Epithelial Tissue

Tissues

Epithelial Tissue

Primary Tissue Types

• Epithelial
• Connective
• Muscle
• Nerve

Overview of Tissues (Table 5.1)

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).