Epithelial Tissue Types and Functions

Epithelial Tissue Overview

• Cells form the main mass of this tissue type
• A minimal amount of extracellular substance is present
• Cells rest on the basal membrane
• Cells create numerous intercellular connections
• Nutrition occurs via diffusion

Division of Epithelia by Function

• Covering epithelia covers and lines surfaces
• Ciliated epithelia use cilia movement to propel unnecessary particles entering the body
• Secretory epithelia contributes to secretory gland formation
• Transporting epithelia transports various chemical molecules across the epithelial layer
• Reproductive epithelia is the origin of gametes

Epithelial Classification

• Epithelial tissue is classified based on the number of cell layers and the shape of the cells
• Layer types are simple or stratified
• Shape types are squamous, cuboidal, and columnar

Simple Squamous Epithelium

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

Simple Cuboidal Epithelium

• Cells resemble cubes having nuclei in the central part
• Lines ducts/tubules with excretory, secretory, or absorptive functions
• Found in kidneys, salivary glands, etc.

Simple Columnar Epithelium

• Cells are tall and column-shaped
• The nucleus can be located basally, centrally, or apically
• Cilia may be present on the surface
• Found in absorptive surfaces (intestine) and in the reproductive system (fallopian tube)

Pseudostratified Epithelium

• Consists of few cell types with varying heights resting on the basal lamina
• Not all the cells reach the organ lumen
• May feature cilia or stereocilia
• Found in the trachea and epididymal duct

Stratified Squamous Epithelium

• Comprises many cell layers
• Cell shapes vary by layer; basal layers are polygonal, transitioning to more flattened towards the apical layer
• Keratinized stratified squamous epithelium has 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 cornea

Stratified Cuboidal Epithelium

• Composed of multiple layers of cube-shaped cells
• Only the most superficial layer exhibits cuboidal cells; other layers may consist of other cell types
• Found in sweat, mammary, circumanal, and salivary glands

Intercellular Junctions

• Tight junctions block molecule passage between adjacent cells; each junction forms encircling ring/belt around the cell called zona occludens
• Adherens junctions involve adhesion molecules and cytoskeletal elements in their formation
• Communicating junctions (ionic-metabolic or nexus) provide a channel for small molecules to pass directly between cells

Cilia Features

• Thin cytoplasmic extensions located on the apical surface of epithelial cells
• Moves synchronously in a wave-like motion to propel mucus/fluid across the epithelial surface

Microvilli Features

• Small finger-like projections of the cell membrane's apical part
• Increases absorptive surface area, enhancing absorption
• Mainly located on the surface of intestinal epithelial cells and kidney tubules
• Wider and longer stereocilia a variation, found e.g. in the epithelium of the epididymal duct

Basal Membrane

• A specialized extracellular matrix separating the epithelium & connective tissue
• Attaches cells to each other/participates in substance exchange
• It has three layers:
  • Lamina lucida contacts epithelial cells
  • Lamina densa provides stabilization
  • Lamina reticularis contains fibrillar collagen moelcules and anchors the epithelial basal membrane to connective tissue

Basal Membrane Composition

• Type IV collagen monomers aggregate spontaneously, forming a regularly spaced, two-dimensional mesh network
• Laminins are large glycoproteins that attach to cell membrane integrins and penetrate the type IV collagen network
• Nidogen and perlecan are small rod-shaped protein/proteoglycan molecules that cross-link laminins to the type IV collagen network to maintain the membrane structure, attach the epithelium, and determine molecule permeability

Glands

• Specialized structures involved in secretion, either individual cells or discrete organs
• All glands are composed of epithelial tissue with the exception of the adrenal medulla and anterior pituitary
• Exocrine glands secrete externally
• Endocrine glands secrete internally (ductless/hemocrynous glands)
• Paracrine glands secrete into the interstitial substance from where it penetrates into cells within the same tissue
• Juxtacrine glands secrete directly into adjacent cells with connected cell membranes

Glandular Secretion Methods

• Merocrine glands secrete via exocytosis and secretory cells retain full integrity during secretion
  • Liver, salivary glands, pancreas, and sweat glands are examples
• Apocrine glands secrete via detachment of the apical part of the cell containing secretion which is followed by regeneration
  • Mammary glands are an example
• Holocrine glands secrete through the death and detachment of the entire cell containing the secretion
  • Sebaceous glands (transforming into sebum) are an example
• Endocrine glands secret their secretion into the bloodstream
  • Pineal, pituitary, thyroid, pancreas, and ovaries are examples

Metaplasia

• Refers to cells that appear functionally and morphologically different from their matrix
• This term is used in pathomorphology and has a relationship to developing tumors
• Cells change original, mature type in response to chronic irritation, pathogen, or carcinogen exposure, as adaptive response