Epithelium Notes Review

Epithelium: Overview

  • The body is composed of four basic tissue types: 44 major tissue types: Epithelium, Connective tissue, Muscle, Nervous tissue.
  • Epithelium is avascular and covers body surfaces, lines body cavities, and forms glands. It creates a barrier between the external environment and underlying connective tissue.
  • Other tissue types: Connective tissue; Muscle; Nervous tissue.

Learning objectives

  • Recognize the 44 major tissue types.
  • Describe the general characteristics of epithelial tissues.
  • Identify the differences between membranous (surface) and glandular epithelia.

Functions of Epithelium

  • Protection: epithelium covering protects underlying tissue from mechanical abrasion/injury, harmful chemicals, invading microbes, and from excessive loss of water.
  • Secretion: epithelial tissue in glands is specialized to secrete specific chemical substances such as enzymes, hormones, and lubricating fluids.
  • Absorption: certain epithelial cells lining the small intestine absorb nutrients from digestion of food; the second largest absorbing area are epithelial cells of kidney tubules.
  • Diffusion: simple SQUAMOUSSQUAMOUS epithelium promotes diffusion of gases, liquids and nutrients (e.g., endothelium of capillaries and lining of lungs).
  • Friction reduction: the smooth, tightly interlocking endothelial cells that line the entire circulatory system reduce friction between the blood and the walls of blood vessels.
  • Cleaning: ciliated respiratory epithelium assists in removing dust particles/foreign bodies from air passages.
  • Sensation: specialized epithelial tissue containing sensory nerve endings is found in the skin, ears, and on the tongue; in nose and eyes are modified neurons.

Characteristics of Epithelium

  • Covers body surfaces (surface epithelium).
  • Forms secretory cells of glands (glandular epithelium).
  • Cells contain cytoplasmic cytokeratin filaments.
  • Cells are capable of mitosis (regeneration).
  • Contacts a basement membrane.
  • Devoid of blood vessels (avascular).
  • Derived from each of the three embryonic germ layers: Ectoderm, Mesoderm, Endoderm.
  • Apical border may have cilia, microvilli or stereocilia (serves to eliminate harmful substances in excretory epithelia, e.g., in the kidney).
  • Cell junctions.

Basement membrane and organization

  • Basement membrane consists of glycoprotein fibers with a basal lamina (from epithelial cells) and a reticular lamina (from connective tissue cells).
  • Secreted by epithelial cells; anchors the basal layer of the epithelium.
  • Vascularization: not present in epithelium (AVASCULAR).
  • In connective tissue below epithelium are the layers called: Dermis (skin), Submucosa, Lamina propria (in mucosa).
  • Epithelial cells are polarized and closely associated to supporting connective tissue.

Embryonic origin of epithelia

  • Ectoderm: gives rise to skin (epidermis) and derivatives (sebaceous and sweat glands); epithelial components of oral and nasal cavities; parts of visual, olfactory, gustatory, and auditory systems.
  • Mesoderm: yields simple epithelia that line body cavities (thoracic, abdominal, pelvic) and cover the external surface of internal organs as serosae; forms endothelium (internal lining of circulatory/lymphatic systems).
  • Endoderm: differentiates into epithelia that line interior of hollow organs and become part of parenchymatous organs, determining organ function.

Apical structures and epithelial polarity

  • Epithelial cells are polarized with distinct apical and basal surfaces.
  • Apical structures:
    • Cilia: motile, move materials across the surface (e.g., nasal epithelium).
    • Microvilli: brush border to increase surface area for absorption (intestinal mucosa) and excretion (renal tubule).
    • Stereocilia: long microvilli, non-motile; long finger-like projections (epididymis and sensory organs of inner ear).
  • Examples: ciliated epithelium in trachea; brush border microvilli in jejunum; stereocilia in epididymis.

Keratinization and surface characteristics

  • Keratinization: dry surfaces (e.g., skin) with an upper layer of dead cells that are tightly attached to provide impermeability and dehydration resistance.
  • Wet surfaces subject to abrasion (e.g., esophagus).

Cell junctions

  • Four main types:
    • Tight junctions: fusion of intramembranous particles between adjacent cells; apical; block passage of small molecules between cells (e.g., intestinal mucosa).
    • Adherens (terminal bars/zona adherens): less tight; cadherins (transmembrane glycoproteins) require Ca^{2+} for binding; connect neighboring cells; common in cuboidal and columnar epithelia.
    • Desmosomes (macula adherens, intercellular bridges): discontinuous junctions with 2 hemidesmosomes anchored to each cell by tonofilaments; contribute to mechanical stability (e.g., epidermis).
    • Gap junctions: conduits between adjacent cells allowing passage of ions and small molecules; present in various epithelia (e.g., cuboidal epithelia in renal tubules).
  • Terminal bars (zona adherens): a terminology for a type of adherens junction visible in EM as a belt-like structure.
  • Cadherins: Ca^{2+}-dependent transmembrane glycoproteins that mediate cell–cell adhesion.

Epithelium in two major forms

  • Surface epithelium: sheets of cells covering external surfaces and lining internal surfaces of vessels and body cavities; enables transport of substances and fluids (examples: endothelium, mesothelium — transport epithelium).
  • Glandular epithelium: secretory cells organized into glands; secretions released into ducts (exocrine) or blood (endocrine); examples include pancreas and thyroid gland.
  • Other specialized examples mentioned (pigment, seminiferous, sensory epithelia such as olfactory).

Surface epithelium characterization: cellular morphology and layers

  • Cellular morphology: squamous, cuboidal, columnar, pseudostratified columnar.
  • Number of cell layers concept:
    • Simple epithelium: one cell layer sitting on a basement membrane.
    • Stratified epithelium: two or more layers with only the basal layer resting on the basement membrane.
    • Pseudostratified epithelium: appears stratified but all cells rest on the basement membrane; not all reach the surface.
  • Basement membrane and connective tissue provide support and anchoring.

Simple epithelia: morphology, location, and function

  • Simple squamous epithelium
    • Structure: single layer of flattened cells.
    • Location: air sacs of lung and lining of heart, blood vessels, and lymphatic vessels; also endothelium lining specific vessels.
    • Function: allows materials to pass by diffusion/filtration and secretes lubricating substances.
  • Simple cuboidal epithelium
    • Structure: single layer of cube-shaped cells; nucleus usually central.
    • Location: lining ducts and secretory portions of small glands; kidney tubules; thyroid follicles; choroid plexus; lens of the eye.
    • Function: secretion and absorption.
  • Simple columnar epithelium
    • Structure: single layer of tall cells; nuclei typically oval and near base; may have microvilli and goblet cells.
    • Location: absorptive/secretory surfaces (glandular stomach, small and large intestine); gall bladder and ducts in some contexts; uterus/bronchi when ciliated; digestive tract when non-ciliated.
    • Function: absorption and secretion; goblet cells produce mucus; some surfaces have microvilli for absorption.
  • Ciliated vs non-ciliated distinction within simple columnar and related tissues.

Pseudostratified columnar epithelium

  • Structure: a single layer of cells varying in height; nuclei at different levels; not all cells reach the free surface.
  • Features: often contains goblet cells (unicellular glands).
  • Location/examples:
    • Respiratory tract: lines much of the upper respiratory system with goblet cells and cilia.
    • Epididymis and ducts deferens in male reproductive system may feature stereocilia (long microvilli) rather than cilia.
  • Function: secretes mucus and, with cilia, moves mucus along the surface.

Illustrative notes from figures

  • A typical table summarizes simple epithelia with location and function:
    • Simple squamous: air sacs of lung; lining heart/blood/lymph vessels; diffusion/filtration and lubrication.
    • Simple cuboidal: ducts/secretory portions of small glands; kidney tubules; secretion and absorption.
    • Simple columnar: ciliated forms in bronchi/uterine tubes/uterus; non-ciliated in digestive tract; absorption and mucus/enzyme secretion.
    • Pseudostratified columnar: ciliated in trachea/upper respiratory tract; mucus secretion; moves mucus.

Surface epithelium: layer count and morphology recap

  • Simple epithelium: single layer; all cells sit on basement membrane.
  • Stratified epithelium: two or more layers; basal layer rests on basement membrane; basal cells proliferate to replace superficial cells.
  • Pseudostratified epithelium: appears layered but is a single layer on basement membrane.

Stratified epithelia: variety and roles

  • Stratified cuboidal epithelium
    • Rare; typically two layers.
    • Lines excretory ducts of glands.
  • Stratified columnar epithelium
    • Rare; contains two or three layers.
    • Lines large ducts of salivary glands.
  • Stratified squamous epithelium
    • Many layers with the apical layer flattened.
    • Exists where abrasion is common.
    • Non-keratinized: apical cells retain nuclei (e.g., esophagus, anal canal).
    • Keratinized: apical cells lose nuclei and accumulate keratin (epidermis).
  • Transitional epithelium (urothelium)
    • Specialized stratified epithelium that lines excretory passages of the urinary system: renal pelvis (peripheral portion), ureters, bladder, urethra.
    • Can stretch; apparent number of cell layers changes with tissue state (stretching vs relaxation).
    • Includes dome-shaped surface cells when relaxed; urothelium forms a protective barrier while allowing distension.
  • Examples in organs:
    • Ureter and bladder show transitional epithelium with dome cells in relaxed state.
    • Bladder undergoes distension with stretching, changing the appearance of layers.

Practical and clinical relevance

  • Epithelial tissues form the first line of defense and play key roles in absorption (digestive tract, kidney), secretion (glands), diffusion (vascular/respiratory systems), and transport
  • Understanding types of epithelia helps interpret histology slides and diagnose tissue defects or disease (e.g., inappropriate keratinization, metaplasia altering barrier function).

Quick reference terms

  • Endothelium: simple squamous epithelium lining heart, blood vessels, and lymphatic vessels.
  • Mesothelium: simple squamous epithelium lining body cavities and covering internal organs.
  • Epithelium: basic tissue type with avascular nature and basement membrane.
  • Basal lamina: part of the basement membrane secreted by epithelial cells.
  • Reticular lamina: part of the basement membrane secreted by connective tissue cells.
  • Cadherins: Ca^{2+}-dependent transmembrane glycoproteins mediating cell–cell adhesion.
  • Tonofilaments: cytoskeletal elements anchoring desmosomes to cells.
  • Goblet cells: unicellular mucus-secreting glands within epithelium.
  • Brush border: microvilli-rich apical surface increasing absorptive area.
  • Keratinization: formation of a dry, protective, keratin-rich surface (e.g., skin).
  • Urothelium: transitional epithelium lining the urinary tract.
  • Ca^{2+}: essential ion for cadherin-mediated adhesion in adherens junctions.