Understanding Epithelial Tissue

Brief Description of Epithelial Tissue

• Main mass is cells with minimal extracellular substance
• Cells rest on the basal membrane
• Numerous intercellular connections exist
• Nutrition occurs via diffusion

Division of Epithelia by Function

• Covering epithelia provide covering and lining
• Ciliated epithelia involve movement by cilia to propel unnecessary particles from the external environment
• Secretory epithelia contribute to the formation of secretory glands
• Transporting epithelia transport various chemical molecules across the epithelial layer
• Reproductive epithelia originate gametes

Classification of Epithelia

• Based on number of cell layers, epithelia are classified as simple or stratified
• Based on shape of cells, epithelia are classified as squamous, cuboidal, or columnar

Simple Squamous Epithelium

• Flattened, polygonal cells with centrally located nuclei
• Lines surfaces involved in passive transport
• Commonly presents in pulmonary alveoli, blood and lymphatic vessels, pericardium, peritoneum, and renal capsules

Simple Cuboidal Epithelium

• Cells whose shape resembles a cube
• Nuclei are centrally located
• Lines ducts and tubules with excretory, secretory, or absorptive functions
• Commonly presents in kidneys and salivary glands

Simple Columnar Epithelium

• Tall, column-shaped cells
• Nucleus can be located basally, centrally, or apically within the cell
• Cilia may be present on the surface
• Presents in absorptive surfaces, eg. intestine, and in the reproductive system, eg. fallopian tube

Pseudostratified Epithelium

• Few cell types with varying heights, resting on the basal lamina but not all reaching the organ lumen
• Can be equipped with cilia or stereocilia
• Commonly presents in the trachea and epididymal duct

Stratified Squamous Epithelium

• Consists of many layers of cells
• Cells have different shapes depending on the layer
• Basal layers are polygonal, becoming more flattened closer to the apical layer
• Keratinized stratified squamous epithelium includes a superficial layer of flat, keratinized cells containing keratin covering the body surface
• Non-keratinized stratified squamous epithelium lacks the horny layer, covering the oral cavity, throat, esophagus, vagina, anus, and anterior surface of the cornea

Stratified Cuboidal Epithelium

• Composed of multiple layers of cube-shaped cells
• Only the most superficial layer is cuboidal, with other layers potentially being other cell types
• Can be located in sweat glands, mammary glands, circumanal glands, and salivary glands

Intercellular Junctions

Tight Junctions

• Block the passage of molecules between adjacent cells
• Form a continuous, encircling ring around the cell, called a belt, or zona occludens

Adherens Junctions

• Molecules of adhesion and cytoskeletal elements participate in their formation

Communicating Junctions

• Also known as ionic-metabolic or nexus junctions
• These are a channel for the passage of small molecules directly between adjacent cells

Cilia

• Thin cytoplasmic extensions located on the apical surface of epithelial cells
• Move synchronously in a wave-like motion
• Propel mucus or fluid across the epithelial surface

Microvilli

• Small finger-like projections of the apical part of the cell membrane
• Increase the absorptive surface area, enhancing absorption processes
• Mainly found on the surface of intestinal epithelial cells and kidney tubules
• Stereocilia, a variation of microvilli, are wider and longer and are found in the epithelium of the epididymal duct

Basal Membrane

• Specialized extracellular matrix separating the epithelium from the connective tissue
• Attaches cells to each other and participates in substance exchange between them and the surroundings

Basal Membrane Layers

• Lamina lucida contacts epithelial cells
• Lamina densa provides stabilization
• Lamina reticularis is a layer of fibrillar collagen molecules and fibers anchoring the epithelial basal membrane to the connective tissue

Basal Membrane Composition

• Type IV collagen monomers spontaneously aggregate, forming a two-dimensional network of regularly spaced subunits, resembling a mesh structure
• Laminins are large glycoproteins that attach to cell membrane integrins and penetrate the network formed by type IV collagen
• Nidogen and perlecan are small rod-shaped proteins and proteoglycans, respectively
• Both molecules cross-link laminins to the type IV collagen network, maintaining the 3D structure of the basal membrane, attaching the epithelium to it, and determining the permeability and size of molecules

Glands

• Specialized structure involved in secretion
• Can be individual cells or discrete organs
• All glands, with exceptions like the adrenal medulla and the anterior pituitary, are composed of epithelial tissue

Extent of Secretion

• Exocrine glands have external secretion
• Endocrine glands have internal secretion, are ductless, and also knowns as hemocrynous glands
• Paracrine glands secrete their secretion into the interstitial substance, penetrating into the cells within the same tissue
• Juxtacrine glands secrete secretion directly into adjacent cells with which their cell membrane is connected

Method of Secretion

• Merocrine secretion occurs via exocytosis through the apical part into the lumen
• Secretory cells maintain integrity during secretion
• Most glands, eg. the liver, salivary glands, pancreas, and sweat glands, are of this type
• Apocrine secretion involves detachment of the apical part of the cell containing the secretion
• A part of the cell is destroyed, while the remaining part regenerates the defect
• Mammary glands are an example
• Holocrine secretion occurs through the death and detachment of the entire cell containing the secretion
• The sebaceous gland, transforming cells into sebum, is an example
• Endocrine secretion occurs via exocytosis through the basal part directly into the bloodstream
• Pineal gland, pituitary gland, thyroid gland, pancreas, and ovaries are examples

Metaplasia

• Transdifferentiation is a term in pathomorphology; tumors may often develop on this basis
• Refers to the appearance of cells functionally and morphologically different from their matrix
• Occurs when cells change their original, mature type in chronic irritation, pathogen, or carcinogen exposure, as an adaptive response