Overview
This lecture covers the tissue level of structural organization, focusing on epithelial tissues: their types, functions, characteristics, classification, and examples of where they are found in the body.
Levels of Structural Organization & Introduction to Tissues
- Cells work together to form tissues; histology is the study of tissues.
- There are four basic tissue types: epithelial, connective, muscle, and nervous.
- Epithelial tissue covers surfaces, lines internal passages, and forms glands.
Functions of Epithelial Tissue
- Provides physical protection against abrasion, dehydration, and chemicals.
- Controls permeability, allowing selective movement of substances.
- Provides sensations (touch, temperature, pain).
- Produces specialized secretions through gland cells (e.g., mucus, sweat, enzymes).
General Characteristics of Epithelial Tissue
- Shows polarity: apical (top/exposed) and basal (bottom/attached) surfaces.
- Always attached to a basement membrane (basal lamina and reticular lamina).
- Avascular: lacks blood vessels; depends on diffusion for nutrients.
- Capable of rapid regeneration due to frequent exposure to damage.
- May have specializations: microvilli (increase surface area for absorption), cilia (move fluids/mucus in one direction).
Intercellular Junctions in Epithelial Tissue
- Gap junctions allow passage of ions and small molecules between cells for coordination.
- Tight junctions create a seal to prevent leakage between cells.
- Basement membrane glues epithelial cells to underlying tissue.
Classification of Epithelial Tissue
- Classified by cell shape (squamous—flat, cuboidal—cube-shaped, columnar—tall/rectangular) and number of layers (simple—one layer, stratified—multiple layers).
- Simple epithelia facilitate diffusion or absorption; stratified epithelia provide protection.
Types and Locations of Epithelial Tissue
- Simple squamous: One flat layer; found in alveoli (lungs), endothelium (vessels), mesothelium (body cavities).
- Stratified squamous: Multiple layers; found in skin (keratinized), mouth, esophagus, vagina (non-keratinized).
- Simple cuboidal: One layer of cube-like cells; lines kidney tubules, thyroid gland.
- Stratified cuboidal: Rare, multiple layers; found in ducts of sweat and mammary glands.
- Transitional epithelium: Changes shape; found in bladder, ureters, renal pelvis.
- Simple columnar: One layer of tall cells; lines stomach, intestines.
- Pseudostratified columnar: Appears layered but is single layer; found in trachea, bronchi, male reproductive tract; often ciliated with goblet cells (mucus secretion).
Glandular Epithelium
- Endocrine glands: Ductless, secrete hormones directly into blood (e.g., thyroid, pituitary).
- Exocrine glands: Have ducts, secrete onto surfaces or into ducts (e.g., sweat, tears, milk).
- Pancreas has both endocrine (insulin, glucagon) and exocrine (digestive enzymes via duct) functions.
Key Terms & Definitions
- Histology — study of tissues.
- Polarity — structural difference between cell surfaces (apical/basal).
- Avascular — lacking blood vessels.
- Microvilli — small projections for absorption/secretion.
- Cilia — hair-like structures for movement of fluids.
- Gap junctions — channels allowing molecules/ion passage between cells.
- Tight junctions — seals preventing leakage between cells.
- Basement membrane — layer anchoring epithelial tissue.
- Keratinization — process providing water resistance in skin cells.
- Transitional epithelium — tissue that can stretch and change shape.
- Goblet cells — cells that secrete mucus.
Action Items / Next Steps
- Review histology images to identify epithelial tissue types in lab.
- Prepare for lab practical and potential exam questions on tissue identification.
- Continue with part 2, focusing on connective tissue.