Epithelial Tissue and Glands

Study Notes

Epithelial Tissue and Glands

The human body is composed of four basic tissue types: nervous, epithelial, muscle, and connective.
Tissues are groups of cells with similar structures that work together to perform specific functions.
Epithelial tissues are present in two forms: as sheets of continuous cells (epithelia) and as glands.
Epithelia cover the external surface and line internal body surfaces.
Glands originate from invaginated epithelial cells and are derived from all three embryonic germ layers, specializing in secretion.
Epithelial tissues have characteristic functions like detection of sensations, protection, absorption, transcellular transport, and secretion.
Epithelial cells demonstrate polarity with a basal pole, apical pole, and lateral surfaces.
Epithelial cells are classified according to their shape and number of layers into simple and stratified epithelia (squamous, cuboidal, columnar, transitional).
Stratified epithelia are classified based on the superficial layer(s) of cells.
Stratified squamous epithelia are classified as keratinized or non-keratinized, depending on the presence of keratin.
Epithelial glands are classified based on the structure (duct or ductless) and mode of secretion (merocrine, holocrine, apocrine).
Merocrine secretion is the most common mode.
Merocrine secretion involves the release of secretory products via exocytosis.
Holocrine secretion involves the disintegration of the entire cell to release the secretion.
Apocrine secretion involves the release of secretory product with a pinched off portion of the apical cytoplasm.
Serous cells are pyramid-shaped, eosinophilic containing digestive enzymes.
Mucous cells contain more flattened basal nuclei and apical secretory granules containing glycoproteins (mucin).
Mixed seromucous glands have both mucous and serous secretory units.

Classification of Epithelial Tissues

Simple epithelia are composed of one layer of cells.
Simple squamous epithelium is a single layer of flattened cells found in vessels, serous cavities, and covering the ovary.
Simple cuboidal epithelium is a single layer of cube-shaped cells lining the kidney tubules and covering the ovary.
Simple columnar epithelium is a single layer of tall, columnar cells found in the digestive tract.
Simple columnar can be absorptive, secretory, or ciliated.
Stratified epithelia are composed of more than one layer of cells.

Covering and Lining Epithelia

Covering epithelia are further classified based on the shapes of cells (squamous, cuboidal, columnar, and transitional).
Squamous cells are thin, flattened cells.
Cuboidal cells have roughly the same height and width.
Columnar cells are taller than they are wide.
Transitional cells can change shape to accommodate distension.

Stratified Epithelia

Stratified squamous epithelium is the most widespread of the stratified tissues, forming the epidermis.
It is keratinized (dry) as in the skin or non-keratinized (moist) as in the lining of the esophagus and vagina.
Stratified cuboidal epithelium is rare and is found in the larger ducts of sweat and salivary glands
Stratified columnar epithelium is very rare and is found in some parts of the male urethra.
Transitional epithelium, also called urothelium, lines the urinary bladder, ureters, and superior part of the urethra.

Polarity and Cell-Surface Specializations

Epithelial cells exhibit polarity, meaning they have distinct domains (basal, apical, and lateral).
Basal surfaces are anchored to basal lamina, which is a specialized extracellular matrix.
Apical surfaces often have specialized structures like microvilli, stereocilia, and cilia for absorption, secretion, and movement.

Intercellular Adhesion & Other Junctions

Several intercellular junctions tightly connect neighboring epithelial cells.
Tight junctions seal the space between cells, preventing the passage of materials.
Adhesive junctions connect cells strongly using cadherins.
Desmosomes act as strong anchoring junctions by binding intermediate filaments.
Gap junctions allow rapid communication and passage of molecules between cells.

Specializations of the Apical Cell Surface

Microvilli are small, finger-like projections that increase the surface area for absorption (common in small intestine).
Stereocilia are similar to microvilli but longer, less mobile, and branched typically found in the epididymis and ductus deferens.
Cilia are long, hair-like projections that move in coordinated waves to propel materials along the epithelial surface.

Transport Across Epithelia

Epithelial cells actively transport molecules and ions to maintain homeostasis across the cell membrane.
Active transport requires energy (e.g., Na+/K+ pumps).
Transcellular transport involves moving substances across the cell, while paracellular transport involves moving substances between cells.

Glands

Glands are classified functionally as exocrine and endocrine.
Exocrine glands secrete their products via ducts to the epithelial surfaces.
Endocrine glands secrete their products directly into the bloodstream or lymphatic vessels.