Histology Lecture 4: Epithelia PDF

IN THE NAME OF GOD THE COMPASSIONATE THE MERCIFUL  Histology Lecture 4 Epithelia  Features of Epithelial Tissues  Closely attached cells with very little extracellular matrix and intercellular spaces  Cell to cell attachments is strong  Junctional complexes attach...

IN THE NAME OF GOD THE COMPASSIONATE THE MERCIFUL  Histology Lecture 4 Epithelia  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 Department of Histology HR Mahmoudzadeh Sagheb Z Heidari MH Noori Mugahi