Epithelial Tissue: Structure and Function

Brief Description

• The primary component of tissue is cells
• There is a minimal quantity of extracellular material
• Cells are positioned on the basal membrane
• Cells establish numerous connections with each other
• Nutrition is facilitated through diffusion

Division by Function

• Covering epithelia provides a protective and lining function
• Ciliated epithelia use cilia to move unwanted particles from the body's external environment
• Secretory epithelia help in the creation of secretory glands
• Transporting epithelia move various chemical molecules across the epithelial layer
• Reproductive epithelia is where gametes are produced

Classification by Cell Layers

• Simple epithelia has one cell layer
• Stratified epithelia has multiple cell layers

Classification by Cell Shape

• Squamous cells are flat
• Cuboidal cells are cube-shaped
• Columnar cells are column-shaped

Simple Squamous Epithelium

• Has flattened, polygonal cells with centrally located nuclei
• Lines surfaces that are passively transported
• Located in parts of the body like: pulmmonary alveoli, blood and lymphatic vessels, pericardium, peritoneum and renal capsules

Simple Cuboidal Epithelium

• Has cells shaped like a cube
• Nuclei are located centrally
• Lines ducts/tubules and have excretory, secretory, or absorptive functions
• Located in parts of the body like: kidneys and salivary glands

Simple Columnar Epithelium

• Has tall, column-shaped cells
• The nucleus can be located at the base, center, or apex of the cell
• Cilia can be present
• Located in the intestine and fallopian tube

Pseudostratified Epithelium

• Has varied cell types with varying heights; not all reach the organ lumen but each rests on the basal lamina
• May contain cilia/stereocilia
• Located in the trachea and epididymal duct

Stratified Squamous Epithelium

• Contains multiple cell layers
• Cells vary in shape depending on the layer: basal layers are polygonal, while apical layers are more flattened
• The keratinized form has a superficial layer comprised of flat, keratinized cells containing keratin and covers the body surface
• The non-keratinized form lacks a horny layer and coats the mouth, throat, esophagus, vagina, anus, and anterior cornea

Stratified Cuboidal Epithelium

• Comprises numerous layers of cube-shaped cells
• Only the surface layer contains cuboidal cells, deeper layers can be other types
• Found in sweat, mammary, circumanal, and salivary glands

Intercellular Junctions

• Tight junctions prevent molecule movement between adjacent cells
• Each of these types forms a continuous, encircling ring known as zona occludens
• Adherens junctions involves molecules of adhesion and cytoskeletal elements
• Communicating junctions establish a channel for tiny molecules to pass directly between cells

Cilia

• Thin cytoplasmic extensions found on epithelial cells' apical surface
• Move rhythmically, propelling fluid/mucus across the epithelial surface

Microvilli

• Small, finger-like projections emerging from the cell membrane's apical part
• Increases the absorptive surface area to absorption
• Predominantly located on intestinal epithelial cells and kidney tubules
• Stereocilia, are a type of microvilli that are longer and wider; found in the epididymal duct epithelium

Basal Membrane

• A specialized extracellular matrix separating the epithelium from connective tissue
• Secures cells together and enables substance exchange
• Consists of three layers:
• Lamina lucida contacts epithelial cells
• Lamina densa provides stabilization
• Lamina reticularis includes fibrillar collagen molecules/fibers, anchoring the epithelial basal membrane to connective tissue

Basal Membrane Composition

• Type IV collagen's monomers join freely, creating a two-dimensional, regularly spaced mesh framework
• Laminins are large glycoproteins adhering to cellular integrins; they enter the matrix produced by type IV collagen
• Nidogen and perlecan, namely a rod-shaped protein and proteoglycan, link laminins, to the type IV collagen matrix

Glands

• Specialized structures involved in secretion
• Can occur in the form of individual cells or distinct organs
• With the exceptions of the medulla and the anterior pituitary, glands are made of epithelial tissue

Extent of Secretion

• Exocrine glands secrete externally
• Endocrine glands secrete internally, like hemocrynous glands
• Paracrine glands release secretions into interstitial fluid, then they permeate into adjacent cells within the same tissue
• Juxtacrine glands secrete secretion directly into adjacent cells with connected cell membranes

Method of Secretion

• Merocrine glands secrete metabolites via exocytosis transporting into the lumen, maintaining cell integrity; examples include liver, salivary glands, pancreas, and sweat glands
• Apocrine glands secrete by detaching the apical portion of the cell, accompanied by cell destruction; mammary glands are one example
• Holocrine glands undergo cell destruction and detachment to release their contents; the sebaceous gland exemplifies
• Endocrine glands secretion occurs via exocytosis through the basal part and into the bloodstream; these glands are pineal, pituitary, thyroid, pancreas, and ovaries

Metaplasia

• Alternatively known as transdifferentiation; utilized in pathomorphology, where tumors are more likely to originate
• Pertains to cells whose morphology and function differ from their original matrix
• Occurs when cells adapt and transform in the presence of irritants, pathogens, or carcinogens