Tissues and Histology Overview

Tissues

• Tissues are groups of cells working together for a specific purpose.
• Histology studies tissues microscopically.
• Pathologists examine tissues for disease indicators.

Types of Tissues

• Four basic tissue types: epithelial, connective, muscular, and nervous.
• Epithelial tissue covers surfaces, lines organs/cavities/ducts, and forms glands.
• Connective tissue protects and supports, binds organs, stores energy, and provides immunity.
• Muscular tissue contracts for movement and heat generation.
• Nervous tissue detects changes and responds with nerve impulses.

Epithelial Tissue

• Types: Surface epithelium (covers) and glandular epithelium (secreting glands)
• Surface epithelium is tightly packed with little intercellular substance.
  • Arranged in single or multiple layers.
  • Avascular (no blood vessels), but has nerves.
  • Regenerative, attached to connective tissue via a basement membrane.
  • Simple epithelium: single layer, functions in diffusion, osmosis, secretion, and absorption.
  • Stratified epithelium: multiple layers, protective in high wear areas.
  • Pseudostratified epithelium: single layer that appears stratified due to nuclei alignment, all cells attach to the basement membrane.
• Cell shapes: squamous (flat), cuboidal (cube-shaped), columnar (rectangular), and transitional (variable).
  • Columnar cells have cilia or microvilli.
  • Transitional cells change shape with organ relaxation/stretching.
• Glandular epithelium: found in glands (skin, digestive, endocrine).
  • Exocrine glands: release products via ducts (e.g., salivary, sweat).
  • Endocrine glands: ductless, release hormones into the bloodstream (e.g., thyroid, pituitary).

Classification of Epithelium

• Based on layers and cell shape.
• Simple Epithelium:
  • Squamous: lining cardiovascular/lymphoid systems, serous membranes, alveoli. Functions in filtration, diffusion, secretion.
    • Endothelium: lines heart, blood vessels, lymphatic vessels.
    • Mesothelium: found in serous membranes.
  • Cuboidal: covers ovaries, lines the lens, forms retinal epithelium.
  • Columnar: one layer of rectangular cells.
    • Nonciliated: lacks cilia, contains microvilli.
    • Ciliated: contains cilia.
  • Pseudostratified columnar: may be ciliated or nonciliated.
• Stratified Epithelium:
  • Squamous: multiple layers, flat top layer.
    • Nonkeratinized: lacks keratin.
    • Keratinized: contains keratin.
  • Cuboidal: multiple layers, cube-shaped top layer.
  • Columnar: multiple layers, rectangular top layer.
  • Urothelium (transitional): cells change shape based on organ form.

Functions of Epithelium

• Protection, secretion, absorption, and excretion.