Epithelial Tissue and Cells

• Epithelium is a type of body tissue that covers internal and external surfaces, lines cavities and hollow organs, and is the major tissue in glands

• Epithelial tissue functions for protection, secretion, and absorption, depending on its location in the body

• Organs are composed of epithelial, connective, muscular, and nervous tissues

• All substances entering or leaving an organ must cross epithelial tissue

• Examples of epithelial tissue include the outer layer of skin, the lining of intestines and the respiratory tract and abdominal cavity, and sweat glands

Epithelial Cells

• Make up epithelial tissue
• They vary in shape and arrangement (single or multiple layers) based on location and function
  • A cell has three main parts:
    • Cell membrane: Surrounds the cell and controls substance movement
    • Nucleus: Contains most of the cell's DNA (genetic material)
    • Cytoplasm: Fluid containing other cell parts with functions

Surface Characteristics of Epithelial Cells

• Microvilli: Non-motile, finger-like structures that increase surface area for absorption
  • Thousands line the small intestine to absorb nutrients and protect the body from intestinal bacteria
• Cilia: Tiny, hair-like, motile structures that move entire cells or substances along the outer surface
  • Epithelial cells lining the respiratory tract have cilia to trap and move dust towards the nostrils
  • Ciliated cells in fallopian tubes help move an egg to the uterus
• Stereocilia: Specialized microvilli resembling cilia, project from certain epithelial cells
  • Needed for hearing and balance in the inner ear

Types of Epithelial Cells Based on Shape

• Squamous epithelium: Flat, sheet-like cells
• Cuboidal epithelium: Cube-like cells with equal width, height, and depth
• Columnar epithelium: Column-like cells, taller than they are wide

Types of Epithelial Cells Based on Arrangement

• Simple: One layer of cells
• Stratified: More than one layer of cells
• Pseudostratified: Closely packed cells of varying sizes appearing in layers, but actually a single layer

Types of Epithelial Cells and Their Locations

• Simple squamous epithelium: Lines blood vessels and body cavities, regulates passage of substances
• Simple cuboidal epithelium: Found in glandular (secreting) tissue and kidney tubules
• Simple columnar epithelium: Specialized for absorption, lines the stomach and intestines
• Stratified squamous epithelium: Protective functions, forms the outer layer of skin (epidermis)
• Stratified cuboidal epithelium: Found in excretory ducts of salivary and sweat glands
• Stratified columnar epithelium: Seen in the mucous membrane lining eyelids, protective and mucus-secreting
• Pseudostratified columnar epithelium: Lines the upper respiratory tract and has lots of cilia

Specialized Epithelial Cells

• Transitional epithelium (Urothelium): Made of multiple cell layers that flatten when stretched and lines the urinary tract, allowing the bladder to expand
• Glandular epithelium: Produces and secretes substances in glands such as hormones, proteins, and water
• Olfactory epithelium: Contains olfactory receptor cells with specialized cilia extensions in the nasal cavity which traps odor molecules, and sends information to the olfactory bulb in the brain

Functions of Epithelial Tissue

• Protection: Protects the body and deeper tissue

• Secretion: Secretes enzymes, hormones, and fluids in glands

• Absorption: Allows absorption of certain substances in the lining of internal organs

• Excretion: Removes waste from the body in the kidneys and sweat glands

• Filtration: Filters out dirt and particles in the respiratory tract and kidneys

• Diffusion: Simple squamous epithelial cells allow selective diffusion for filtration, absorption, and secretion

• Sensory reception: Sensory nerve endings in epithelial tissue enable sensory stimuli reception

Conditions Affecting Epithelial Tissue

• A major concern is the potential for malignancy development
• Adenocarcinoma affects glandular epithelial tissue and is the most common type of cancer affecting organs

Epithelium vs. Endothelium vs. Mesothelium

• All three are types of epithelial cell layers that line organs and form an outer layer of skin
• Epithelium generally lines pathways open to the external environment such as the repiratory and digestive system
• Endothelium lines fully internal pathways like blood vessels and cornea
• Mesothelium lines major body cavities (peritoneum, pleuar, pericardium), and it secretes a lubricant film called serous fluid

Cell Nuclei

• Differ in shape and maybe elliptic, spherical or flattened, with corresponding nuclear shapes
  • Columnar cells elongated nucleus, squamous have flattened
  • Cuboidal or pyramidal have more spherical cell nuclei
• Nutrients for these cells must diffuse across the basement membrane
• Nuclei allow one to determine the number of cell layers
• Adjoin layer of blood vessel connective tissue where epithelial cells get nutrients and O2
• Connective tissue underlying epithelia and lining organs in the digestive, respiratory and urinary systems is called lamina propria

Papillae

• The area of contact papillae from two tissues is increased by small evaginations
• Project from the connective tissue into the epithelium
• Occur most frequently in epithelial tissues subject to friction (skin or tongue covering)

Basal Pole

• Contacting the ECM and connective tissue

• Functions differently from the opposite, apical pole

• Basal surface rests on basement membrane

• Basement membrane is a thin extracellular, felt-like sheet of macromolecules between the epithelia and connective tissue, filtering substances reaching epithelial cells from below

• Basal lamina is a thin electron-dense sheetlike layer of fine fibrils nearest the epithelial cells

• The reticular lamina has diffuse and fibrous properties beneath this layer the basal lamina

• Dense basal lamina is 20-100 nm thick and anchored to type III collagen reticular lamina

• Hemidesmosomes bind to basal surface of the epithelial cell

• ECM components specific to basal laminae are Type IV collagen, Laminins, Nidogen and Perelman

• Proteins such as type III and VII collagen adhere to basal lamina through anchoring fibrils

• Basement membrane functions include structural support, attachment to underlying connective tissue, organization of integrins, maintaining cell polarity and mediating cell-cell interactions

• Epithelial cells have specialized intercellular junctions with different functions

  • Tight or occluding junctions: seal between adjacent cells
  • Adherent or anchoring junctions: create sites of strong cell adhesion
  • Gap junctions: act as channels for communication between adjacent cells

Tight junctions

• The most apical one forms a band encircling each cell

• The seal results from transmembrane proteins such as claudin and occludin

• Intercellular seal of tight junctions ensures through-cell (transcellular path) crossing instead of between-cell (paracellular pathway) crossing

• Epithelia with few fused sealing strands are more permeable to water and solutes than with many fused strands

• T Tight also serve as fences, restricting lipid and protein movements between apical and basolateral cell poles

• Maintains membrane domains (apical and basolateral) with different components, receptors and proteins

Adherens Junctions

• They encircle the epithelial cells below tight junctions
• Cell adhesion depends on cadherins, linking to actin filaments with actin-binding proteins

Actin Filaments

• Linked to adherens junctions form the terminal web in epithelial cells
• The tight and adherent junctions function like plastic bands holding canned drinks

Desmosome Anchoring Junction

• It resembles single spot-weld and doesn't form a belt around the cell (disc-shaped lateral structures matching neighboring structure)

• It contains members of the cadherin family called desmogleins and desmocollins

• Cytoplasmic ends cluster transmembrane proteins bind intermediate filament binding plakoglobin, catenin-like proteins, linking to plakin proteins in electron-dense plaque

• Desmoplakins bind intermediate filament proteins of cytokeratin (tonofilaments) firm adhesions for tensile strength

Gap Junctions

• They mediate intercellular communication instead of adhesion

• Consists in every mammalian tissue

• Aggregate transmembrane protein complexes in plasma membrane

• Permit exchange of nutrients and signal molecules between cells

• Communicating channels formed by abutting particles (connexons) with six protein subunits(connexins)

• Connexin gap junction proteins form connexons with hydrophilic pole

• Small molecules measuring less than 1.5nm can be transferred intercellularly through gap junctions

• They link to basal epithelial cells surface through anchoring hemidesmosomes and integrate with laminin

• Resemble a half-desmosome, linking cytokeratin and intermediate filaments

Focal adhesion

• Represent basal anchoring junction in epithelial repair or reorganization

• Resemble hemidesmosomes and involve integrins linked to actin filaments

• Integrins of focal adhesions bind paxillin to focal adhesion kinases for cellular adhesion, mobility in fibroblasts

• Apical ends of cuboidal epithelial cells have projecting structures to increase surface area for absorption

Cytoplasmic Projections

• Movements and activity of actin filaments
• Extensions are temporary and variable in length, shape and quantity

Microvilli -

• Apical cell surfaces become filled with array of projecting microvilli, in epithelia for absorption and uniform length

• They constitute brush border/striated border in cells and projecting into the lumen

• Each microvillus contains actin filament bundles with actin-binding proteins to microfilament

• Microvilli are dynamic myosin-based proteins with receptors for absorptive optimal conditions with channels, receptors, other proteins,

• Actin filaments insert into terminal web of cortical microfilaments at microvilli base

Stereocilia

• An uncommon type of apical process is seen on absorptive epithelial cells in the male
• It increases cell surface area, facilitates absorption and detects motion in inner ear sensor cells

Cilia

• Long, motile apical structures that are larger than microvilli and contain internal arrays with microtubules

• Cell types have a short projection called primary cilium not specialized for mobility

• Cilia are complex for detection of light, odors, motion and the flow of liquid across cells

• Epithelial cells lining the respiratory tract have well developed cilia

  • They have a core structure nine outer microtubules shared with few protofilaments arrayed around a central pair of singlet microtubules
  • This nine plus two assembly has an axoneme

• Microtubules of axonemes are continuous with cytoplasmic basal bodies of the cell membranes

• Cilia exhibit rapid beating that moves currents of suspended matter in direction of the epithelium

• Ciliary motion successions by changes in axoneme conformation

• In accessory portions each cilium is relatively rigid and elastic

• Complexes with axonemal dynein is bound to tubules/doublets to bend the axoneme and produce the Cilia beating motion

Types of Epithelia

• Include two main types of groups

• Covering (lining) and secretory glandular (epithelia)

• Represents arbitrary divisions

• Some also secrete lining epithelial cells

• Glandular cells distributed in lining cells

• Simple consist of one cell layer

• Classified based on cell shape: squamous, cuboidal, or columnar

• Epithelia are further classified as squamous/flat cell, cuboidal or columnar

• Can prevent dehydration with an underlying epidermis

Transitional/urothelium

• Lines much of the urinary tract and is stratified

• Superficial cells rounded/dome shaped for hypertonic urine protection

• Can adjust relationships with one another to transition

• Psuedostratified columnar epithelia may appear multilayered. Irregular and all attached to basement membrane with all irregular apical surfaces for stratified appearance.

• Lines trachea for cilia

• Secretory functions performed by various Epithelial cells found within the gland

• Macromolecules found with other major functions and organs

• Glands release proteins/lipids/complex carbos

• Uniscellular glands Found in simple columnar

• Lubricating mucos aide in the functioning

• Develop from covering epithelia

• Growth an cell proliferation leads to differentiation

• Exocrine connection with surface epithelium that delivere material

• Endocrine cells have lost e connection

• Capillaries absorb products

Glandular epithelium

• Simple glands can be ducts (not branched) or compounds ( ducts with two or more branches)
• Tublar( short/ long or coiled and rounded or acinar for type

Merocrine secretion

• Method of protein or glycoprotein secretion from membrane - bound vesicles or granules

Holocrine secretion

• Cells accumulate continuously as they enlarge and undergo erminal differentiation
• Completes cell disruption to release

Aprocrine secretion

• Product accumulates at apical ends and releases portions as well
• Droplets rebest case

Exocrine

Exocrine- either mucous material with distinct properties vs serous Serous cells synthesize proteins-glycolated enzymes/etc which and fill with secretory granules in maturation stages and stain for acidity

Mucous layers

• Complex golgi and apical layers for highly glycosylated proteins- mucins

• Hyophilic musics washed for poor eosin staining and olgosacharides remaining

• The lumens have connecting tissue to surrounds tubing and ducts

Seromucouus

• Have both and water mucus
• Exocrine and contractile contain contractile myo

Endocrine

• Lack and special hormone

Endocrine

• Hormone goes to target by bloodstream often

• There may also be paracrine and autocrine secretion

• Importantly paracrine also singlularly and in small functions

Transport

• Actively transports ions against concentration

• Water transcellular/membrane flow through with tight junction/paracellular

• Kidney has major body function of water balance

• Tubules of becoming especially transcellulary

• apical free run of base for fluid to the membrane

Diagram

• A highly specialized for absorption in proximal convolutedtubule
• Brush border is that increases area

internalize

• All cells now internalize extracellulary with cytoplasmic
• Cytosis/domains between underlies processes

##Renewal

Cells

• Cells undergo continuous newal with mitotic an stem cell growth

• Highly renewal is fast in glands

• Contact and are in basal cell in stratified

• Some niches need stem cels to

• The epithelium/hairfolliicle also dervic stem cells

• Gap junctions permit intercellular exchange of molecules with small (<1.5 nm) diameters

• Some molecules mediating signal transduction, such as cyclic nucleotides and ions, move rapidly through gap junctions, allowing cells in many tissues to act in a coordinated manner rather than as independent units

• In heart and visceral muscles, gap junctions help produce rhythmic contractions

• On the basal epithelial surface, cells attach to the basal lamina by anchoring junctions called hemidesmosomes

• These adhesive structures resemble a half-desmosome ultrastructurally, but unlike desmosomes the clustered transmembrane proteins that indirectly link to cytokeratin intermediate filaments are integrins rather than cadherins

• The integrins of hemidesmosomes bind primarily to laminin molecules in the basal lamina