Epithelial Tissue Structure and Function

• Organs consist of four basic tissue types: epithelial, connective, muscular, and nervous.
• Tissues contain cells and extracellular matrix (ECM) and associate in proportions that characterize each organ.
• Connective tissue has cells producing abundant ECM.
• Muscle tissue has elongated cells specialized for contraction and movement.
• Nervous tissues have cells with long, fine processes specialized to transmit nerve impulses.
• Most organs have parenchyma (cells for the organ's specialized functions) and stroma (supporting cells).
• Stroma is always connective tissue except in the brain and spinal cord.
• Epithelial tissues are composed of aggregated polyhedral cells adhering strongly to each other and to the ECM.
• Epithelia forms cellular sheets that line cavities and cover the body surface.
• Epithelia line external and internal surfaces of the body.
• Substances entering or leaving an organ must cross an epithelium.

Functions of Epithelial Tissues

• Covering, lining, and protecting surfaces (e.g., epidermis).
• Absorption (e.g., the intestinal lining).
• Secretion (e.g., parenchymal cells of glands).
• Certain epithelial cells are contractile (myoepithelial cells) or specialized sensory cells (taste buds, olfactory epithelium).

Characteristic Features of Epithelial Cells

• Epithelial cell shapes vary from columnar to cuboidal to squamous.
• Cell size and morphology depend on function.
• Nuclei shape corresponds to cell shape, e.g., elongated in columnar cells, flattened in squamous cells.
• The number and shape of the cell nuclei are important indicators of shape and density as membranes are indistinguishable using light microscopy.
• Primary morphologic classification criterion is the number of nuclei.
• Most epithelia are adjacent to connective tissue with blood vessels for nutrient supply since epithelium itself does not contain blood vessels.
• The connective tissue underlying epithelia of digestive, respiratory, and urinary systems is called the lamina propria.
• Papillae (small evaginations) project from connective tissue into the epithelium to increase contact area.
• Papillae are common in tissues subject to friction (skin, tongue).
• Epithelial cells show polarity, with uneven distribution of organelles and membrane proteins.
• The region contacting the ECM and connective tissue is the basal pole.
• The opposite end, facing a space, is the apical pole.
• Regions adjoining neighboring cells are lateral surfaces, often folded to increase area.

Basement Membranes

• The basal surface rests on the basement membrane, a thin, extracellular sheet of macromolecules.
• It acts as a semipermeable filter for substances reaching epithelial cells from below.
• Glycoproteins and other components stain and visualize under the light microscope.
• Transmission electron microscopy (TEM) resolves two parts: basal lamina (thin, electron-dense, nearest epithelial cells) and reticular lamina (more diffuse, fibrous).
• "Basement membrane" denotes the entire structure, while "basal lamina" refers to the fine extracellular layer seen ultrastructurally.
• The basal lamina macromolecules are from epithelial cells and form a sheet array:
• Type IV collagen creates a two-dimensional network of subunits in the appearance resembling a window mesh.
• Laminin are large glycoproteins attaching to transmembrane integrins and projecting through the collagen mesh.
• Nidogen and perlecan cross-link laminins to the type IV collagen for the three-dimensional structure, binding epithelium and determining its filtration abilities.
• Basal laminae (external laminae) with similar compositions exist around muscle cells, nerves, and fat-storing cells.
• They serve as semipermeable barriers regulating exchange between cells and connective tissue.
• The reticular lamina contains type III collagen, bound to the basal lamina by anchoring fibrils of type VII collagen.
• Both are produced by cells of the connective tissue.
• Basement membranes support epithelial cells and attach epithelia to connective tissue.
• Basal lamina components organize integrins and proteins, maintaining polarity and localizing activities.
• They mediate cell-to-cell interactions and also act as a scaffold allowing epithelial repair and regeneration to take place more efficiently.

Intercellular Adhesion & Other Junctions

• Epithelial cells adhere to each other and basal laminae, particularly under mechanical forces.
• Lateral surfaces have specialized intercellular junctions as follows.
• Tight or occluding junctions form a seal between adjacent cells restricting passage of molecules.
• Adherens or anchoring junctions mediate strong cell adhesion.
• Gap junctions provide channels for communication between adjacent cells.
• Tight junctions are the most apical of the junctions.
• Adjacent membranes at tight junctions appear fused since they are very tightly apposed.
• The intercellular seal is due to interactions between transmembrane proteins claudin and occludin.
• Cryofracture reveals tight junctions as branching strands around the cell's apical end.
• Tight junctions ensure molecules cross an epithelium traveling transcellularly rather than paracellularly.
• Epithelial tight junctions hinder movement of membrane lipids and proteins between apical and lateral/basal surfaces thereby maintaining distinct membrane domains.

Epithelial Cell Junctions

• Adherens junctions (zonula adherens) encircle the cell below tight junctions for firm anchoring.
• Cell adhesion is by cadherins (transmembrane glycoproteins) binding to each other with Ca2+.
• Inside the cell, cadherins bind catenins linking to actin filaments in the "terminal web."
• Tight and adherent junctions encircle epithelial cells like plastic bands on a six-pack.
• Desmosomes (macula adherens) resemble "spot-welds" not belts, and are disc-shaped for strong attachment
• Desmosomes contain cadherin family members (desmogleins and desmocollins).
• Cytoplasmic ends of these bind plakoglobins (catenin-like proteins) that link to larger desmoplakins in an electron-dense plaque.
• Desmoplakins bind intermediate filament proteins rather than actins, and attach to cable-like tonofilaments connecting throughout the epithelium.
• Gap junctions facilitate intercellular communication, and exist in abundance.
• Cryofracture shows that the gap junctions consist of transmembrane protein complexes that form circular patches in the plasma membrane.
• Transmembrane proteins, connexins, form hexameric connexons, with a hydrophilic pore.
• Connexons in adjacent cell membranes align producing channels between cells, permitting intercellular exchange Molecules that are smaller than <1.5 nm can pass through a cell junction.
• Molecules such as signaling cyclic nucleotides and ions move rapidly for coordinated action across the tissue in the heart and visceral muscles, gap junctions help produce contractions.
• Hemidesmosomes anchoring junctions attach cells to the basal lamina on the basal epithelial surface.
• They attach to the basal lamina through integrins linked to the basal lamina.
• Focal adhesions, or focal contacts are another basal anchoring junction found in epithelial repair.
  • They link to bundled actin filaments via integrins and initiate a cascade of intracellular protein phosphorylation affecting cell adhesion, mobility, and gene expression.

Specializations Of The Apical Cell Surface

• Apical structures increase surface area or move substances along the epithelial surface.
• Microvilli are visible with an electron microscope, are usually reflective of the activity of actin filaments, and are variable in size.
• Microvilli in absorptive epithelia project and are densely packed.
• They give a brush or striated border in structures like the small intestine.
• Microvilli contain bundled actin filaments capped, also bound to plasma membrane with actin-binding proteins.
• Microfilament arrays are dynamic undergoing movement and maintaining absorption with channels.
• Actin filaments insert into the terminal web of cortical microfilaments (just below the microvilli)

Stereocilia Morphology and Function

• Stereocilia is a less common apical process.
• It is commonly seen on absorptive cells of the male reproductive system.
• Like microvilli, stereocilia increase surface area for absorption.
• Specialized stereocilia detect motion (important in inner ear sensory cells)
• Stereocilia resemble microvilli with microfilaments, connections to the terminal web, long length, and similar diameter.
• However, stereocilia are less motile than microvilli, and may branch distally.

Cilia: Structure and Function

• Cilia are long, motile apical structures being larger than microvilli

• They contain microtubules (not microfilaments).

• Most cell types, if not all, have primary cilium projections that is not motile

• Primary cilium projections are enriched with receptors and signal transduction complexes.

• These detect light, odors, motion, also liquid flow

• Motile cilia are abundant on cuboidal or columnar cells of many epithelia.

• Typical cilia are longer and wider than microvilli averaging 5-10µm long and 0.2µm in diameter.

• Each cilium shows a core arrangement of nine peripheral microtubule doublets organized around two central microtubules in a structure called the axoneme.

• As with other microtubules, kinesin and cytoplasmic dynein motors move molecule components along peripheral microtubules.

• Microtubules of axonemes are continuous with basal bodies being cytoplasmic structures just below the membrane

• Basal bodies have structures similar to centrioles with microtubules forming rootlets anchoring the structure

• Cilia exhibit beating patterns moving fluid and matter in one direction. Ciliary motion occurs with conformational changes of the axoneme

• Accessory proteins make each cilium stiff, yet elastic.

• Complexes with axonemal dynein slide adjacent doublets producing the beating action of the cilia.

• The long flagellum that extends from each fully differentiated sperm cell shares an axenomal structure, its movements also follow similar mechanisms.

Types Of Epithelia

• Epithelia are divided into covering or lining epithelia and secretory or glandular epithelia which dictates respective functions.

Covering or lining Epithelia: Classification by Cell Layer

• Cells of covering epithelia are organized into layers covering surfaces.
• Covering or lining also line the cavities of organs.
• Covered/Lining are epithelia classified by organization, cell layers/morphology in the outer layer
• Simple epithelia have one cell layer(squamous, cuboidal, columnar)
• Stratified epithelia contain two or more layers
• Stratified epithelium are named after the cell shape of the outer layers(squamous, cuboidal, columnar)
• Thin cells of stratified squamous epithelia can be keratinized (packed with intermediate filaments) Keratinized squamous epithelium are dry and protective helping to prevent dehydration from the tissue
  • Cells form irregular shapes that flatten and accumulate keratin Nonkeratinzed stratified squamosi epithelia are moist and contain retent nuclei
• Stratified cuboidal and stratified columnar epithelia are rare Stratified cuboidal epithelium is present in excretory ducts of the ductor glands Stratified columar epithelium is present in the eylids to provide protection secrection
• Unique transitional epithelium or urotheliam allows distention through morphological cellular relationships.

Additional Epithelial Classifications

• Pseudostratified columnar ethelium is attached to the basement membrane
• Tall and irregulars, not all extend reach the reach the free surface.
• Good example: upper respiratory tract.

Secretory Epithelia & Glands

• Epithelial cells functioning to produce and secrete macromolecules may occur in epithelia with other majorfunctions, those primarily with the ability to produce major macromolecules or comprise specialized organcalled glands

Secretory cells may have the ability to

• synthesize, store, release proteins (pancreas)
• Lipid-rich (adrenal, sabaceous gland)
• Proteins/Carbphydrates complex
  • The epithelia of mammary gland secrete all three substances The cells of some glands primarily secret water and are of little synthetic value.
• These secrete ions that transfer from the blood. Secretory cells can be called unicellular and are cubodial and line simple epithelia (simple columnar, pseudostratified epthelia)

Glands are a devloped cell that grows into connective tissue

• Exocrine glands are remeain connected to epitheilum
• Exocrine glands are responsible for the tubular ducts to bring secreted material.
• Endocrine glands do not connect to the ogirinal epithelium due and lack ducts that allow it to have absorbed hormone products.

Epithelium of exocrine gland organized an a continuouss system of small portions and transport secretion In both endocrine glands and exocrine glands. the secretory units are suprpoted conectiver tisse stromia. Larger ducts include larger conective layers For the exocrine glands and in structure, the size, and morphological classifications vary

Exocrine and Endocrine Gland Morphology:

Classifications:

• Glands be able to simple- ducts dont branch our more.
• • They can grow, forming branches, and have tubular form that are branched compound gland- having two, or less branches but the scretory portion may be branching. glands have the ability to branch/form secretion. The basic types of three used for common/specialized secretion

Explore the structure and function of epithelial tissues, including connective tissues, cell shapes, and key components like the basement membrane. Understand how these features contribute to the overall function of organ systems. Learn about cell polarity and the roles of different layers within the epithelial structure.