Tissue Introduction & Epithelial Tissue

Tissue Introduction & Epithelial Tissue

• Human bodies develop from single to multicellular cells, specializing.
• The body is an interconnected system; malfunction in one cell group can be catastrophic.
• Cells specialize into tissues, then into organs.
• Tissues are groups of similar cells in structure and function.

Types of Tissues

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

Function of Epithelial Tissue

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

Characteristics of Epithelial Tissue

• Continuous sheets (like tiles).
• Apical surface: Top surface of epithelial cells, bordering an open space (lumen).
• Basement membrane: Underside of epithelial cells, anchors to connective tissue.
• Avascular: Lacks blood vessels; nourished by connective tissue.
• Regenerates and repairs quickly.

Classification of Epithelial Tissue

• Cell Shape:
  • Squamous: Flattened, like fish scales.
  • Cuboidal: Cube-shaped.
  • Columnar: Column-shaped.
• Cell Layers:
  • Simple: One layer of cells.
  • Stratified: Multiple layers of cells.
  • Named for the cell type at the apical surface.

Simple Squamous Epithelium

• Structure: Single layer of flattened cells.
• Function: Absorption and filtration; not good protection due to single cell layer.
• Location: Walls of capillaries, air sacs in lungs, and serous membranes in body cavities.

Simple Cuboidal Epithelium

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

Simple Columnar Epithelium

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

Pseudostratified Epithelium

• Structure: Irregularly shaped cells with nuclei at different levels; appear stratified but all reach the basement membrane.
• Function: Absorption and secretion; goblet cells produce mucus; cilia (larger than microvilli) sweep mucus.
• Location: Respiratory linings and reproductive tract.

Stratified Squamous Epithelium

• Structure: Multiple layers, usually cuboidal/columnar at the bottom, squamous at the top.
• Function: Protection; keratin (protein) accumulates in older cells, making skin waterproof and tough.
• Location: Skin (keratinized), mouth, and throat.

Transitional Epithelium

• Structure: Multiple layers; cells at the base are cuboidal/columnar, surface varies; changes between stratified and simple as tissue stretches.
• Function: Allows stretching (change in size).
• Location: Urinary bladder, ureters, and urethra.

Glands

• One or more cells making and secreting a product.
• Secretion: Protein in aqueous solution (hormones, acids, oils).
  • Endocrine: No duct, releases secretion into blood vessels (often hormones, like thyroid, adrenal, and pituitary).
  • Exocrine: Contains ducts, empties onto epithelial surface (e.g., sweat, oil, salivary, mammary).

Shapes of Exocrine Glands

• Branching: Simple (single, unbranched duct) and compound (branched duct).
• Shape: Tubular (tube-shaped), alveolar (flask/sac-shaped), and tubuloalveolar (has both tubes and sacs).

Modes of Secretion

• Merocrine: Released by exocytosis, without altering the gland (e.g., sweat and salivary glands).
• Holocrine: Gland ruptures, releasing secretions and dead cells (e.g., sebaceous glands).