Epithelial Tissue Structure and Function

Epithelial Tissue

• Epithelium is a tissue type characterized by tightly bound cells forming a sheetlike or tubular structure with minimal extracellular material.
• Cells in epithelia have two distinct sides: apical (facing the free surface) and basal (facing the basement membrane and underlying connective tissue).
• Epithelia are specialized for various functions, including:
  • Absorption
  • Transcytosis
  • Pinocytosis (at the apical side)
  • Exocytosis (at the basolateral side)
• Most epithelial cells exhibit continuous renewal, with:
  • Stem cells located in specific areas
  • Variable rates of cell turnover depending on the specialized epithelium

Basement Membrane

• Consists of two parts: basal lamina and reticular lamina
• Basal lamina is a thin meshwork composed of type IV collagen and laminin
• Basal lamina is produced by epithelial cells
• Reticular lamina contains type III collagen and anchoring fibrils of VII collagen
• Reticular lamina components are secreted by cells of the adjacent connective tissue
• Functions of basement membrane:
  • Attaches epithelia to connective tissue
  • Regulates (filtrates) substances passing from connective tissue into epithelia
  • Provides a guide or scaffold during tissue regeneration after injury
  • Compartmentalizes epithelial cells from other tissues

Intercellular Junctions in Epithelia

• Intercellular junctions are well-developed in epithelia and consist of three major types with different functions.

Tight or Occluding Junctions

• Formed by interacting transmembrane proteins such as claudin and occludin.
• Linear arrangements of these linked proteins surround the apical ends of cells.
• Prevent paracellular passage of substances between cells.

Adherent or Anchoring Junctions

• Formed by interacting proteins of the cadherin family.
• Points of strong attachment holding together cells of the epithelium.
• May form zonula adherens that encircle epithelial cells just below their tight junctions.
• May also form scattered, spot-like attachment sites called desmosomes or maculae adherens.
• Attached to cytoplasmic keratins.

Hemidesmosomes

• Composed of transmembrane integrins.
• Attach cells to proteins of the basal lamina.

Gap or Communicating Junctions

• Points of cell contact where both plasma membranes have numerous hexameric complexes of transmembrane connexons.
• Each forming a channel allowing passage of small molecules from one cell to the other.

Apical Structures of Epithelial Cells

• Microvilli are small membrane projections with cores of actin filaments that increase the apical surface area of epithelial cells for absorption.
• Stereocilia are a type of microvilli with specialized mechanosensory functions, found in cells of the inner ear, and for absorption in tissues of the male reproductive tract.
• Stereocilia are longer than microvilli.

Cilia

• Cilia are larger projecting structures with a well-organized core of microtubules in a 9 + 2 arrangement called the axoneme.
• The axoneme is responsible for ciliary movement, which propels material along an epithelial surface.
• Ciliary movement is caused by restricted, dynein-based sliding of microtubules.

Morphological Types of Epithelia

• Epithelia with one cell layer are classified as simple, with cells varying in height from thin/squamous to cuboidal to tall/columnar.

Simple Epithelia

• Characterized by a single cell layer
• Cell heights range from very thin (squamous) to very tall (columnar)

Stratified Epithelia

• Comprise two or more layers of cells
• Almost all stratified epithelia are stratified squamous
• Outer cell layers are thin and flattened
• Cells migrate from basal to surface layers, changing shape and becoming filled with keratin intermediate filaments

Functions of Stratified Squamous Epithelia

• Protect underlying tissues from excess water loss (dehydration) and microbial invasion
• Found in the epidermis, covering the body surface

Pseudostratified Epithelia

• Appear to have several cell layers
• All cells attach to the basal lamina, but not all extend to the free epithelial surface
• Thick epithelial layers

Transitional Epithelium (Urothelium)

• Found only in the lining of the urinary system
• Stratified epithelium with large rounded surface cells
• Protective against urine

Epithelial Secretion and Glands

• Epithelial cells have a major function in synthesizing and secreting specialized products, forming organs called glands.

Types of Glands

• Exocrine glands have epithelial ducts that carry secretions to specific sites, with two types:
  • Simple glands: unbranched ducts
  • Compound glands: branched ducts
• Endocrine glands lack ducts, and secreted substances (hormones) are carried throughout the body by interstitial fluid and blood, with specificity produced by hormone receptors of target cells.

Secretory Mechanisms

• Three basic secretory mechanisms:
  • Merocrine: uses exocytosis
  • Holocrine: terminally differentiated cells filled with lipid product are released
  • Apocrine: apical, product-filled areas of cells are extruded

Mucous Glands

• Exocrine glands producing mucus, or individual cells called goblet cells, are called mucous glands
• Oligosaccharide components of mucus stain poorly with routine dyes but stain well with PAS stain

Epithelial Tissue

• Epithelial tissue is a type of tissue where cells are tightly bound together to form a sheet-like or tubular structure with minimal extracellular material between cells.
• Each cell has an apical side facing the free surface and a basal side facing the basement membrane and underlying connective tissue.
• Epithelial cells are often specialized for absorption, transcytosis, pinocytosis, or exocytosis.

Basement Membrane

• The basement membrane is a thin extracellular layer of specialized proteins, consisting of two parts: a basal lamina and a reticular lamina.
• The basal lamina is a thin meshwork of type IV collagen and laminin produced by epithelial cells.
• The reticular lamina contains type III collagen and anchoring fibrils of VII collagen, secreted by adjacent connective tissue cells.

Intercellular Junctions

• Intercellular junctions are well-developed in epithelial tissue and consist of three major types: tight junctions, adherent junctions, and gap junctions.
• Tight junctions prevent paracellular passage of substances between cells.
• Adherent junctions hold cells together, forming strong attachment points.
• Gap junctions allow passage of small molecules between cells.

Apical Structures of Epithelial Cells

• Microvilli are small membrane projections that increase epithelial cells' apical surface area for absorption.
• Stereocilia are long microvilli with specialized mechanosensory function in cells of the inner ear and for absorption in male reproductive tract tissues.
• Cilia are larger projecting structures that propel material along an epithelial surface.

Morphological Types of Epithelia

• Epithelia can be classified as simple, stratified, pseudostratified, or transitional.
• Simple epithelia have one cell layer, with varying cell heights.
• Stratified epithelia have two or more cell layers, often with outer layers being thin and flattened.
• Pseudostratified epithelia appear to have multiple layers, but all cells attach to the basal lamina.
• Transitional epithelia are found in the urinary system and have large rounded surface cells.

Epithelial Secretion and Glands

• Epithelial cells synthesize and secrete specialized products, with organs composed primarily of these cells called glands.
• Exocrine glands have epithelial ducts carrying secretions to specific sites, with secretory portions forming round or elongated structures.
• Endocrine glands lack ducts, and secreted substances are hormones carried by the interstitial fluid and blood.
• Glands have three basic secretory mechanisms: merocrine, holocrine, and apocrine.