Untitled

• Organs of the human body are composed of four basic tissue types: epithelial, connective, muscular, and nervous

Epithelial Tissue Definition

• Epithelial tissues consist of closely aggregated polyhedral cells that adhere strongly to each other and to a thin layer of ECM
• They form cellular sheets lining organ cavities and covering the body surface
• Epithelia line all external and internal body surfaces; substances entering or leaving organs must cross this tissue

Epithelial Tissue Functions

• Include covering, lining, and protecting surfaces like the epidermis
• It also facilitates absorption, for example, the intestinal lining
• Secretion, for example, parenchymal cells of glands

Characteristic Features of Epithelial Cells

• Epithelial cell shapes and sizes vary from tall columnar to cuboidal to low squamous
• Cell size and morphology are dictated by function
• Columnar cells have elongated nuclei, squamous have flattened nuclei, and cuboidal/pyramidal have spherical nuclei
• Nuclei number and shape indicate cell shape and density
• Nuclei also help determine the number of cell layers in an epithelium
• Most epithelia are adjacent to connective tissue containing blood vessels for nutrients/O₂
• The connective tissue underlying epithelia lining the digestive, respiratory, and urinary systems is the lamina propria
• The contact area between tissues is increased by papillae projecting from connective tissue into the epithelium
• Papillae are common in tissues subject to friction, like skin or tongue coverings

Polarity of Epithelial Cells

• Epithelial cells show polarity with uneven distribution of organelles and membrane proteins
• The region contacting ECM and connective tissue is the basal pole
• The opposite end, usually facing a space, is the apical pole

Basement Membranes

• The basal surface of all epithelia rests on a thin, extracellular sheet of macromolecules called the basement membrane
• It functions as a semipermeable filter
• Glycoproteins and components are stained and visualized with a light microscope

Basement Membranes Components

• Nearest the epithelial cells is the basal lamina, a thin, electron-dense layer of fine fibrils
• Beneath this layer is a more diffuse and fibrous reticular lamina
• The basal lamina's macromolecules include:
  • Type IV collagen, which self-assembles into a 2D network of evenly spaced subunits
  • Laminin, large glycoproteins attaching to transmembrane integrin proteins and projecting through the type IV collagen mesh
  • Nidogen and perlecan, cross-linking laminins to the type IV collagen network while determining porosity and molecule size
  • Basal laminae are barriers regulating macromolecular exchanges among various cells

Functions of Basement Membranes

• Filters
• Structural support for epithelial cells
• Attaches epithelia to underlying connective tissue
• Components help organize integrins and other proteins
• Facilitates cell-to-cell interactions
• Serves as a scaffold for epithelial repair and regeneration

Intercellular Adhesion and Junctions

• Epithelial cells adhere strongly to neighboring cells and basal laminae
• Lateral surfaces have specialized intercellular junctions with different functions
• tight or occluding junctions form a seal between adjacent cells
• Adherent or anchoring junctions are sites of strong cell adhesion
• Gap junctions are channels for communication between adjacent cells

Junction Order in Epithelia

• Tight junctions, called zonulae occludens, are the most apical
• "Zonula" indicates that the junction forms a band encircling each cell
• They help to ensure molecules cross an epithelium via a transcellular path
• They also serve as fences restricting membrane lipid and protein movements, maintaining membrane domains

Adherens Junctions

• Encircles epithelial cells immediately below the tight junction
• Attaches a cell to its neighbors
• Cell adhesion is mediated by cadherins
• Cadherins bind catenins that link to actin filaments

Desmosomes

• Resemble a single "spot-weld"
• Disc-shaped structures matched with identical structures at adjacent cell surfaces
• Desmosomes provide firm cellular adhesion and strength throughout the epithelium

Gap Junctions

• Mediate intercellular communication, rather than adhesion or occlusion
• Cryofracture preparations show that these consist of aggregated transmembrane protein complexes, forming circular patches

Gap Junctions Proteins

• Connexins form hexameric complexes called connexons, each with a central hydrophilic pore
• Connexons in adjacent cell membranes align to produce connexons between cells
• They permit intercellular exchange of smaller molecules

Hemidesmosomes and Focal Adhesions

• On the basal epithelial surface, cells attach to the basal lamina via anchoring junctions called hemidesmosomes
• Those resemble a half-desmosome ultrastructurally
• Integrins of hemidesmosomes bind primarily to laminin molecules in the basal lamina
• Focal adhesions are present in cells undergoing epithelial repair/reorganization

Specializations of the Apical Cell Surface

• The apical ends of many columnar and cuboidal epithelial cells have specialized projecting structures
• These function to increase the apical surface area for absorption or move substances

Microvilli

• Cytoplasmic projections often filled with an array of projecting microvilli
• Densely packed microvilli are visible as a brush or striated border projecting into the lumen

Stereocilia

• Less common apical process
• Increase the cells' surface area, facilitating absorption
• Important components of inner ear sensory cells with a motion-detecting function

Cilia

• Long, highly motile apical structures
• larger than microvilli,
• contain internal arrays of microtubules
• Cilia exhibit beating patterns that move a current of fluid and suspended matter

Types of Epithelia

• Divided into two groups: covering (or lining) epithelia and secretory (glandular) epithelia
• Covering or lining epithelia are organized into one or more layers that cover the surface or line organ cavities, classified by cell layers and morphology

Epithelial Classification

• Simple epithelia contain one cell layer and stratified epithelia have two or more layers
• Simple epithelia are classified as squamous (thin cells), cuboidal (similar width and thickness), or columnar (taller than wide)
• Most stratified epithelia are classified by the superficial outer layer
• Those classified as squamous, cuboidal, or columnar cell types

Stratified Squamous Epithelium

• The surface cells can be "keratinized" (packed with keratin filaments) or "nonkeratinized" (relatively sparse keratin)
• Keratinized stratified squamous epithelium helps prevent tissue dehydration
• Stratified squamous nonkeratinized epithelium lines moist internal cavities without dehydration concerns
• Stratified cuboidal and stratified columnar epithelia both are relatively rare
• Unique transitional epithelium or urothelium lines much of the urinary tract is characterized by large, dome-like umbrella cells.
• Pseudostratified columnar epithelium has tall, irregular cells attached to the basement membrane, with nuclei at different levels

Secretory Epithelia and Glands

• Epithelial cells synthesizing and secreting macromolecules may occur as part other or comprise specialized organs known as glands
• Scattered secretory cells, called unicellular glands, are common in epithelia, with the goblet cell being an easily seen example