Epithelium.pdf

 Epithelium-a continuous sheet of closely adherent cells covering a naturally occuring surface in the body  It may consist of single layer of cells or multiple layers  The epithelium of different organs may be made up of cells specialized for absorption, secretion, or ion transport  B...

 Epithelium-a continuous sheet of closely adherent cells covering a naturally occuring surface in the body  It may consist of single layer of cells or multiple layers  The epithelium of different organs may be made up of cells specialized for absorption, secretion, or ion transport  Basal Lamina- where lowermost cell rest, comprises a very fine meshwork of filaments in an amorphous matrix  Classification of Epithelia  Several types of epithelium are based upon the number of cell layers, the shape of the cells, and the presence or absence of specializations of the free surface  Simple epithelium- single layer of cells  Stratified epithelium- multiple layers of cell  Modifiers such as squamous,cuboidal, or columnar are added to indicate shape of the cells  Ciliated-term added if there is a slender motile processes on their free surface Classification of Epithelia 1. Simple Epithelium- lines the thoracic, abdominal and pericardial cavities of the body 2. Stratified Epithelium- constitutes the epidermis of the skin, and is found in the lining of the oral cavity, the esophagus and the vagina 3. Pseudostratified Epithelium- found in the male urethra and the duct of the parotid gland 4. Transitional Epithelium- lines the urinary bladder - Found throughout the urinary tract, from the calyces of the kidneys to the urethra  Simple squamous epithelium- closely adherent cells that have polygonal outline in surface view. And in section,are fusiform. Lines the thoracic, abdominal and pericardial cavities of the body  Mesothelium-simple squamous epithelium lining a body cavity  Endothelium- lining blood vessels  Simple cuboidal epithelium- polygonal in surface view, and in section, have a rectangular or square outline. Lines the ducts of many glands and some of the tubules of kidney  Simple columnar epithelium- tall narrow, rectangular profile, and their nuclei are all aligned at the same level. Found lining stomach  Stratified squamous epithelium- multiple layers, & the cell shape changes from the basal to the uppermost layer of the epithelium. Found in epidermis of the skin, lining of the oral cavity, esophagus, and vagina  Keratinized stratified squamous epithelium- found on the palms of the hand and soles of the feet  Stratified columnar epithelium- the basal cells are cuboidal, or low columnar, & the cells of the upper row are tall columnar. The nuclei are aligned in 2 distinct rows. This type of epithelium is uncommon & found only in the conjunctiva of the eye, the cavernous urethra,& in the large excretory ducts of some glands  Pseudostratified columnar epithelium- all cells rest on the basal lamina, but some are columnar in form and extend upwards to the free surface, while others located between them have tapering upper end, which extends only part way to the surface. The nuclei of these 2 kinds of cells are aligned at different levels, creating a false impression of stratification. They are found in the male urethra and duct of parotid gland  Ciliated pseudostratified epithelium- occurs in a portion of the trachea, primary bronchi, auditory tube , & lining a portion of the tympanic cavity of the inner ear  Transitional epithelium- lines the urinary bladder. A special epithelium has evolved to adapt to the changes in volume of this organ during its filling and emptying  Epithelial Polarity  A cell is polarized, if it functions are preferentially directed towards one end  Morphological evidence of cell polarity is found in: a) apical specializations that amplify the area of their free surface b) supranuclear position of the Golgi complex c) the accumulation of secretory products in the apical cytoplasm  Diagram of the Polarity of the Columnar Epithelial Cell  Function  Components  Apical Plasma Membrane  Apical Plasma Membrane  1)regulation of nutrient and  Glycolipids ,cholesterol, H/K water uptake 2)Regulated ATPase, & anion channels secretion 3)Protection  Lateral Plasma Membrane  Lateral Plasma Membrane  1)Cell contact and adhesion  1)Cell adhesion molecules 2)Cell communication 2) Junctional Complex  Basal-lateral Membrane  Basal-lateral Membrane  1)Signal reception &  1)Binding sites for transduction 2) generation of constituents of basal lamina ion gradients 3) Constitutive 2)receptors for hormones & secretion other signaling molecules that regulate the function of cell  Cell Cohesion  Cell adhesion molecules(CAM)-family of glycoproteins in the cell membrane that maintains the cohesion and arrangement of cells in an epithelium  Cadherins- are responsible for the mutual recognition & cohesion of similar cells  4 Kinds of Junctional Complexes  Zona occludens(tight junction)-belt- like membrane specialization that encircles the columnar cells just below the surface of the epithelium  Zonula occludens- enable cells to exercise selectivity;serve as a barrier between the apical and basolateral domains of the plasma membrane; prevent movement of integral proteins from one to the other  2 possible pathway across an epithelium  1)Trancellular Pathway- involves the active uptake of substances at the free surface by pinocytosis, followed by transport of the vesicles across the cell and release of their content at the cell base  2)Paracellular Pathway- molecules passively diffuse through the intercellular spaces of epithelium  Zonula adherens- encircles the apical portion of the cells below the zonula occludens  Site of cell-cell adherence  Have a role in stabilizing the epithelium by connecting the terminal webs of adjoining cells  Desmosomes(macula adherens)- they are small , round, junctional specializations on the sides of adjoining epithelial cells  Site of cell-cell attachments  Contribute to the stability of the epithelium as a whole by linking the cytoskeletons of adjoining cells  Hemidesmosomes- found in stratified squamous epithelium. Enable the epithelium to withstand the blows and shearing stresses to which the skin is exposed, without becoming separated from the basal lamina  Cell Communication  Gap Junction(nexus)-a junctional complex that is primarily concerned with cell to cell communication  They are abundant in smooth and striated muscle and ensures their simultaneous contraction  Connexons- project into the intercellular space, of opposing membrane, where they are linked end to end  Ions, amino acids, cyclic AMP & other molecules less than 2nm in diameter would be able to pass freely through this channel Basal Lamina  Extracellular layer found beneath all epithelia  Lamina lucida- transparent layer adjacent to epithelium. It is composed of large proteoglycan,laminin, & atypical collagen (type IV)  Lamina densa- meshwork of fine filaments of another uncommon type of collagen (type VII)  Anchoring fibers of this collagen extend downwards from the basal lamina, looping around type I collagen fibers in the underlying extracellular matrix Basal Lamina Surface Specializations  Brush border- exhibit fine vertical striations,that are refractile at apical border of columnar epithelia. Occurs on epithelia specialized for absorption. In epithelia specialized for rapid absorption of electrolytes & water the brush border is supplemented by basal infoldings of the plasmalemma, which amplify its surface area  Made up of closely spaced, slender cell process 1-2 micrometer long, and 80-90nm in diameter called microvilli  Glycocalyx-very fine branching filaments extending from their tips may form a furry coat over the border  Cilia-motile cel processes 7-10 micrometer in length and 0.3micrometer in diameter, which are present in great numbers on the free surface of certain epithelia  They are arranged in rows, and all beat in the same direction  Epithelial Renewal  Epithelial cells have a limited lifespan,and some are continually lost by exfoliation, or by programmed cell death, and are replaced by mitotic division of other cells  Absorptive Epithelia- composed of cells specialized for uptake of ions and small molecules from the lumen, and their movement across the epithelium to the extracellular fluid beneath the basal lamina  Secretory Epithelia-are specialized for the synthesis of a specific product and its release onto an external or internal surface  Exocrine Glands  Glands that deliver their secretion onto the surface of an external or internal surface of the body  Glands are classified according to the arrangement of their ducts and secretory units  A) Simple glands- single unbranched duct  B) Compound glands- highly branched duct  Modifiers are added to indicate whether the secretory portion is straight, coiled or branch  Ex: 1.simple tubular (skin) 2. simple branched tubular(sebaceous) 3. simple coiled tubular(penile and urethra) Classification of Exocrine Glands  According to the configuration of their duct system: simple acinar , or branched acinar (stomach)  Compound tubuloacinar glands- secretory units at the ends of a branching system of ducts are short tubules that have acini along their sides and at their ends (sublingual)  Goblet cells- solitary secretory cells may be found widely distributed among the absorptive cells of an epithelium such as that lining the small intestine  They have a narrow basal region & greatly expanded apical region, containing stored secretory product  Different types of glands according to the physical properties of their secretory product  Mucous glands- produce a viscous surface layer of mucopolysaccharide  Serous glands- produce a watery secretion, which is often rich with enzymes  Merocrine- product is released by fusion of membrane of granules with cell membrane( ex : apocrine sweat gland)  Holocrine- whole cells are exfoliated and the secretion consist consists of residuesof those cells as well as the product they have synthesized( ex: sebaceous)  Apocrine- secretory cells pinch off their apical portion containing the secretory product( ex: mammary gland)  Endocrine glands  Do not deliver their secretion through a system of ducts; their product is a protein or steroid hormone that is released into the surrounding extracellular fluid and diffuses into the blood, which carries it to distant target organs that have specific receptors for that hormone