Histology I: Epithelial Tissue Overview

Histology I (Epithelial Tissue)

• Epithelial tissue covers surfaces or lines cavities throughout the body
• Constructed of densely packed cells with minimal intercellular spaces
• Rests on a basement membrane for support
• Exhibits a high regeneration rate
• Displays polarity (apical, lateral, and basal surfaces)
• Lateral and basal surfaces have specialized junctions
• Apical surface may have microvilli, stereocilia, cilia, or flagella
• Avascular; obtains nutrients via diffusion through the basement membrane from underlying connective tissue
• Originates from all three embryonic germ layers: ectoderm, endoderm, and mesoderm

Types of Tissues

• Epithelial Tissue: Composed of aggregated polyhedral cells; small extracellular matrix; lines surfaces and forms glands.
• Connective Tissue: Composed of several fixed and wandering cells; abundant extracellular matrix; supports and protects
• Muscle Tissue: Composed of elongated contractile cells; moderate extracellular matrix; facilitates movement
• Nervous Tissue: Composed of elongated cells with fine processes; very small extracellular matrix; transmits nerve impulses

Function of Epithelium Tissue

• Transports molecules (diffusion, carrier-mediated, and vesicle-mediated)
• Absorbs substances (endocytosis, pinocytosis)
• Secretes various molecules (exocytosis)
• Protects against abrasion and injury.
• Exhibits selective permeability maintaining different compositions on the cell's exterior and interior surfaces (e.g., kidney)

Covering, Lining, and Surface Epithelium

• Simple Epithelium: One layer of cells
• Stratified Epithelium: Multiple layers of cells. This type are grouped by cell shape and include simple squamous, simple cuboidal, simple columnar, pseudostratified columnar, stratified squamous (non-keratinized & keratinized), stratified cuboidal, stratified columnar, and transitional.

Types of Epithelial Cells

• Based on cell shape and number of layers, several types of epithelial cells have unique structures and functions in various areas of the body. Examples include squamous, cuboidal, and columnar cells, stratified and simple cells, as well as transitional cells.

Secretory or Glandular Epithelium

• Exocrine glands: Possess ducts that deliver secretions to body surfaces or cavities.
• Endocrine glands: Lack ducts; secrete hormones directly into the bloodstream.
• Mixed glands: Contain both exocrine and endocrine components (e.g., pancreas)

Classification of Exocrine Glands

• Number of cells: Unicellular (e.g., goblet cells) and multicellular.
• Mechanism of secretion: Merocrine, apocrine, and holocrine.
• Branching of duct: Simple or compound.
• Shape of secretory part: Tubular, alveolar, or tubuloalveolar. Categorization of exocrine glands according to these factors helps understand their specific structures and roles in different body systems.

Apical Epithelial Surfaces

• Microvilli: Tiny, finger-like projections increasing surface area (e.g., intestinal lining, kidney tubules)
• Stereocilia: Especially long and non-motile microvilli (e.g., inner ear, epididymis)
• Cilia and Flagella: Hair-like extensions that beat to move substances (e.g., respiratory tract, female reproductive tract)
• Specialized arrangements of cytoskeletal components enable different types of functional cell movement.
• Cilia have characteristic 9+2 axoneme arrangement providing functionality for moving secretions.

Intercellular Junctions

• Tight Junctions: Close cells together.
• Adherens Junctions: Strengthen tight junctions.
• Desmosomes: Link cytoskeletons, strengthening cells.
• Gap Junctions: Allow cell-to-cell communication.
• Hemidesmosomes: Anchor cells to the basement membrane. Specialized connections mediate cell interactions and maintain tissue integrity.

This quiz covers the fundamental elements of epithelial tissue. Focus on its characteristics, functions, types, and origins from the embryonic germ layers. Test your understanding of how this tissue supports various bodily functions.