Epithelial Tissue

Tissues

Epithelial Tissue

Primary Tissue Types

  • Epithelial
  • Connective
  • Muscle
  • Nerve

Overview of Tissues (Table 5.1)

Tissue TypeCompositionFunctionsSubtypes
Epithelial TissueTightly packed cells with minimal extracellular matrixCovers body and organ surfaces, lines body cavities and organ cavities, forms glandsSimple Epithelium (simple squamous, simple cuboidal, simple columnar, pseudostratified columnar), Stratified Epithelium (stratified squamous, stratified cuboidal, stratified columnar, transitional)
Connective TissueContains cells, protein fibers, and ground substanceBinds, supports, and protects other tissues and organsConnective Tissue Proper (Loose: areolar, adipose, reticular; Dense: regular, irregular, elastic), Supporting Connective Tissue (Cartilage: hyaline cartilage, elastic cartilage, fibrocartilage; Bone), Fluid Connective Tissue (blood, lymph)
Muscle TissueCells that may be cylindrical, branching, or spindle-shaped; contain contractile proteins (myofilaments)Moves the skeleton, organ walls, or body structures; contractile, conductive, elastic extensible, and excitable
Skeletal, Cardiac, Smooth
Nervous TissueContains neurons and glial cellsNeurons transmit nerve impulses and process information; glial cells support, protect, and nourish neuronsNone

Epithelial Tissue Characteristics

  • Adjacent to a free surface: covering or lining
  • Lacks intercellular material
  • Non-vascular (no blood vessels)
  • Nerve supply
  • Mitotic (capable of cell division)
  • Rests upon basement membrane

Epithelial Tissue Functions

  • Protection
  • Secretion
  • Absorption
  • Filtration

Epithelial Tissue Classification

  • Shape
    • Squamous
    • Cuboidal
    • Columnar
    • Transitional
  • Number of layers
    • Simple
    • Stratified
    • Pseudostratified

Epithelium Structure

  • Apical surface (facing the free space)
  • Lateral surface (connecting to adjacent cells)
  • Basal surface (resting on the basement membrane)
  • Basement membrane (connective tissue interface)

Epithelium Classification by Layers

  • Simple epithelium: One layer of cells
  • Stratified epithelium: Two or more layers of cells

Epithelium Classification by Shapes

  • Squamous cells: Flattened cells
  • Cuboidal cells: Cells about as tall as they are wide
  • Columnar cells: Tall, narrow cells

Types of Epithelia

  • Simple squamous: One layer of flattened cells
  • Simple cuboidal: One layer of cells about as tall as wide
  • Simple columnar: One layer of tall, narrow cells; can be ciliated or nonciliated
  • Pseudostratified columnar: One layer of tall cells that appears stratified
  • Stratified squamous: Multiple layers; upper layers of flattened cells; can be keratinized or nonkeratinized
  • Stratified cuboidal: Two or more layers; cells about as tall as wide
  • Stratified columnar: Two or more layers of tall, narrow cells
  • Transitional: Multiple layers of cells that change shape when stretched

Table 5.2 Simple Epithelia

  • (a) Simple Squamous Epithelium
    • Structure: Single layer of thin, flat cells; relatively flat single nucleus bulges at its center.
    • Function: Thinnest possible barrier for rapid diffusion and filtration; secretion in serous membranes.
    • Location: Air sacs in lungs (alveoli); lining of blood and lymph vessels (endothelium); serous membranes of body cavities (mesothelium).
  • (b) Simple Cuboidal Epithelium
    • Structure: Single layer of cells about as tall as they are wide; spherical, centrally located nucleus.
    • Function: Absorption and secretion; forms secretory tissue of most glands and small ducts.
    • Location: Lining of kidney tubules; thyroid gland follicles; surface of ovary; secretory regions and ducts of most exocrine glands.
  • (c) Nonciliated Simple Columnar Epithelium
    • Structure: Single layer of nonciliated cells taller than they are wide; oval-shaped nucleus oriented lengthwise in basal region of cell; apical surface may have microvilli; may contain goblet cells.
    • Function: Absorption and secretion.
    • Location: Lining of most of the gastrointestinal tract (stomach, small intestine, and large intestine).
  • (d) Ciliated Simple Columnar Epithelium
    • Structure: Single layer of ciliated cells taller than they are wide; oval-shaped nucleus oriented lengthwise in basal region of cell; may contain goblet cells.
    • Function: Secretion of mucin and movement of mucus along apical surface by cilia; oocyte movement through uterine tube.
    • Location: Lining of larger bronchioles (air passageways) of the lung and uterine tubes.
  • (e) Ciliated Pseudostratified Columnar Epithelium
    • Structure: Single layer of cells with varying heights; all cells connect to the basement membrane, but not all cells reach the apical surface; has goblet cells and cilia.
    • Function: Protection; secretion of mucin and movement of mucus along apical surface by cilia.
    • Location: Lining of the larger airways of respiratory tract, including nasal cavity, part of pharynx, parts of larynx, trachea, and bronchi.
  • (f) Nonciliated Pseudostratified Columnar Epithelium
    • Structure: Single layer of cells with varying heights; all cells connect to the basement membrane, but not all cells reach the apical surface; lacks goblet cells and cilia.
    • Function: Protection.
    • Location: Rare-lining of part of the male urethra and epididymis.

Table 5.3 Stratified Epithelia

  • (a) Keratinized Stratified Squamous Epithelium
    • Structure: Multiple cell layers; basal cells are cuboidal or polyhedral and alive, while apical (superficial) cells are squamous, lack a nucleus and organelles, are filled with keratin, and are dead.
    • Function: Protection of underlying tissue from abrasion.
    • Location: Epidermis of skin.
  • (b) Nonkeratinized Stratified Squamous Epithelium
    • Structure: Multiple cell layers; basal cells are cuboidal or polyhedral, while apical (superficial) cells are squamous; all cells are living; each cell contains a nucleus and organelles and lacks keratin; superficial cells kept moist.
    • Function: Protection of underlying tissue from abrasion.
    • Location: Lining of oral cavity, part of pharynx, part of larynx, esophagus, lining of vagina, and anus.
  • (c) Stratified Cuboidal Epithelium
    • Structure: Two or more layers of cells; cells at the apical surface are about as tall as they are wide.
    • Function: Protection and secretion.
    • Location: Ducts of most exocrine glands and ovarian follicles.
  • (d) Stratified Columnar Epithelium
    • Structure: Two or more layers of cells; cells at the apical surface are taller than they are wide.
    • Function: Protection and secretion.
    • Location: Large ducts of salivary glands; conjunctiva covering the eye; lining of the membranous part of male urethra.
  • (e) Transitional Epithelium (relaxed)
    • Structure: Epithelial appearance varies, depending upon whether tissue is relaxed or distended (stretched); relaxed transitional epithelium has cuboidal or polyhedral cells and the apical cells are large and rounded, some cells are binucleated
    • Function: Accommodates volume changes (by distending or relaxing) in the urinary bladder, ureters, and part of urethra
    • Location: Lining of urinary bladder, ureters, and part of urethra.

Simple Epithelia

  • Best suited for absorption, secretion, or diffusion.
  • Examples:
    • Simple squamous epithelium of capillary wall and alveolus wall: Allows for rapid diffusion of gases between an alveolus of the lung and a capillary.
    • Simple cuboidal epithelium in convoluted tubules of the kidney: Functions in absorption and secretion of materials between filtrate and the blood.
    • Simple columnar epithelium lining the small intestine: Microvilli and single layer of cells facilitate absorption of nutrients, and goblet cells secrete mucus.
    • Pseudostratified ciliated columnar epithelium in the upper respiratory tract: Protection, secretion of mucus; cilia propel mucus along the epithelial surface.

Stratified Epithelia

  • Best suited for physical protection.
  • Examples:
    • Nonkeratinized stratified squamous epithelium lining the oral cavity and esophagus: Multiple layers of cells withstand abrasion from ingested materials.
    • Keratinized stratified squamous epithelium in the epidermis of the skin: Multiple layers of keratinized cells protect underlying tissue from abrasion; offers greater protection than nonkeratinized epithelium.
    • Transitional epithelium lining the urinary bladder, ureters, and part of the urethra: Distensible, multilayer tissue protects deeper tissue from urine, distends, and relaxes to accommodate urine volume changes.

Glandular Epithelia

  • Endocrine Glands : secrete hormones directly into the bloodstream (not explicitly detailed in the provided transcript).
  • Exocrine Glands: secrete substances into ducts that open onto a surface.

Multicellular Exocrine Glands: Structure

  • Structure:
    • Simple: unbranched duct
      • Simple Tubular
      • Simple Branched Tubular
      • Simple Coiled Tubular
      • Simple Acinar
      • Simple Branched Acinar
    • Compound: branched duct
      • Compound Tubular
      • Compound Acinar
      • Compound Tubuloacinar

Mode of Secretion

  • Merocrine: Secrete by exocytosis (e.g., pancreas, salivary glands, most sweat glands).
  • Apocrine: Accumulate products at the tip and cell pinches off with its contents (e.g., mammary gland).
  • Holocrine: Accumulate products; the cell dies and is discharged with its contents, later replaced by underlying cells (e.g., sebaceous glands).