Overview
This lecture covers tissues and tissue systems, explaining their components, classifications, and how understanding tissues is key to building organ systems in anatomy.
Introduction to Tissues
- Tissues are groups of cells with similar structures and/or functions.
- Two or more cells form a tissue.
- Tissues consist of cells and the extracellular matrix (ECM).
Extracellular Matrix (ECM)
- Cells produce ECM, creating their own supportive environment.
- ECM contains mostly water (extracellular fluid), ground substance (complex carbohydrates, proteins, lipids, inorganic ions), and protein fibers.
- Protein fibers in ECM: collagen (strong, flexible, inelastic), elastin (weak, elastic, stretchable), and reticular (strong, inelastic, network-like).
Types of Matrix
- Hard matrix: rich in inorganic components and collagen (e.g., bone).
- Liquid matrix: mostly fluid (e.g., blood plasma).
- Gelatinous matrix: intermediate consistency (e.g., cartilage).
Intercellular Junctions
- Desmosomes (anchoring junctions): provide strength where stress/shear occurs, common in epithelial tissue.
- Tight junctions: form barriers to transport, encircle cells' apical surfaces, block fluid movement between cells.
- Gap junctions (connexons): allow direct cytoplasmic exchange between cells, common in some muscle and nervous tissue.
Four Basic Tissue Types
- Epithelial tissue (ET): covers/lines surfaces, forms glands.
- Connective tissue (CT): supports, binds, and protects.
- Muscle tissue (MT): contracts for movement.
- Nervous tissue (NT): transmits signals.
Epithelial Tissue Characteristics
- Highly cellular with very little ECM.
- Avascular and mostly not innervated.
- Rests on a basement membrane attached to underlying connective tissue.
Classification of Epithelial Tissue
- By layers: simple (one layer), stratified (multiple layers).
- By shape: squamous (flat), cuboidal (cube), columnar (tall), pseudostratified (appears layered), transitional (stretchable; found in urinary bladder).
- By function: protection (cover/line organs), absorption (columnar cells), secretion/excretion (cuboidal cells), diffusion (squamous cells), reception of stimuli (specialized cells).
- Found as membranes (mucous, serous, cutaneous) and glandular epithelium (exocrine glands).
Epithelial Membranes
- Mucous membranes: line body openings, contain goblet cells producing mucus.
- Serous membranes: line closed cavities, produce lubricating serous fluid, include pericardium, pleura, and peritoneum.
- Cutaneous membrane: skin.
Connective Tissue Characteristics
- Few cells, mainly fibroblasts, lots of ECM.
- Highly vascularized (except cartilage).
- Not associated with a basement membrane.
- Functions: binding/support, protection, infection defense, fat storage.
Classification of Connective Tissue
- Typical CT: areolar (loose), dense (regular/irregular), adipose (fat).
- Cartilage: fibrocartilage (dense, shock absorption), elastic cartilage (stretchable), hyaline cartilage (most common, strong, gel-like).
- Specialized CT: bone (hard matrix), blood and lymph (liquid matrices).
- Cartilage is non-vascularized, surrounded by perichondrium; cells include chondrocytes (maintain), chondroblasts (form), and chondroclasts (destroy) matrix.
Key Terms & Definitions
- Tissue — group of similar cells performing a common function.
- Extracellular Matrix (ECM) — material produced by cells, surrounding them, consisting of fluid and fibers.
- Collagen — strong, flexible, inelastic protein fiber.
- Elastin — stretchable, elastic protein fiber.
- Reticular fiber — inelastic, network-forming protein fiber.
- Desmosome — protein-based anchoring cell junction.
- Tight Junction — seal preventing intercellular fluid movement.
- Gap Junction (Connexon) — protein channel connecting cytoplasm of adjacent cells.
- Basement Membrane — ECM layer between epithelium and connective tissue.
- Chondrocyte — cartilage cell.
- Perichondrium — connective tissue covering cartilage.
Action Items / Next Steps
- Review handouts provided on the course website.
- Read textbook sections on tissue types and intercellular junctions.
- Prepare for discussion on muscle and nervous tissue in upcoming lectures.