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