Epithelial Tissue

• Epithelial tissue is one of the four basic tissue types
• The other three tissue types are connective, muscle, and nervous tissue
• Epithelial tissue is composed of cells that line cavities of organs and cover the body's surface
• Epithelial tissue originates from all three germ layers

Tissue Hierarchy

• Specialized cells make up tissue
• Tissue combine to form organs
• Organs work together as systems
• System cooperate into organisms

Four Tissue Types

• Connective
• Epithelial
• Muscle
• Nervous

Epithelial Tissue Functions

• Epithelial tissue's functions are:
  • Covering, lining, and protecting surfaces, as with the epidermis
  • Absorption, exemplified by the intestinal lining
  • Secretion, such as parenchymal cells of glands
• Specific cell types of this tissue:
  • Contractile cells, like myoepithelial cells
  • Sensory cells, like taste buds and olfactory epithelium

Epithelial Tissue Distinguishing Characteristics

• Cellularity: A characteristic is being composed almost entirely of cells
• Polarity: A characteristic is having apical and basal surfaces, or top and bottom
• Attachment: A characteristic is being supported by connective tissue via a basement membrane
• Avascularity: A characteristic is containing no blood vessels and obtaining nutrients by diffusion
• Regeneration: A characteristic is rapidly replacing lost cells by cell division

Epithelial Cell Polarity

• Epithelial cells show polarity
• Organelles and membrane proteins are distributed unevenly within the cell
• The region contacting connective tissue is called the basal surface
• The opposite end, usually facing a space, is the apical surface
• The two poles of epithelial cells differ in structure and function
• Cuboidal or columnar cells adjoin neighboring cells at lateral surfaces
• The Apical surface has microvilli, cilia and stereocilia
• The Lateral surface has specilized junctions
• The Basal Surface has cellular interdigitations and basal lamina and receptors

Interactions with Connective Tissue

• Epithelia are attached to and supported by connective tissue
• Nerves and blood vessels are supported, which are necessary for the maintenance and functioning of the epithelia
• The connective tissue underlying epithelia lining digestive, respiratory, and urinary organs is called the lamina propria

Basement Membranes

• All epithelial cells that contact subjacent connective tissue have a basal surface with a specialized, felt-like sheet of extracellular material
• The sheet of extracellular material is known as the basement membrane
• Glycoproteins and other components can be stained and made visible beneath epithelia using a light microscope

Intercellular Junctions

• Membrane-associated structures provide adhesion and communication between cells in the intercellular junction
• Epithelial cells strongly adhere to neighboring cells and basal laminae
• Lateral surfaces exhibit specialized intercellular junctions serving various functions

Junctional Complexes

• Tight junctions seal cells to prevent leakage
• Adherens junctions join actin bundles between cells
• Desmosomes join intermediate filaments between cells
• Gap junctions allow small molecules to pass between cells
• Hemidesmosomes anchor intermediate filaments to the basal lamina

Tight Junctions (Zonulae Occludens)

• Tight junctions (zonulae occludens) are the most apical of the junctions
• In epithelia, junctions are present in an order at the apical end
• The junction forms a band encircling each cells
• Water and solutes are more permeable in epithelia with fewer fused sealing strands, such as the proximal renal tubule
• Epithelia lining the urinary bladder, which have many fusion sites, are less permeable to water and solutes
• Molecules crossing an epithelium do so "through" them via a transcellular path instead of "between" them (paracellular pathway), due to tight junctions

Gap Junctions

• Gap junctions mediate communication between cells
• Gap junctions consist of aggregated transmembrane protein complexes forming circular patches in the plasma membrane
• Gap junction proteins, called connexins, form hexameric complexes called connexons.
• Connexons each have a central hydrophilic pore of about 1.5 nm in diameter

Gap Junction Permeability

• Gap junctions enable the intercellular exchange of molecules less than 1.5 nm in diameter
• Cyclic nucleotides and ions that mediate signal transduction move rapidly through gap junctions.
• Gap junctions that allow cells to act in coordination rather than as independent units
• Gap junctions in heart and visceral muscles aid rhythmic contractions

Zonula Adherens (Adherens Junction)

• Zonula Adherens (Adherens Junction) encircles the epithelial cell, immediately below the zonula occludens
• A adherent junction firmly anchors a cell to its neighbors
• Cell adhesion is mediated by cadherins, which are transmembrane glycoproteins
• At cytoplasmic ends, cadherins bind catenin, which is linked to actin filaments
• Zonula adherens' actin filaments are linked
• It plays a role in cytoplasmic motility and other functions as a cytoskeletal feature at the apical pole

Desmosomes (Macula Adherens)

• Desmosomes are single "spot-welds" rather than belts around a cell
• Maculae adherens or desmosomes are disc-shaped structures at the surface of one cell that are matched with identical structures at an adjacent cell surface
• Desmosomes contain desmoglein and desmocollin, which are larger members of the cadherin family
• Cable-like filaments of cytokeratin are common in desmosomes of epithelia
• Desmosomes provide firm adhesion because their intermediate filaments are very strong

Hemidesmosomes

• Hemidesmosomes are the junctions that attach the basal domain of an epithelial cell to the subjacent basal lamina.
• Can be seen with a Transmission Electron Microscope
• Half-desmosomes are resembled structurally by these adhesive structures at the ultrastructural level
• Integrins, not cadherins, are abundant in desmosomes
• Laminin and collagen type IV are extracellular macromolecules, that the transmembrane integrin proteins in Hemidesmosomes bind to

Specializations of the Apical Cell Surface

• The apical ends of tall or cuboidal epithelial cells face an organ's lumen
• Often have specialized projecting structures
• These structures either increase surface area for absorption or facilitate substance movement
• Microvilli
• Stereocilia
• Cilia

Microvilli

• Microvilli are cytoplasmic projections that are visible with an electron microscope
• Projections called microvilli are present in an array on apical surfaces of cells specializing in absorption
• They increase the cell's surface area
• Apical surfaces of cells lining the small intestine are densely covered with microvilli
• Visible as a striated or brush border
• Each microvillus contains bundles of actin filaments capped and cross-linked to each other
• Each microvillus has a length of about 1 µm and a width of 0.1 µm
• Hundreds or thousands present on each absorptive cell, overall increasing surface area by 20- or 30-fold

Stereocilia

• Restricted to absorptive epithelial cells lining the epididymis and ductus deferens' proximal sections
• Stereocilia are a less common type of apical process
• Like microvilli, stereocilia increase cell surface area for absorption
• Specialized stereocilia are components of inner ear motion-detecting sensory cells
• Stereocilia contain arrays of actin filaments and actin-binding proteins and resemble microvilli
• Typically much longer than microvilli, and may show distal branching along their length

Cilia

• Cilia are projecting structures larger than microvilli and contain internal arrays of microtubules
• Typical cilia are two times wider (0.2µm) and longer (5-10µm) than microvillus
• Motile cilia exist only in epithelia
• They're abundant on the apical domains of many cuboidal or columnar cells
• Some cell types have at least one nonmotile primary cilium of variable length
• These are enriched receptors and signal transduction complexes for detection of light, odors, motion, and flow of liquid
• Each cilium's core structure consists of nine peripheral microtubular doublets in which few tubulin protofilaments are shared
• The nine doublets form an array around two central microtubules
• This 9 + 2 assembly of microtubules is the axoneme
• Epithelial cilia exhibit rapid beating that propel fluid and suspended matter in one direction
• The long flagellum extending from each fully differentiated sperm cell has an axonemal structure identical to that of cilia and moves with similar mechanism

Kartagener Syndrome

• Kartagener Syndrome inherited as autosomal recessive trait.
• Patient develop bronchiactasis, sinusitis and situs invertus.
• Defect in ciliary motility due to absent or irregular dynein arms.
• Lack of ciliary activity interferers with bronchial clearance.
• Males have infertility.

Types of Epithelial Tissues (by Function)

• Covering and lining epithelium examples, epidermis of skin, lining of blood vessels and ducts, lining respiratory, reproductive, urinary & GI tract
• Glandular epithelium examples, secreting portion of glands, thyroid, adrenal, and sweat glands

Types of Epithelia (by Morphology)

• Criteria include cell layers and cell shape
• Cell Layers can be Simple of Stratified
• Cell shape can be Squamous, Cuboidal, Columnar

Cell Layers and Shape

• Simple epithelia contain one cell layer
• Stratified epithelia contain two or more layers
• Simple epithelia, based on cell shape, are classified as:
  • squamous
  • cuboidal
  • columnar
• Squamous epithelia are thin and flattened cells
• Cuboidal epithelia have a cell width and thickness that are similar.
• Columnar epithelial cells are taller than they are wide

Stratified Epithelia

• Stratified epithelia are classified according to the cell shape of the superficial layer(s)
• Can be squamous, cuboidal, or columnar
• The superficial cells of stratified squamous epithelia may have an amount of keratin:
  • "keratinized"
  • "nonkeratinized"

Stratified Squamous Epithelium

• Stratified squamous keratinized epithelium is found mainly in the epidermis of skin
• Helps prevent dehydration from the tissue
• Its cells form many layers, with the less differentiated cuboidal cells near the underlying connective tissue
• Stratified squamous nonkeratinized epithelium lines wet cavities (eg, mouth, esophagus, and vagina) where water loss is not a problem

Epithelia Examples

• Stratified cuboidal and stratified columnar epithelia are both relatively rare
• Stratified cuboidal epithelium is restricted to excretory ducts of salivary and sweat glands
• Stratified columnar epithelium can be found in the conjunctiva lining the eyelids
• It protects and secretes mucus in the eyelids
• Transitional epithelium, or urothelium, lines the urinary tract
• Characterized has dome-like umbrella cells as a superficial protective barrier, is water proof, and can withstand significant stretch

Pseudostratified Columnar Epithelium

• Here tall, cells are attached to the basement membrane but their nuclei are at different levels, and not all cells reach the surface, so they appear stratified
• The upper respiratory lining is a good example, where cells are heavily ciliated

Simple Squamous Epithelium

• Simple Squamous – Endothelium (En)
• Simple Squamous (Mesothelium) by TEM

Simple Cuboidal Epithelium

• Simple Cuboidal Epithelium (longitudinal section)
• Simple Cuboidal by TEM

Cell Structures

• Simple Columnar (cross section)
• Goblet Cells reside within Simple Columnar epithelium
• Pseudostratified Epithelium with Cilia and Goblet Cells (Respiratory Epithelium)
• Stratified Squamous non-Keratinized Epithelium
• Stratified Squamous Keratinized (thin)
• Transitional Epithelium with rounded binucleated surface cells