Tissues and Tissue Membranes

Four Major Types: Epithelial, Connective, Muscular, Nervous/Neural.
Tissues are collections of cells and cell products performing specific, limited functions.

General: Lines body surfaces and cavities; forms many glands.
Functions: Physical protection, permeability control, sensation, specialized secretions (glandular epithelium).
Characteristics:
- Cellularity: Cells fit closely together via cell junctions.
- Polarity: Exhibits apical (exposed) and basal (attached) surfaces.
- Attachment: Lower surface bound by a basement membrane.
- Avascularity: Lacks blood vessels; nutrients obtained by diffusion and absorption.
- Regeneration: Regenerates easily if well nourished.
Apical Surface Structures:
- Microvilli: Increase absorption or secretion.
- Cilia: Move fluids.
Basement Membrane: Composed of Lamina lucida (secreted by epithelia) and Lamina densa (produced by connective tissue).
Intercellular Connections (Cell Junctions):
- Tight junctions (Occluding junctions): Prevent passage of water/solutes between cells.
- Desmosomes (Anchoring junctions): Anchor cells together, resist mechanical stress.
- Hemidesmosomes: Anchor cells to connective tissue.
- Gap junctions (Communicating junctions): Allow rapid communication and ion passage between cells.
Repair & Replacement: Replaced by division of germinative cells (stem cells) near the basal lamina.

By Cell Layers:
- Simple: 1 layer of cells.
- Stratified: More than 1 layer of cells.
By Cell Shape:
- Squamous: Flattened cells.
- Cuboidal: Cube-shaped cells.
- Columnar: Column-like cells.
Types:
- Simple Squamous: Flat, forms membranes, lines lungs, heart, blood vessels; aids diffusion.
- Simple Cuboidal: Cube-like, common in glands, kidney tubules, covers ovaries.
- Simple Columnar: Tall cells, often with goblet cells (produce mucins/mucus), lines digestive tract.
- Pseudostratified Columnar: Appears stratified but is 1 layer; often ciliated in respiratory tract; functions in absorption/secretion.
- Stratified Squamous: Flat cells at surface, protective, found in skin, mouth, esophagus, anus, vagina.
- Stratified Cuboidal: Rare, found in ducts of large glands (mammary, sweat glands).
- Stratified Columnar: Rare, surface cells columnar (e.g., salivary gland duct).
- Transitional: Cells change shape with stretching; lines urinary system organs.

Gland: One or more cells secreting a product.
Types of Glands:
- Endocrine glands: Ductless; secrete hormones into interstitial fluid/bloodstream.
- Exocrine glands: Secrete products onto epithelial surfaces via ducts (e.g., sweat, tears, oil glands).
Modes of Secretion:
- Apocrine: Secrete products by shedding cytoplasm (e.g., mammary glands).
- Merocrine: Secrete products by exocytosis in vesicles (e.g., pancreas, most sweat/salivary glands).
- Holocrine: Cells fill, rupture, and die to release products (e.g., sebaceous/oil glands).

Functions: Structural framework, fluid transport, organ protection, support/interconnection, energy storage (triglycerides), body defense.
Basic Components:
- Specialized Cells: (See below).
- Ground Substance: Water, adhesion proteins, polysaccharide molecules; fills space, slows pathogens.
- Extracellular Protein Fibers: Collagen, Elastic, and Reticular fibers.
Cells of Connective Tissue:
- Red Blood Cells (Erythrocytes): O2 and CO2 transport.
- White Blood Cells (Leukocytes): Lymphocytes (immune cells), Neutrophils (phagocytic).
- Mast Cells: Stimulate inflammation (release histamine, heparin, leukotrienes, prostaglandins).
- Adipocytes (Fat cells): Store a single, large fat droplet.
- Mesenchymal cells: Stem cells, differentiate into other cell types.
- Melanocytes: Synthesize melanin (brown pigment).
- Macrophages: Large, amoeba-like cells; engulf pathogens and damaged cells (phagocytic).