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BIOL 211 Chapter 4 Notes: Tissues

Nervous tissue

  • Internal communication system of the body: brain, spinal cord, and nerves.

Muscle tissue

  • Contracts to cause movement.
  • Muscles attached to bones → skeletal muscle.
  • Muscles of the heart → cardiac muscle.
  • Muscles of the walls of hollow organs → smooth muscle.

Epithelial tissue

  • Forms boundaries between different environments; protects; secretes; absorbs; filters.
  • Examples: skin surface (epidermis); lining of GI tract organs and other hollow organs.
  • Main functions: Protection; Absorption; Filtration; Excretion; Secretion; Sensory reception.
  • Two main types based on location and function:
    • Covering/lining epithelia: Found on external and internal surfaces of organs; forms protective barrier.
    • Glandular epithelia: Located within glands; synthesizes and secretes a protein or lipid product.

Connective tissue

  • Supports, protects, binds other tissues together.
  • Examples: Bones; Tendons (and ligaments); Fat and other soft padding tissue.

Preparation of tissue samples

  • To view tissue under a microscope, tissue must be:
    • Extracted from body
    • Fixed: cells killed, preserved, and structurally stabilized
    • Sectioned: tissue sliced thin enough to transmit light or electrons
    • Stained: treated with dyes to enhance contrast between subcellular organelles, different cell types, and/or components of the extracellular matrix
    • Common histological stain: hematoxylin & eosin ("H&E")

Epithelial tissue: Characteristics

  • Epithelial tissue is a sheet of cells that covers body surfaces or cavities.
  • Main functions include: Protection; Absorption; Filtration; Excretion; Secretion; Sensory reception.
  • Polarity: Cells have apical and basal surfaces.
    • Apical surface = "upper" or free surface of cell.
    • Most apical surfaces are smooth; some bear microvilli (e.g., brush border of intestinal lining) or cilia (e.g., lining of trachea).
    • Basal surface = "lower" surface of cell.
    • Basal surface of basal layer attaches to the basal lamina.
    • Basal lamina = a distinctive layer of extracellular matrix, composed of glycoproteins and collagen.
    • Basal lamina is produced by epithelial tissue; Reticular lamina is produced by underlying connective tissue.

Epithelial tissue: Additional properties

  • Composed of closely packed cells forming continuous sheets held together by tight junctions and desmosomes.
  • Avascular but innervated: No blood vessels in epithelial tissue; nourished by diffusion of nutrients from blood vessels in underlying connective tissues; nerve fibers penetrate epithelia.
  • High rate of regeneration: Epithelia are constantly exposed to friction and hostile substances, leading to damage; regeneration is stimulated by loss of apical-basal polarity and broken lateral contacts; in stratified epithelia, the basal layer constantly divides to replace damaged or sloughed cells.

Classification of Epithelia

  • Ask two questions to classify: 1) How many layers?
    • 1 layer = simple epithelium
    • More than 1 layer = stratified epithelium
  • If stratified, name the epithelium according to the shape of the apical layer of cells.

Epithelial cell shape (for classification)

  • Shapes of cells in the apical layer (when applicable):
    • Flat (width > height) → Squamous
    • Width ≈ Height → Cuboidal
    • Tall (height > width) → Columnar
  • Note: If the epithelium is stratified, name it according to the shape of the apical layer of cells (not the basal layer).