Epithelial Tissue Overview

Epithelial Tissue

• A sheet of contiguous cells lining internal and external body surfaces.
• Derived from all three embryonic germ layers (endoderm, mesoderm, ectoderm).
• Functions include protection, transcellular transport, secretion, absorption, sensation, and gamete maturation.
• Classified as surface epithelium, glandular epithelium, and neuroepithelium. Also called tunica mucosa.

Cellular Morphology

• Key parts of epithelial cells include apex (apical), supra, peri, para, width, height, lateral, infra, base (basal), and basolateral.
• Diagram provided illustrates these structural components.

Basic Characteristics

• Epithelial cells are tightly bound by junctional complexes.
• Lack of significant extracellular space.
• Possess a basement membrane (basal lamina and lamina reticularis).
• Supported by underlying connective tissue.
• Avascular tissue.

Morphological Classification of Epithelial Tissue

• Epithelial tissue is classified as simple or stratified. -Simple epithelium includes squamous, cuboidal, columnar, and pseudostratified columnar. -Stratified epithelium includes non-keratinized stratified squamous, keratinized stratified squamous, cuboidal or columnar stratified, and transitional.
• Specific types, like squamous epithelium, are further categorized (e.g., endothelium and mesothelium).

1(a) Squamous Epithelium

• Flattened, low-profile polygonal cells.
• Nucleus is centrally located and bulging in apical view.
• Protruding nucleus to apical surface, sometimes anuclear in lateral view.
• Facilitates passive diffusion.
• Found in the endothelium (blood vessels), mesothelium (body cavities), loop of Henle, pulmonary alveoli, and tunica intima of blood vessels.
  • Images of Loop of Henle and Venules/Pneumocytes are provided.

1(b) Cuboidal Epithelium

• Square-shaped cells, with width and height approximately equivalent.
• Centrally located nucleus in the cytoplasm.
• Modified with microvilli; present in tubules, ducts, and glands.
• Some cells resemble pyramids within glands.
• Crucial in secretion, absorption, and protection.
• Found in the proximal and distal convoluted tubules, thyroid follicle, ovary, and bile canaliculi.
  • Images of Thyroid Follicles and Distal Convoluted Tubules are provided.

1(c) Columnar Epithelium

• Rectangular cells with greater height than width.
• Oval nucleus situated basally.
• Apical modifications may include microvilli, stereocilia, and cilia.
• Important for transport, secretion, absorption, and protection.
• Located in the digestive tract, gallbladder, uterine tubes, and bronchioles.
  • Images of Duodenal Mucosa and Uterine Mucosa are provided.

1(d) Pseudostratified Columnar Epithelium

• Single layer of columnar cells with nuclei arranged in multiple levels.
• All cells contact the basement membrane.
• Some cells (basal reserve cells) don't reach the apical surface.
• Modifications like cilia and stereocilia may be present.
• Located in the trachea and bronchi, and epididymis.
  • Images of Trachea and Epididymis are provided.

2(a) Non-Keratinized, Stratified Squamous Epithelium

• Only the basal layer contacts the basement membrane.
• Stratum basale consists of cuboidal cells that actively proliferate.
• Deeper layers with increasing keratinization .
• Moistened by glandular secretions from underlying tissue (submucosa).
• Influenced by species and diet.
• Found in oral cavity, esophagus, pharynx, and vagina.
  • Images of Esophagus and Vagina are provided.

2(b) Keratinized, Stratified Squamous Epithelium

• Consists of multiple strata (layers: basal, spinous, granulosum, lucidum, corneum).
• Stratum basale cells are cuboidal/columnar, responsible for cell proliferation.
• Stratum spinosum has polygonal cells, with prominent intracytoplasmic tonofibrils.
• Stratum granulosum shows flattened cells, producing keratin.
• Stratum lucidum is a clear layer, present in thick skin.
• Stratum corneum is a layer of dead, keratinized cells.
• Found in skin and paws.
  • Images of thin and thick skin are provided.

2(c) Stratified Cuboidal or Columnar Epithelium

• Fewer locations in the animal body.
• Only the deep layer contacts the basement membrane.
• Restricted to 2-3 layers.
• Composed of cuboidal or columnar cells depending on the type.
• Found in papillary ducts, sweat glands, and major salivary glands (parotid and mandibular).
  • Images of Papillary Duct and Salivary Duct of Parotid Glands are provided

2(d) Transitional Epithelium

• Specialized stratified epithelium in the urinary passage (urothelium).
• Basal layer is cuboidal or columnar followed by polygonal cells.
• Superficial layer comprises rounded cells with tight junctions.
• Adaptable to pressure, distension, and constant changes.
• Located in the renal pelvis, ureter, bladder, and urethra. - Images of Ureter and Urinary Bladder are provided.

Glandular Epithelium

• Invagination of epithelial tissue into deeper connective tissue.
• Involved in synthesis and secretion of specific substances.
• Materials stored in secretory granules.
• Composed of parenchyma (secretory unit) and stroma(surrounding connective tissue).
• Two major types: exocrine and endocrine.

Histogenesis of Glandular Epithelium

• Four stages of gland formation (Primitive Epithelial Ingrowth, Ductal Elongation, Ductal Branching, Acinar Proliferation) are illustrated.

  -Acinar Differentiation and Acinar Survival are additional aspects of gland formation.

1. Exocrine Glands (Types)

• Unicellular:

  • Goblet cells are the simplest type.
  • Secrete mucus for protection and lubrication in digestive and respiratory tracts.
  • Two distinct regions (stem and theca).

• Multicellular:

  • Composed of clusters (acinus) forming a secretory unit.

  • Collagenous capsule surrounds the secretory organ.

  • Septa divide the lobes/lobules.

  • Myoepithelial cells aid in secretion.

  • Examples include sweat, salivary, and mammary glands.

    • Images of Sweat Glands and Mammary Glands are provided.

  • Types of Secretion (Serous, Mucous, Seromucous):

  • Serous: Protein-rich watery secretion, spherical and dense granules, apical microvilli, basal infoldings, are examples of parotid, lacrimal gland, and pancreatic exocrine glands (see images).

  • Mucous: Mucin-rich, foamy, pyramidal cells, Well-defined plasma membrane around lumen, flattened nuclei, present in esophageal glands and pyloric gland (see images).

  • Seromucous: A combination of serous and mucous (e.g., mixed glands) producing both types of secretion (example, parotid glands).

    • images of parotid, esophageal, pyloric glands.

Exocrine Gland Classification

• Images illustrate simple (tubular, coiled, branched tubular, branched acinar) and compound (tubular, acinar, tubuloacinar) gland structures.

2. Endocrine Glands

• Ductless glands.
• Release hormones into blood vessels.
• Often arranged in cords or follicles.
• Different classifications including autocrine, paracrine, and endocrine (see images).

Mode of Secretion

• Merocrine: Secretion via exocytosis, cells remain intact. (e.g, salivary glands)
• Apocrine: Apical cytoplasm is shed with secretion. (e.g, mammary glands)
• Holocrine: Entire cells disintegrate and become the secretion. (e.g, sebaceous glands)(Images illustrate Merocrine, Apocrine, and Holocrine.)