Introduction to Histology & Epithelial Tissue

Introduction to Histology & Epithelial Tissue

• Histology studies the tissues of organisms.
• Tissues are aggregations of similar cells and surrounding materials working together for a shared function.
• Understanding tissues is vital in fields like physical therapy, physician's assistants, cardiac health, and pharmacology.

Tissue Components

• Tissues are composed of cells and an extracellular matrix (ECM).
• The ECM includes fibrous proteins (such as collagen and elastic fibers) and ground substance.

Ground Substance

• Also known as tissue fluid (or extracellular fluid, or interstitial fluid)
• Amorphous gel-like substance, consisting of water, gases, minerals, nutrients, and waste products.
• Cells utilize this fluid for gaining essentials and releasing waste products.
• Acts as a protective barrier and absorbs shocks.

Cellular Junctions

• Tight junctions seal spaces between cells, preventing leakage (ex: digestive juices).
• Desmosomes provide space between cells, while resisting mechanical stress.
• Gap junctions create pores, linking cytoplasm and enabling simultaneous actions (such as muscle contractions).

Four Primary Tissue Categories

• Epithelial tissue covers surfaces and lines cavities.
• Connective tissue connects and supports.
• Nervous tissue communicates.
• Muscular tissue contracts and produces movement.

Epithelial Tissue

• Characteristics include closely adhering cells forming linings, and exocrine glands.
• Their function includes protection, secretion, absorption, filtration, and sensation.
• Lumen is the cavity/open tube of an organ/gland.

Epithelial Tissue - Common Features

• Closely packed cells with many tight junctions and desmosomes.
• Low ECM.
• Polarity (apical/free surface faces the lumen, basal surface connects to the basement membrane).
• Avascular (lacking blood vessels), innervated (containing nerves), and high regeneration rate.

Specialized Epithelial Features

• Based on cell shape: Squamous (flat), cuboidal (cube-shaped), columnar (taller than wide).
• Based on layers: Simple (single layer), stratified (multiple layers), pseudostratified (looks like multiple layers, but all cells contact the basement membrane)

Simple Squamous Epithelium

• Very thin, single layer of cells.
• Important for rapid gas, nutrient diffusion.
• Found in lungs and blood vessels; lines body cavities to reduce friction.
• Specialized examples are endothelium (blood vessels) and mesothelium (body cavities).

Simple Cuboidal Epithelium

• Single layer of cube-shaped cells.
• Thicker barrier than simple squamous, allowing more organelles (for active transport, secretion).
• Found in the kidney tubules and various secretory glands.

Simple Columnar Epithelium

• Single layer of tall cells (large surface area).
• Even thicker barrier than cuboidal, accommodating more organelles (for function).
• Contains microvilli for absorption and goblet cells for mucus secretion.
• Found in lower digestive tract.

Goblet Cells

• Produce mucus which traps particles (airway), protects tissues, and lubricates gut contents to move food efficiently through the GI tract.

Pseudostratified Columnar Epithelium

• Single layer of tall cells that appear layered, but all cells touch the basement membrane.
• Contains goblet cells and cilia, supporting function in respiratory tract.

Stratified Squamous Epithelium

• Multiple layers of flat(ish) cells.
• High turnover, apical cells are flatter than basal cells.
• Keratinized (containing keratin protein for durability) or non-keratinized (for lubrication/protection).
• Keratinized examples include skin epidermis; non-keratinized examples include vagina and oral lining.

Keratinized Stratified Squamous Epithelium

• Characterized by keratin-producing cells.
• Apical cells are dead and packed with keratin, making the tissue a tough and protective barrier
• Example: Epidermis of skin

Non-keratinized stratified squamous epithelium

• Apical layers of cells remain living, lacking keratin.
• Found in wet environments such as the vaginal canals and upper digestive tract

Stratified Cuboidal Epithelium

• Multiple layers of cube-shaped cells, primarily for secretion functions
• Example: sweat glands, ovaries, seminiferous tubules

Urothelium (Transitional Epithelium)

• Layers of cells that change shape (dome to flat) based on bladder filling.
• Protects against osmotic damage from urine
• Containing special uroplakin proteins, these cells create an impermeable barrier to prevent urine leakage
• Found in the urinary bladder and ureters.