Epithelium
| Epithelium Type | Distinctive Histological Features | Key Locations / Examples | Functional Purpose |
| Simple Squamous | * Single layer of flat, scale-like cells. * Centrally located, flattened nuclei. * Minimal cytoplasm forming an ultra-thin barrier. | * Alveoli of lungs (Type I pneumocytes) * Endothelium (lining blood/lymph vessels) * Mesothelium (pleura, peritoneum, pericardium) * Bowman’s capsule(kidneys) | Rapid diffusion, filtration, and reduction of friction. |
| Simple Cuboidal | * Single layer of cube-shaped cells. * Large, spherical, centrally located nuclei. * Look like a string of beads in cross-section. | * Renal tubules(kidneys) * Ovary surface(germinal epithelium) * Secretory ducts of small glands (e.g., thyroid follicles, salivary glands) | Secretion and absorption. |
| Simple Columnar | * Single layer of tall, rectangular cells. * Oval nuclei usually lined up near the basal surface. * Can be ciliated or non-ciliated. | * Non-ciliated: Stomach, small intestine, large intestine, gallbladder. * Ciliated: Fallopian tubes (oviducts), small bronchi of lungs. | * Absorption and secretion of mucus (often features goblet cells or microvilli/brush border). * Cilia propel eggs/mucus. |
| Pseudostratified Columnar | * Single layer of cells but appears stratified because nuclei sit at different levels. * All cells touch the basement membrane, but not all reach the apical surface. * Almost always ciliated. | * Trachea and upper respiratory tract(classic respiratory epithelium) * Epididymis and vas deferens (with non-motile stereocilia) | Protection, secretion of mucus, and ciliary sweeping (mucociliary escalator). |
| Epithelium Type | Distinctive Histological Features | Key Locations / Examples | Functional Purpose |
| Stratified Squamous (Keratinized) | * Multiple cell layers. * Superficial apical layers are flat, dead, and packed with tough, water-resistant keratin protein. * Cells lack nuclei at the surface. | * Epidermis of the skin | Severe protection against abrasion, desiccation (drying out), and pathogen entry. |
| Stratified Squamous (Non-keratinized) | * Multiple cell layers. * Superficial apical layers remain flat but are alive and moist. * Surface cells retain their nuclei. | * Oral cavity(buccal mucosa, tongue) * Esophagus(upper to lower portions) * Vagina and ectocervix * Pharynx and anus | Protection against mechanical abrasion in wet environments. |
| Stratified Cuboidal | * Usually just 2 to 3 layers of cube-shaped cells. * Relatively rare in the human body. | * Ducts of sweat glands * Ducts of mammary glands and salivary glands | Structural reinforcement of larger exocrine ducts. |
| Stratified Columnar | * Multiple layers where only the outermost (apical) layer consists of tall columnar cells. * Rare tissue type. | * Large ducts of salivary glands * Conjunctiva of the eye * Portions of the male urethra | Protection and secretion. |
| Epithelium Type | Distinctive Histological Features | Key Locations / Examples | Functional Purpose |
| Transitional (Urothelium) | * Stratified epithelium that changes appearance based on tension. * Relaxed state: Superficial cells look large, rounded, and dome-like ("umbrella cells"). * Stretched state: Flattens out to look squamous-like. | * Urinary bladder * Ureters * Portions of the urethra | Allows distension and stretching without tearing, while protecting underlying tissues from caustic urine. |
Salivary Duct System Transitions
Salivary glands have a highly structured branching system, and the epithelial lining changes continuously from the secretory units to the oral cavity:
- Acinus (Secretory unit): Simple cuboidal epithelium.
- Intercalated ducts: Simple low cuboidal epithelium.
- Striated ducts: Simple columnar epithelium (with basal striations).
- Interlobular / Excretory ducts: Pseudostratified columnar or stratified columnar.
- Main Excretory Duct (e.g., Stensen's/Parotid Duct): Stratified squamous epithelium (non-keratinized) as it approaches the terminal opening to blend seamlessly with the lining of the oral mucosa.