Epithelial Tissue Study Materials

Features of Epithelial Tissues  Closely attached cells with very little extracellular matrix and intercellular spaces  Cell to cell attachments is strong  Junctional complexes attach cells together in different ways  Epithelium present in two form: 1. Flat or curved sheets that lining surfaces (G.I. Tract and vasculature) 2. Glands  Epithelial tissues are avascular  All epithelia have a basal lamina and supported by a connective tissue Functions of Epithelia Covering and lining surfaces (skin) Protection (skin) Absorption (intestine) Secretion (glands of skin & GI tract) Sensation (taste buds, olfactory epithelium) Contractility (myoepithelium of glands) Epithelial Cells Lateral Membrane Specialization ⊕ Lateral membrane specialization reveal the presence of terminal bar ⊕ Terminal bar is where the cells are in close contact to each other and attach by junctional complexes consist of: • Tight junction (zonulae occludentes) • Belt desmosome (zonulae adherents) • Gap junction • Desmosome (maculae adherents) Tight junction (zonulae occludentes) Tight junction is located between adjacent plasma membrane and form a belt around the cell Outer leaflets of cell membrane fuse together Transmembrane proteins Occludin, Claudin and Cadherin bind membrane together Proteins form tight junction strands Tight junction prevent seal paracellular pathway Prevent movement of membrane protein between apical and basolateral domain Belt desmosome (zonulae adherentes) Zonulae adherentes are located just basal to tight junction in the form of a belt Plasma membranes separated by 15 – 20 nm and occupied by Cadherins Cadherin is an integral protein In cytoplasmic aspect a bundle of actin filaments bind to cadherin Actin filaments bind together and to cell membrane by means of - actinin and vinculin Desmosome (maculae adherentes) Desmosome randomly distributed along lateral cell membrane and are spot like Attachment plaques are disk like structures located opposite each other on cytoplasmic aspect of cell membrane Desmoplakins and Pakoglobins are some proteins forming plaque Intermediate filaments insert into plaque like a hairpin Inercellular space is up to 30 nm and contains filamentous materials include of Desmoglein Gap Junction Gap junction composed of an ordered array of 6 subunits Membrane separated at gap junction by a 2 nm space Each subunit is a transmembrane protein named connexins Connexins assembled to form a structure which is called connexons When two connexons register together forming the functional unit of intercellular communication Gap junction are not limited to epithelia, neurons smooth and cardiac muscle cells, have also gap junction Basal Surface Specializations Basal surface specializations include of basal lamina, plasma membrane enfolding and hemidesmosome Basal lamina is a supportive structure composed of collagen type IV, laminin and proteoglycans Plasma membrane enfolding increase the surface area and it is well developed in ion-transporting cells Form by deep invagination of plasma membrane Many mitochondria are located in finger like enfoldings Ion pumps are brought into association with mitochondria Hemidesmosome Hemidesmosome is an attachment specialization It is like of one-half desmosome Hemidesmosome located at basal surface of the cells in certain epithelia It attaches the basal membrane to the underlying basal lamina Attachment plaques composed of desmoplakin and other proteins Keratin tonofilaments insert into these plaques but not looping Apical Specializations of Epithelial Cells • Microvilli – in kidney and small intestine • Stereocilia – epididymis, hair cells of organ of Corti • Cilia and Flagella – respiratory tract, oviduct, sperm Microvilli Microvilli are finger like projections 1 µm in length Comprise the brush border(PCT in kidney) and striated border(Intestinal absorptive cells) Each microvilli contains about 30 actin filaments that extend to terminal web Myosin-I and calmodulin connect actin filaments to plasma membrane Villin cross link actin filaments to each other Terminal web is a combination of actin, spectrin and intermediate filaments at the cortical zone of epithelial cells Stereocilia Stereocilia are long microvilli They are found in epididymis and in cochlea of inner ear They are nonmotile Increase surface area in epididymis Signal generation in ear Cilia and Flagella Cillia are motile long projections which have 7 to 10 µm length In respiratory system and oviduct cells have many cilia They covered by plasmalemma and contains an axoneme Axoneme consists of 9 + 2 arrangements of microtubules( 9 doublets around 2 central singlets) Doublets have two subunits A and B Subunits A posses 13 and B 10 protofilaments Nexin connects adjacent doublets to each other Radial spokes project from subunit A to central sheath Cilia and Flagella Dynein which has ATPase activity radiate from subunit A to subunit B of neighboring doublet Basal body located at the base of cilia Basal body is like centriole It is composed of 9 triplets Flagella is found just in spermatozoa Types of Epithelia Covering epithelia Glandular epithelia Epithelial Cell Shapes Squamous ( nucleus flattened) Cuboidal (nucleus spherical) Columnar (nucleus long, irregular) Types of Covering Epithelia • Simple – one layer of cells • Stratified – more than one layer of cells Types of Simple Epithelium • Simple squamous – endothelium of vessels; mesothelium • Simple cuboidal – kidney tubules, thyroid follicle, serosa of some organs • Simple columnar – small intestine, colon, stomach Types of Simple Epithelium Types of Stratified Epithelium Stratified squamous keratinized • The free surface cells are dead and filled with keratin • Skin Stratified squamous nonkeratinized  Supermost cells are nucleated  It is usually wet  Oral cavity, esophagus, vagina Stratified cuboidal )It is consists of two layers of cuboidal cells )Sweat gland duct Stratified columnar • Two or more layers of cells • Conjunctiva, duct of pancreas Transitional Epithelium It is a kind of stratified epithelium Basal cells located on basal lamina and are cuboidal Above the former are several layer of polyhedral cells Superficial cells are dome shaped when relaxed When stretched layers reduced to only a few layers of flattened cells The ability of the surface cells to expand and relax lies in stores of plasma membrane as membrane vesicle in the apical cytoplasm at relax state As the organ fills with urine, the surface cells mobilize their membrane vesicles rapidly Urinary bladder Pseudostratified epithelium This epithelium is composed of a single layer of cells All cells are in contact with basal lamina Cells have different height Only some cells reach the free surface Respiratory tract, nasal cavity Specialized Epithelia Neuroepithelium: taste buds, organ of Corti, olfactory epithelium Myoepithelium: secretory glands such as salivary glands, sweat glands and mammary glands Myoepithelial Cell • Satellite or spindle shaped • Base of acinar and duct cells in glands • Contain actin and myosin Types of Glandular Epithelia Unicellular glands Exocrine: goblet cells Endocrine: DNES - - Multicellular glands – Exocrine glands: have ducts • (salivary glands, lacrimal glands, mammary glands, etc.) – Endocrine glands: have no ducts • cords of cells with vessels (adrenal cortex, parathyroid, anterior pituitary) • follicular (thyroid) Unicellular Exocrine Gland Unicellular glands are the simplest form of a gland Goblet cells are the primary example Their basal region is thin Apical portion is expanded which is called Theca The theca filled with secretory granules Classification of Multicellular Exocrine glands Based on Architecture • Simple: one unbranched duct – – – – – Tubular Coiled tubular Branched tubular Acinar(Alveolar) Branched acinar • Compound: branched ducts – Tubular – Acinar – Tubuloacinar Classification of Multicellular Exocrine glands Based on Mechanism of Secretion Merocrine: membrane-bound granules by exocytosis; pancreas, salivary glands Holocrine: secretions released by bursting of whole cell; sebaceous glands Apocrine: secretions released as droplet with cytoplasm from apical surface; mammary glands and apocrine sweat glands Classification of Multicellular Exocrine glands Based on Type of Secretion Mucous glands (goblet cells, minor salivary glands) This glands secrete mucinogens composed of large glycosylated proteins When hydrated swell and become a viscous gel like lubricant named mucin which is a major component of mucus Serous glands (pancreas) Secrete a watery enzyme-rich fliuid Mixed glands (Submandibular gland) Contain mucous acini and serous acini and mucous acini with serous demilunes Unicellular Endocrine Glands This secretory cells produce paracrine or endocrine hormones They are among other cells in digestive tract and respiratory sytem Form diffuse neuroendocrine system They also called APUD cells Some of them is called Argentaffin or Argyrophil cells because they stain with silver salts Multicellular Endocrine Glands Release their products into blood or lymphatic vessels Secretory cells of endocrine glands are in the form of cords or follicles In the follicle type; hormone accumulate in a cavity