Biology: Tissues and Cell Junctions

Epithelial Tissues

• Non-ciliated simple columnar epithelium:
  • Exhibit elongated nuclei
  • Found on highly absorptive surfaces (e.g., lining of the gastrointestinal tract)
  • Microvilli increase surface area for absorption
  • Goblet cells secrete mucus
• Ciliated simple columnar epithelium:
  • Cilia are much larger than microvilli (up to 300 per cell)
  • Cilia beat to propel fluid or material over the epithelial surface
  • Found in the lining of the fallopian tubes in the female reproductive system
• Pseudostratified columnar epithelium:
  • Appears to have several layers, but all cells contact the basement membrane
  • Nuclei are at different levels (not columnar)
  • Ciliated variety is almost exclusive to the lining of the upper respiratory tract
• Stratified cuboidal epithelium:
  • Thin, usually only 2-3 cells thick
  • Found in ducts of sweat glands
  • Provides protection to other tissues
  • Stratified columnar epithelium is very rare
• Stratified squamous epithelium:
  • Keratinized variety forms the superficial layer of the skin (epidermis)
  • Non-keratinized variety lines wet surfaces (e.g., mouth, esophagus, and vagina)
  • Protects against abrasion, water loss, UV, and defense against microbes
• Transitional epithelium:
  • Variable appearance
  • Almost exclusively lines the urinary system (e.g., bladder, ureters, and urethra)
  • Allows urinary organs to stretch and maintain a protective lining

Epithelia vs. Connective Tissues

• Surfaces of epithelial cells and basement membrane:
  • Apical surfaces may contain cilia or microvilli
  • Lateral surfaces may contain junctions
  • Basal surface binds to the basement membrane
• Function of epithelia:
  • Form selective barriers
  • Provide secretory surfaces
  • Provide protective surfaces

Classification of Epithelial Tissues

Classified by Shape of Surface Cells

• Squamous = flat
• Cuboidal = cube-shaped
• Columnar = tall column
• Transitional = shape varies with tissue stretching (changes shape)

Classified by Arrangement of Cells into Layers

• Simple: One layer, all cells in contact with basement membrane
• Pseudostratified: Appears to have layers, but all cells contact the basement membrane
• Stratified: Two or more layers, only basal layer in contact with basement membrane

Simple Squamous Epithelium

• Found lining surfaces involved in passive transport (diffusion)
• Examples: gases (lungs), liquids (walls of blood capillaries), epithelial layer of serous membranes (mesothelium), and lining of pleural, pericardial, and peritoneal cavities (visceral/parietal layers)

Simple Cuboidal Epithelium

• Functions in secretion and absorption
• Usually lines small ducts and tubules
• Examples: covers the surface of the ovary, lines kidney, salivary, and pancreatic tubules

Basement Membrane

• Consists of two extracellular layers:
  • Basal lamina: contains laminin and collagen
  • Reticular lamina: contains fibroblasts that secrete collagen

Connective Tissues

• Dense irregular connective tissue:
  • Consists predominantly of fibroblasts and freely branching elastic fibers
  • Allows stretching of certain tissues like the elastic arteries (e.g., aorta)
• Cartilage:
  • Tissue with poor blood supply that grows slowly
  • Hyaline (glass-like) cartilage is the most abundant type
  • Covers the ends of long bones and parts of the ribs, nose, trachea, bronchi, and larynx
  • Provides a smooth surface for joint movement
• Fibrocartilage:
  • Consists of chondrocytes located in a threadlike network of elastic fibers
  • Makes up the malleable part of the external ear and the epiglottis
• Elastic cartilage:
  • With thick bundles of collagen fibers, is a very strong, tough cartilage
  • Supports the huge loads up and down the long axis of the body
  • Makes up the intervertebral spaces and the knee joints

Histology

• Histology is the science that deals with the study of tissues
• Pathologists specialize in laboratory studies of cells and tissues for diagnoses

Cell Junctions

• Contact points between membranes of tissue cells
• Five main types: tight junctions, adherens, desmosomes, hemidesmosomes, and gap junctions

Adherens Junctions

• Contain plaque protein which anchors membrane proteins to cytoskeleton
• Cadherins: transmembrane glycoproteins join adjacent cells
• Form adhesion belts around cells
• Helps resist cells separating under mechanical stress (e.g., during peristalsis)

Tight Junctions

• Web-like strands of transmembrane proteins
• Fuse together at the outer surfaces of cells
• Seal passageways between cells
• Found in the stomach, intestines, and bladder
• Prevent leakage and stop substances from passing between cells

Desmosomes and Hemidesmosomes

Desmosomes

• Like adherens, contain plaque protein and Cadherins
• Plaque attaches to intermediate filaments in cells (Keratin)
• Intermediate span cells, providing stability

Hemidesmosomes

• Do not link adjacent cells
• Contain Integrin transmembrane glycoproteins (not cadherins) which attach to intermediate filaments inside cells
• Anchor to the basement membrane by binding to Laminin

Four Basic Tissue Types

• Epithelial:
  • Covers body surfaces and lines hollow organs, body cavities, ducts, and forms glands
• Connective:
  • Protects, supports, and binds organs
  • Stores energy as fat and provides immunity
• Muscular:
  • Generates the physical force needed to make body structures move and generate body heat
• Nervous:
  • Detects changes in the body and responds by generating nerve impulses

Cytoskeleton

• Network of protein filaments throughout the cytosol
• Provides structural support for the cell
• Three types:
  • Actin/Myosin: generates movement and mechanical support
  • Intermediate filaments: stabilize organelle position and attach cells together
  • Microtubules: hollow tubes - TUBULIN, determines cell shape, and movement of organelles and vesicles

Learn about the definition and types of tissues, as well as the different types of cell junctions in this biology quiz. Understand the importance of histology in diagnosing diseases.