Epithelial Tissue and Glands

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.

Medical Applications of Epithelial Tissue

• Many cancers arise from epithelial tissue (e.g., carcinoma).
• Adenocarcinomas are cancers derived from glandular epithelial tissues.
• The distribution proportion of tumors based on tissue types changes with age.