skin, providing barrier and protection.
• Synovial Membranes: Line joints and secrete synovial fluid for lubrication.

Muscular and Nervous Tissues

• Muscle Tissue: Unique contractile cells (myocytes) generate movement and heat; includes skeletal, cardiac, and smooth muscles.
• Nervous Tissue: Contains neurons that conduct impulses and glial cells that support neurons.### Skeletal Muscle Tissue
• Long, multinucleate fibers displaying striations.
• Responsible for voluntary movements, posture maintenance, and heat generation.
• Connects to bones through tendons.

Cardiac Muscle Tissue

• Features branched fibers with a single nucleus and striations.
• Contains gap junctions that facilitate rapid electrical signal conduction.
• Functions involuntarily and collectively forms the myocardium, or heart wall.

Smooth Muscle Tissue

• Composed of long, non-striated fibers within irregularly-shaped cells, each containing a single nucleus.
• Cells have a thickened middle and are interconnected by gap junctions to enable involuntary muscle contractions.
• Essential for peristalsis, airway constriction, and contractions of the urinary bladder and gallbladder.

Nervous Tissue

• Comprises two main types of cells:
  • Neurons, which consist of a cell body, dendrites, and an axon; they generate electrical signals or nerve impulses.
  • Neuroglia, which do not generate impulses but provide support, including synthesizing myelin, a coating that enhances the speed of electrical signal transmission in neuronal axons.