Epithelial Tissues Overview

Epithelial Tissues

• Epithelial cells form sheets (epithelia) covering body surfaces, both internal and external.

• Functions include secretion, absorption, and protection.

• Cell junctions (occluding, anchoring, communication junctions) bind cells together and maintain integrity, linking cytoskeleton and ensuring communication between cells.

• The basement membrane, a layer of extracellular matrix, anchors the epithelial layer to underlying tissues. It's composed of type IV collagen and laminin.

• Surface adaptations (microvilli, stereocilia, basolateral folds, cilia, keratin) increase surface area, move substances, and provide protection

• Microvilli: Finger-like projections enhancing absorption in kidney tubules and intestines.

• Stereocilia: Extremely long microvilli; found in epididymis.

• Basolateral folds: Involved in fluid/ion transport; found in renal tubular cells and secretory ducts.

• Cilia: Motile projections; transport materials over cell surfaces, found in respiratory and female reproductive systems.

• Keratinization: Condensation of cytoskeleton proteins in outer epithelial layers forms a protective barrier (keratin), found in skin epithelium.

• Simple epithelium: Single layer of cells contacting the basement membrane.

• Pseudostratified columnar: Appears layered but all cells reach the basement membrane but not all reach the surface. Found in pharynx, trachea, bronchi, and urethra.

• Endothelium: Simple squamous epithelium lining the heart, blood vessels, lymphatics.

• Mesothelium: Simple squamous epithelium lining body cavities.

• Simple columnar: Tall, column-shaped cells, with or without surface modifications (cilia/microvilli), found in secretion/absorption regions (stomach, intestine, gallbladder, cervix).

• Stratified epithelium: Multiple layers of cells with only the basal layer contacting the basement membrane.

• Transitional epithelium/Urothelium: Specialized stratified epithelium found only in urinary tract.

Glands

• Epithelial cells specialize in secreting macromolecules (enzymes, mucin, hormones).
• Modes of secretion:
  • Merocrine: Exocytosis from cell apex (salivary, sweat, pancreatic glands)
  • Apocrine: Pinching off apical cytoplasm (mammary glands)
  • Holocrine: Shedding of entire cell (sebaceous glands)

Connective Tissues

• Connective tissues support other tissue types.

• Parenchymal cells: Perform major functions of a tissue (e.g., hepatocytes in the liver).

• Supporting cells: Provide structural framework.

• Fibroblasts: Secrete extracellular matrix components (collagen, elastin) in most tissues.

• Chondrocytes: Secrete cartilage extracellular matrix components.

• Osteoblasts: Secrete bone extracellular matrix components.

• Myofibroblasts: Secrete ECM and have a contractile function.

• Adipocytes: Lipid-storing cells crucial for energy storage, endocrine function, and protection.

Fibrillar Proteins

• Provide tensile strength and support in extracellular matrix.
  • Collagen: Most important fibrillar component, synthesized by fibroblasts and other support cells. Forms filaments, fibrils, and meshworks.
  • Elastin: Stretchable and resilient sheets/fibers, forming elastic fibers (produced by fibroblasts).
  • Fibrillin: Part of elastic fiber structure.
  • Fibronectin: Mediator glycoprotein linking cells and ECM components (like collagen).

Basement Membrane

• Composed of type IV collagen, laminins, perlecan, and nidogens (entactins).

• Laminated layers from interface of the cell membrane to extracellular matrix: Lamina lucida, Lamina densa, Fibroreticular lamina

Specialized Cell Types

• Fibroblasts: Active in growth and secretion of extracellular matrix; repair roles.

• Chondroblasts/chondrocytes: Cartilage formation.

• Osteoblasts/osteocytes: Bone formation and maintenance.

• Cartilage types: Hyaline (temporary skeleton, growth plates, joints), Elastic (external ear, auditory tube), Fibrocartilage (intervertebral discs, tendon attachments)

• Adipose tissue types:

  • Unilocular/white fat: Energy storage.
  • Multilocular/brown fat: Heat production.

Muscle Cells

• Muscle cells contract via actin and myosin interaction.

• Types:

  • Skeletal muscle: Striated, voluntary, myofibrils.
  • Cardiac muscle: Striated, involuntary, intercalated discs.
  • Smooth muscle: Involuntary, spindle-shaped, mitosis possible.

• Structures for skeletal muscle fibers: Sarcolemma, sarcoplasm, sarcoplasmic reticulum.

• Muscle fiber organization: Endomysium (surrounding individual muscle fibers), Perimysium (surrounds fascicles), Epimysium (surrounds anatomical muscle).

• Hypertrophy/Hyperplasia: Skeletal, cardiac, and smooth muscle adaptations to increased demand.

Immune Cells and Tissues

• Immune responses occur in secondary immune organs (lymph nodes, spleen) and MALT (mucosa-associated lymphoid tissue).
• B lymphocytes: Bone marrow origin, lymphoid organs and circulation.
• T lymphocytes: Bone marrow origin, thymus maturation, lymphoid organs and circulation.
• Natural killer (NK) cells: Activated cytotoxic lymphocytes, eliminate infected/cancer cells.
• Specialized macrophages: Alveolar macrophages (lungs), Kupffer cells (liver), microglia (brain), sinusoidal cells (spleen).
• Lymph nodes: Filter lymph, harbor immune cells (B/T cells), and non-specific phagocytic cells.
• Spleen: Mounts immune responses, filters blood.
• MALT subtypes: GALT (gut), BALT (bronchus), NALT (nasal), SALT (skin).

Cardiovascular System

• Heart wall layers: Epicardium (outer), Myocardium (middle, muscle), Endocardium (inner).

• Coronary arteries/veins: Supply/drain blood to/from the heart.

• Endocardium layers: Outer (contacting myocardium – collagen, Purkinje fibers), Middle (thickest, collagen/elastic fibers), Inner (endothelial cells).

• Purkinje fibers: Specialized cardiac muscle fibers for signal conduction.

• Blood vessels: Arteries (carry blood away from heart), Veins (carry blood to heart).

• Blood vessel layers (tunica):

  • Tunica intima (endothelium, internal elastic lamina)
  • Tunica media (smooth muscle, elastic tissue)
  • Tunica adventitia (fibroblasts, collagen)

• Capillaries: Smallest vessels for gas exchange.

• Postcapillary venules: Smallest venules resembling capillaries, containing more pericytes.