Epithelial Tissue Classification Quiz

Tissues

• Tissues are groups of cells organized to perform one or more functions.
• The cell is the basic structural unit of the body.
• In multicellular organisms, tissues are responsible for maintaining body functions due to the collaborative efforts of their individual cells.

Four Basic Tissue Types

• Epithelium (90% of tumors)
• Connective tissue
• Muscular tissue
• Nervous tissue

Epithelial Tissue

• Covers body surfaces, lines body cavities, and forms glands.
• Two main types:
  • Covering and lining: Covers outer body surfaces (skin), inner body surfaces (lumen of hollow organs - intestines), and cavities (peritoneal, pericardial, pleural), blood vessels, ducts, and tubules. They separate the underlying connective tissue from the environment of internal cavities.
  • Glandular: Forms glands.

Origin and Distribution of Epithelium

• Ectoderm: Epidermis of skin, cornea epithelium, oral cavity, sebaceous and mammary glands, anterior pituitary
• Endoderm: Alimentary tract, liver, pancreas, gastric glands, intestinal, Endocrine glands (lose connection to surface)
• Mesoderm: Endothelium (lining of blood vessels), Mesothelium (lining serous cavities)

Epithelia Specialized for Functions

• Absorption (intestine)
• Secretion (pancreas)
• Transport (eye, endothelium in vessels)
• Excretion (kidney)
• Protection (against mechanical damage and dehydration)
• Sensory reception (pain, taste, smell)
• Contraction (myoepithelium)

Surface Specializations of Epithelia

• Microvilli: Intestinal absorptive cells
• Cilia: Respiratory epithelium
• Basal lamina: All epithelium
• Intercellular canaliculus: Hepatocytes
• Secretory canaliculus: Gastric parietal cells
• Flagella

Characteristics of Epithelial Tissue

• No extracellular substance between cells
• Cells are tightly attached to each other by special junctions.
• Basal surface attached to an underlying basement membrane (extracellular protein-polysaccharide-rich layer).
• Polarity: Two surfaces—apical (outer) and basal (inner).

Epithelial Tissues and Basement Membrane

• Epithelial cells have an apical surface (free surface) and a basal surface that contacts the basement membrane.
• The basement membrane is a layer made of connective tissue that lies under the epithelium.

Additional Characteristics of Epithelial Tissue

• Avascular: No blood vessels; nourished by diffusion.
• Frequently mitotically active: Capable of dividing quickly.

Morphological Classification of Epithelial Tissues

• Classification based on the number of cell layers and shape of surface cells.
  • Simple: One cell layer thick.
  • Stratified: Two or more cell layers thick.
• Shape of Surface Cells:
  • Squamous: Flat cells
  • Cuboidal: Cube-shaped cells
  • Columnar: Tall, column-shaped cells

Epithelia: Histological Identification

• Simple squamous: Single layer of flat cells; lines blood vessels, organs.
• Simple cuboidal: Single layer of cube-shaped cells; lines kidney tubules, liver cells, etc.
• Simple columnar: Single layer of tall, thin cells; lines intestine, etc.
• Stratified squamous: Multiple layers of cells; outer layers flat; covers skin, gums.

Simple Squamous Epithelium

• Single layer of flat cells, very thin.
• Lines blood vessels (endothelium), closed body cavities (mesothelium), alveoli in the lungs.
• Function: Controls diffusion, osmosis, and filtration.

Mesothelium and Endothelium

• Mesothelium: Lines closed body cavities.
• Endothelium: Lines blood vessels

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells.
• Lines tubules and ducts

Simple Columnar Epithelium

• Single layer of tall, thin columnar cells.
• Lines stomach, intestine; Often has microvilli (finger-like projections) for absorption.

Pseudostratified Columnar Epithelium

• Appears layered but all cells contact basement membrane, not all reach free surface.
• Nuclei at varying depths.

Stratified Squamous Epithelium

• Several cell layers thick
• Two types:
  • Keratinized: Outer layers of dead cells filled with keratin; skin
  • Nonkeratinized: Moist, living cells at apical surface; cornea

Stratified Cuboidal Epithelium

• Surface cells are cuboidal
• Lines sweat gland ducts

Stratified Columnar Epithelium

• Surface cells are columnar;
• Lines large ducts of glands

Transitional Epithelium

• Multilayered; surface cells vary in shape, round to flat when stretched.
• Lines hollow organs of urinary tract.

Glandular Epithelia

• Secretion is the primary function.
• Secretion is a cyclical process with 4 phases:
  • Diffusion of metabolites into cells
  • Proper synthesis
  • Release of substances from cells
  • Cell restoration

Mechanisms of Glandular Secretion

• Merocrine: Secretion released via exocytosis; cells remain intact
• Apocrine: Secretion released with a portion of the cytoplasm; cells remain intact.
• Holocrine: Entire cell disintegrates to release secretions.

Gland Parts

• Parenchyma: Secretory cells
• Stroma: Connective tissue framework

Gland Types

• Exocrine: Secretes through ducts onto a surface or into a cavity
  • Unicellular (e.g., goblet cells)
  • Multicellular (e.g., salivary, sweat)
    • Classified by secretion type: mucous, serous, mixed
    • Classified by structure: tubular, alveolar (acinar), tubuloalveolar
• Endocrine: Secretes hormones into blood.

Functions of Epithelial Tissue

• Transport across the epithelium
• Protection of underlying tissues
• Secretion and excretion of various substances
• Communication (e.g. hormones, nerve signaling, etc)
• Absorption / passage of substances across.