Untitled Quiz

Epithelial Tissue

• Epithelial tissue is one of the four basic tissues in the body, the others being connective, muscular, and nervous tissues.
• It forms a sheet of aggregated, similar cells.
• It covers body surfaces, lines hollow organs, and forms glands.
• It originates from one of the three embryonic germ layers (ectoderm, mesoderm, and endoderm).
• It is composed of many similar cells close together, with little intercellular substance.
• Cells adhere to one another via specialized cell junctions.
• All epithelia have an apical surface, which faces the body surface or the lumen of a hollow organ.
• Epithelia are separated from underlying connective tissue by a basement membrane.
• Epithelial tissue is avascular; blood and lymph vessels do not penetrate it.
• Nutrients are delivered via diffusion.
• While not highly vascular, it is highly innervated.
• Epithelial cells have a high capacity for regeneration.
• Epithelial tissue can change from one form to another in a process called metaplasia.
• Functions include protection, secretion, excretion, absorption, sensory reception, and contraction.

Classification

• Epithelial tissue is classified into four types: epithelial membranes, glandular epithelium, myoepithelium, and neuroepithelium.
• Epithelial membranes are further classified based on cell layers (Simple vs. Stratified) and cell shape (Squamous, Cuboidal, Columnar).
  • Simple Epithelium: a single layer of cells.
  • Stratified Epithelium: two or more layers of cells.
• Shapes of epithelial cells include:
  • Squamous: scale-like.
  • Cuboidal: cube-like
  • Columnar: pillar-like.

Simple Epithelium

• Simple Squamous Epithelium

  • One layer of thin, flat, scale-like cells.
  • Cells have an irregular shape with a slightly serrated border.
  • Nuclei are centrally located (spherical or oval).
  • Found in lung alveoli, glomerular capsule, linings of blood and lymph vessels (endothelium), serous membranes (mesothelium), subarachnoid and subdural spaces (mesenchymal epithelium), anterior chamber of the eye, and perilymphatic spaces of the ear.

• Simple Cuboidal Epithelium

  • One layer of cube-like cells (width and height are nearly equal).
  • Cells are squares in cross-section with centrally located spherical nuclei.
  • Located in glands and organs with secretory or absorptive functions: e.g. thyroid follicles, glands' ducts, kidney tubules.

• Simple Columnar Epithelium

  • One layer of tall, narrow cells with height greater than width.
  • Nuclei are oval and located near the base of the cells.
  • Found in stomach lining, oviducts, bronchioles, small intestine, and gall bladder (absorptive function).

• Pseudostratified Columnar Epithelium

  • Composed of single layer of irregular cells.
  • All cells rest on the basement membrane, but not all reach the luminal surface.
  • Nuclei located at different levels, creating a false impression of stratification.
  • Cells reaching the surface are either ciliated or goblet cells, with short cells acting as progenitors for tall cells.
  • Located in trachea, auditory tube, male urethra, and epididymis.

Stratified Epithelium

• Stratified Squamous Epithelium

  • Several layers of cells; with only superficial cells having squamous shape.
  • Types: non-keratinized and keratinized.
    • Non-keratinized: surface cells are living with nuclei, and kept moist by local glandular secretions. Found in oral cavity, pharynx, esophagus, anal canal, and vagina.
    • Keratinized: surface cells are dead and lose their nuclei. They contain keratin, a water-resistant protein forming a non-living covering. Found in skin, epidermis of thick skin (palm and sole), hooves, horns, and feathers.

• Stratified Cuboidal Epithelium

  • Two or more layers of cells, with superficial cells being cuboidal, usually seen as two-layered epithelium.
  • Found in large ducts of sweat and salivary glands.

• Stratified Columnar Epithelium

  • Several layers of cells, with only superficial cells being columnar.
  • Found in distal portion of the urethra, ducts of parotid and mandibular glands, lacrimal sac, and parts of the lacrimal duct.

• Transitional Epithelium

  • A stratified epithelium found only in the urinary tract (lines the ureter and urinary bladder = urothelium).
  • Highly specialized to stretch and withstand urine toxicity.
  • 4 to 6 cells thick.
  • Differences (compared to other stratified squamous epithelium): Cells at the surface are not squamous; deepest cells are columnar or cuboidal; middle layers are made of polyhedral or pear-shaped cells; surface cells are large and often shaped like an umbrella.

Glandular Epithelium

• Classification of Glandular Epithelium: based on the presence or absence of ducts (exocrine vs. endocrine) and the microscopic structure of the glands (e.g. tubules or alveoli).

  • Exocrine: have duct systems that convey secretions to sites of utilization (eg. salivary glands).
  • Endocrine: secrete products into the blood stream with no duct system (eg. thyroid glands).
  • Paracrine: secrete products into the local extracellular space, affecting surrounding cells (eg.enteroendocrine cells of GIT)
  • Unicellular: a single secretory cell (e.g. goblet cell)
  • Multicellular: clusters of cells (e.g. salivary glands and pancreas)
  • Types Based Morphology: Simple tubular, Simple acinar/alveolar, Simple tubuloalveolar, Simple branched tubular, Simple branched alveolar, Simple branched tubuloalveolar, Compound tubular, Compound alveolar, Compound tubuloalveolar

• Types of Glands Based on Secretion:

  • Merocrine: cells remain intact during secretion
  • Apocrine: apical parts of the cells are lost during secretion
  • Holocrine: products are secreted with whole cell destruction secretion with the whole cell disintegrating in glands such as sebaceous glands

• Serous glands: Produce thin, watery secretions.

  • Pyramidal shaped secretion cells.
  • Nuclei are spherical and centrally located
  • Cytoplasm has two zones: basophilic (rER) and eosinophilic (zymogen granules.

• Mixed glands: Such as seromucous.

  • Primarily mucous secretory units with crescent-shaped clusters of serous cells (serous demilunes) located on the periphery of the mucous units.
  • Serous secretion reaches the lumen through intercellular canaliculi located between the mucous cells.

Myoepithelial Cells

• Myoepithelial cells are specialized flattened cells with cytoplasmic processes.
• Lie between secretory cells and the basement membrane.
• Processes form a basket-like network (basket cells) around the secretory units of certain glands.
• Cytoplasm is rich in actin and myosin filaments.
• Contraction forces secretory products into the duct system.
• Found in salivary, sweat, and mammary glands

Neuroepithelium

• A special type of epithelium designed to receive sensory stimuli.
• Examples: taste buds and olfactory epithelium.
• Related to digestive and respiratory systems.