Introduction to Epithelial Tissue

Tissue Introduction & Epithelial Tissue

• Human bodies develop from single to multicellular, cells specialize.
• The body is an interdependent system, failure of even one group of cells can be catastrophic.
• Cells become tissues, then organs.
• Tissues are groups of cells with similar structure and function.

Types of Tissues

• Epithelium: Coverings & linings of surfaces
• Connective: Support (bone, ligaments, fat)
• Muscle: Movement
• Nervous: Control (brain, nerves, spinal cord)

Function of Epithelial Tissue

• Protection: Skin protects from sunlight, bacteria, & physical damage.
• Absorption: Lining of small intestine absorbs nutrients into blood.
• Filtration: Lining of kidney tubules filters wastes from blood plasma.
• Secretion: Glands produce perspiration, oil, digestive enzymes & mucus.

Characteristics of Epithelial Tissue

• Forms continuous sheets (like tiles).
• Apical Surface: Top surface of cells bordering an open space (lumen).
• Basement Membrane: Underside of epithelial cells, anchoring them to connective tissue.
• Avascular: Lacks blood vessels, nourished by connective tissue.
• Regenerates quickly.

Classification of Epithelial Tissue

• Cell Shape: Squamous (flattened like fish scales), Cuboidal (cubes), Columnar (columns)
• Cell Layers: Simple (one layer), Stratified (many layers).
• Naming is based on the cell type at the apical surface.

Simple Squamous Epithelium

• Structure: Single layer of flattened cells.
• Function: Absorption & filtration. Less protection due to single cell layer.
• Location: Walls of capillaries, air sacs in lungs, and forms serous membranes in body cavities.

Simple Cuboidal Epithelium

• Structure: Single layer of cube-shaped cells.
• Function: Secretion & transportation in glands, filtration in kidneys.
• Location: Glands and ducts (pancreas & salivary), kidney tubules, covers ovaries.

Simple Columnar Epithelium

• Structure: Elongated layer of cells with nuclei at same level.
• Function: Absorption, protection, secretion. Forms mucous membranes when open to body cavities.
• Special Features: Microvilli (increase surface area & absorption), goblet cells (produce mucus).
• Location: Linings of entire digestive tract.

Pseudostratified Epithelium

• Structure: Irregularly shaped cells with nuclei at different levels—appear stratified, but all cells reach the basement membrane.
• Function: Absorption & secretion. Goblet cells secrete mucus, cilia sweep mucus.
• Location: Respiratory linings & reproductive tract.

Stratified Squamous Epithelium

• Structure: Many layers, typically cuboidal or columnar at bottom with squamous at top.
• Function: Protection. Keratin (protein) hardens & waterproofs skin.
• Location: Skin (keratinized), mouth, & throat.

Transitional Epithelium

• Structure: Many layers, specialized cells that change between stratified & simple forms based on tissue stretching. Cells at base are cuboidal or columnar; surface cells vary.
• Function: Allows stretching (size change).
• Location: Urinary bladder, ureters, & urethra.

Glands

• Single or multiple cells producing & secreting products.
• Secretion: Protein in aqueous solution (e.g., hormones, acids, oils).
  • Endocrine: No ducts, release secretions into blood vessels (e.g., hormones, thyroid, adrenal & pituitary).
  • Exocrine: Contain ducts, empty onto epithelial surface (e.g., sweat, oil, salivary, mammary).

Shapes of Exocrine Glands

• Branching: Simple (unbranched duct) vs. Compound (branched).
• Shape: Tubular (tube-like) vs. Alveolar (flask or sac-like). vs. Tubuloalveolar (both).

Modes of Secretion

• Merocrine: Exocytosis, no gland alteration. (e.g., sweat & salivary glands)
• Holocrine: Gland ruptures, releasing secretion and dead cells. (e.g., sebaceous oil glands)