Epithelial+Tissues


 * 25 August 2006**
 * Epithelial Tissues**
 * Dr. Robert Crissman, Ph.D.**

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=**Levels of Organization**=


 * Cells – basic unit of all animals
 * Tissues – clumps of similar cells grouped together to perform a specific function
 * Organs – two or more tissues grouped together to perform a specific function, i.e. kidney
 * Systems – two or more organs grouped together to perform a specific function, i.e. urinary system

=**Basic Tissue Types**=


 * [|Epithelial]
 * Layer of cells that cover the body’s surfaces. Selective barrier to aid or prevent materials from transferring the surface they cover.
 * Muscle
 * A group of cells that change shape, specialized for contraction. Responsible for locomotion, constriction, pumping and peristalsis
 * Nerve
 * A group of cells characterized by their excitability. Organism can react to stimuli
 * Two types: neurons and neuroglial cells.
 * [|Connective Tissue]
 * A group of cells that connect other tissues together in the body, structurally and metabolically.

**Common Structure of Epithelium**

 * Predominately cellular, making a contiguous sheet of cells forming a layer or membrane
 * Have numerous intercellular junctions to hold together to a single, continuous layer
 * Polar cells – divide cells into two different regions: basolateral (bottom) and apical (surface) portions
 * All epithelial cells have a free surface
 * Sits on a basement membrane
 * Avascular – no blood vessels
 * Blood supply from connective tissue

**Embryological Origin**

 * Epithelia derived from all three germ layers, though most of epithelia are derived from ectoderm and endoderm
 * [|Ectoderm] gives rise to epidermis
 * [|Endoderm] gives rise to the linings of the G.I. tract, respiratory tract, and distal parts of the urogenital tract.
 * [|Mesoderm] gives rise to the linings of internal body cavities
 * [|Mesothelium] – lines the pericardial, pleural and peritoneal cavities
 * [|Endothelium] – lines the blood and lymphatic vessels and heart

**Functions of Epithelia**

 * Functions as selectively permeable barriers, aiding or prevents materials from transversing the surface they cover
 * Protection from mechanical, dehydration, and chemical damage
 * Secretion – produce and secrete materials into space they are bound
 * Form glands which are part of epithelium basic tissue type
 * Absorption – G.I. tract
 * Transport – endothelium, lung
 * Acts as a sensory surface
 * Regenerate and self-repair

=**Classification of Epithelium by Structure**=


 * There are two parts of name based on structural charactereistics:
 * Number of layers of cells
 * Unilaminar or simple – single layer
 * Multilaminar or stratified – more than one layer
 * Shape of cell
 * Squamous – thin flattened cells
 * Cuboidal – cells are cubical
 * Columnar – tall narrow cells

**Examples of Simple Epithelium Tissue**

 * Simple squamous epithelium
 * Endothelium and mesothelium
 * Simple Cuboidal epithelium
 * Ducts of glands, organ coverings, kidney tubules
 * Simple Columnar epithelium
 * Parts of the G.I. respiratory and reproductive tracts
 * Pseduostratified columnar epithelium
 * Appears stratified but is not; nuclei located at different levels but all cells sit on basement membrane
 * Some cells are tall and some are short with nuclei at different levels
 * Most of respiratory tract, trachea, epididymis, nasal cavity

**Examples of Stratified Epithelium Tissue**

 * Stratified tissue shapes described by shape of cell on surface
 * Stratified squamous epithelium
 * Keratinized – containing fibrous protein called keratin
 * Builds up in cells and kills cells – no nucleus in surface layers
 * Epidermis of skin
 * Non-keratinized – Same appearance as keratinized but have nuclei at surface
 * Wet inner surfaces of oral cavity, esophagus, vagina, conjunctiva of eye
 * Stratified squamous mucosal epithelium
 * Why stratified? Because there is a high rate of cell loss and is necessary to have multiple layers
 * Stratified cuboidal epithelium
 * Two layers of cuboidal cells
 * Transitional epithelium
 * Looks like in transition between stratified cuboidal and stratified squamous
 * Lines most of urinary tract – uroepithelium
 * Undergoes considerable stretching
 * Non-distended uroepithelium – many layers of cells (5-10); large dome shaped cells on surface (cuboidal)
 * Distended uroepithelium – only a few layers (2-3); stretched layers almost like squamous

**Specialized Epithelium**

 * Specialized Epithelium – named on specialized functions
 * Sensory epithelium
 * Gustatory – taste
 * Covers tongue and contains taste buds
 * Olfactory – smell
 * Covers part of nasal passage and contains neuroepithelial cells
 * Stato-acustic – hearing
 * Covers part of inner ear and sense sound and proprioception
 * Germinal epithelium
 * Lines seminiferous tubules of testis – produces germ cells
 * Stratified but shapes are difficult to determine

=**Polarity of Epithelium**=


 * Epithelial cells have different domains separated by junctional complex

**Apical Domain**

 * Apical domain
 * Free luminal surface and contains membrane proteins such as carbohydrates forming glycocalyx important for recognition and adhesion
 * Capable of transport
 * Membrane specializations

**Microvilli**

 * Microvilli – small finger-like projections; non-motile; increase cell surface area
 * 1-2 μm, appear in 100’s or 1000’s on cells
 * Together, they form the brush border or striated border
 * Usually large amounts of fluid is being absorbed
 * Internal support system of actin filaments (25-30 filaments)
 * Villin – protein at tip that joins filaments
 * Fimbrin – protein cross-links actin filaments
 * Actin filaments embedded in terminal web at base of microvillus

**Stereocillia**

 * Large finger-like projections; non-motile; increase cell surface area
 * Same internal structure as microvilli but larger
 * Found in [|epididymis]

**Cilia**

 * Cilia – finger-like projections; motile; moves fluids on surface
 * Larger than microfillia
 * Core consists of microtubles called axoneme arranged in 9+2 pattern
 * 2 singlets with 9 peripheral doubles consisting of subunits B and A with 2 arms of dynein proteins
 * Attached to basal body by 9 triplets – basal body is thought to produce microtubules of axoneme
 * Dynein ATPase provides energy that causes arms to transiently bind to the B subunit, sliding the tip to cause bending
 * When Dynein releases subunit B, cilium snaps back upright
 * Beating of cilia are coordinated within one cell and with adjacent cells so all cilia beat in one direction
 * When cilia becomes immotile, can result in primary ciliary dyskenesis, immotile cilia syndrome, Kartagener’s syndrome
 * Structurally lacking Dynein arms and can’t clear respiratory tract of mucus, causing respiratory infections, pneumonia
 * Frequently infertile (eggs can’t get transferred through fallopian tube, resulting in atopic pregnancies) and have malrotation of the heart

**Flagella**

 * Flagella – largest single projection; motile; moves cells
 * Similar to cilia only larger
 * Tail of sperm

**Basolateral Domain**

 * Basolateral domain
 * Consist of lateral sides and base of cell
 * Biochemically different than apical domain
 * Characterized by junctional complex (a.k.a. Terminal Bar) and gap junctions
 * Junctional complex located near the top of the cell where adjacent cells attach to each other and runs circumference of cell
 * Three different junctions: zonula occludens, zonula adherens, macula adherens

**Zonula Occludens**

 * Zonula occludens (a.k.a. tight junctions)
 * Most superficial to apical surface
 * Fusion of membranes via transmembrane proteins claudins and occluding
 * Creates impermeable barrier – materials cannot pass between cells and have to get through epithelium cell
 * Extends around entire circumference of cell
 * Tight (i.e. blood brain barrier) vs. leaky (i.e. capillary), depending on function
 * Marks division of apical and basolateral membranes
 * prevents movement of membrane proteins from one domain to another

**Zonula Adherens**

 * Zonula adherens (a.k.a. Adhesion belt)
 * Intercellular gap between membranes, 15-20nm
 * Cadherins – transmembrane adhesion molecule or linker protein, extending across gap to hold membrane together
 * Bind to cytoskeleton – actin filaments form dense aggegates on cytoplasmic side linker proteins (vinculin and α-actinin) bind cadherins to actin
 * Function: mechanical attachment of adjacent cells

**Macula Adherens**

 * Macula Adherens (a.k.a. Desmosomes)
 * “spot weld”
 * Gap between membranes, 30 nm
 * Dense attachemt plaques on cytoplasmic side consist of desmoplakin and plakoglobin
 * Transmembrane proteisn, two cadherins, desmoglein and desmocolin, extend across gap to attach to each other
 * Intermediate filament attached to attachment plaques
 * Function: mechanical attachment of adjacent cells
 * Clinical relevance: Phemphigus vulgaris
 * Autoimmune disease – antibodies produced against cadherin and destroy desmoglein
 * Patients desmosomes are destroyed in skin, lose integrity of epithelial, causing severe blistering
 * Infections are life threatening

**Gap Junctions**

 * Junction found in the lateral domain
 * Functions in cell-to-cell communication
 * Memrabne separated by a narrow gap 2-3 nm thick
 * Disc of numerous pores or channels across membranes
 * Each pore is called a connexon, about 1.5 nm in diameter
 * Connexons are structured like a barrel of 6 subunits of connexin
 * Only small ions, small amino acids, cAMP, nucleotides and some vitamins can pass through
 * Larger molecules like proteins, nucleic acids and polysaccharides cannot pass
 * Some are signaling molecules distributed during cell migration and coordination of cilia beating
 * Point of low electrical resistance
 * Can pass along electrical impulse for contraction of smooth muscle to create a peristaltic wave in gut wall

**Basal Domain**

 * Plasma membrane enfolding - **tessellation**
 * Increases surface area for increased passage of fluids
 * **Basal lamina** – dense layer located outside cell just below basal membrane
 * Hold epithelium to connective tissue
 * **Hemidesmosomes**
 * Appears as half a desmosome on cytoplasmic side
 * Integrin is a transmembrane protein that binds cell membrane to basal lamina

=**Glands**=


 * Included as epithelium because they all form as an outgrowth of epithelium
 * Classified as exocrine versus endocrine glands
 * If connection to surface epithelium remains as a duct, then the gland is exocrine
 * Secretions go to the surface of the epithelium
 * If the connection to the surface epithelium is lost and does not have a duct, then the gland is an endocrine gland
 * Secretions go directly to the connective tissue and into the blood vessels
 * Glands are also classified by structure/morphology, mode of secretion, and type of secretion

**Structural Classification**

 * First structural characteristic is unicellular versus multicellular
 * Unicellular glands – consist of single secretory cells embedded in epithelial sheet
 * Multicellular glands – consists of numberous cells and can be divided intoa duct and a secretory portion

**Classification by Gland**

 * Glands named according to the (1) branching of the duct and (2) the shape of the secretory portion
 * Duct can be simple or compound if it branches
 * Secretory unit can be tubular or alveolar/acinar (and sometimes both)
 * Tubular glands can be very long and coiled
 * **Compound tubuloalveolar – salivary glands**
 * Surrounded by connective tissue capsule, subdivided by septa to form lobes and lobules

**Classification by Mode of Secertion**

 * Classified by mode of secretion
 * Can be holocrine, merocrine, apocrine, and cytocrine
 * Holocrine – disintegrating cell and contents is secreted; cell is dead
 * i.e. sebaceous glands
 * **Merocrine** – secretion is through vesicle fuses with membrane and extrudes contents without cytoplasmic loss
 * Most common form of secretion
 * Apocrine – Pinched off portion of the cell is secreted
 * Lipid portion of milk of mammary glands
 * Cytocrine – secrete a whole living cell
 * i.e. ovaries and testis

**Classification by Secretion**

 * Classification by class of secretion
 * Mucus – thick viscous secretion containing mucinogens
 * Slippery, lubricant
 * Cells stain lightly with foamy cytoplasm due to extracted mucus
 * i.e. globlet cells, sublingual salivary gland
 * Serous – Watery secretion, rich in ions or enzymes
 * Acidophilic (red stain) due to presence of secretory machinery
 * i.e. pancreas or sweat glands
 * Mixed Glands – combination of serous and mucus secretory glands
 * Contains serous demilunes, cresent or half-moon on outside of mucous alveolus
 * Sebum – oily secretion, rich in lipids
 * Acts as lubricant
 * i.e. sebaceous glands, meibomian glands of eyelids
 * Ceruminous – waxy secretion
 * Acts for protection of the ear
 * Ceruminous glands of external auditory canal

=**Objectives**=