His To Epithelial

Epithelial Tissue

Ma. Minda Luz M. Manuguid, M.D.

Epithelia: General Considerations 

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Epithelium is avascular tissue composed of cells that cover the exterior surfaces of the body and line both internal closed cavities and body tubes that communicate with the exterior (the alimentary, respiratory, and gastrointestinal tract). Epithelia form the receptors of certain sensory organs. Epithelia form the secretory portions (or parenchyma) of glands and their ducts (a.k.a. glandular epithelium or exocrine glands).

Functions of Epithelia  

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Mechanical barrier Permeability barrier Absorption Filtration Secretion Sensory receptors

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Skin Blood – brain barrier GIT mucosa Glomerulus Exocrine glands Retina, Olfactory epithelium

Epithelial Tissue General Features 

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Epithelial tissue lines body surfaces Avascular; dependent on underlying connective tisssue for oxygenation & nutrition Cells are polarized, with apical & basal poles Cells lie on a basement

Epithelial Tissue Classification Classification / Nomenclature depends on A. number of cell layers : 1. simple- single layer of epithelial cells 2. stratified- two or more cell layers

B. morphology of epithelial cells: 1. squamous- flattened, plate-like cells with central round to ovoid nuclei 2. cuboidal- cube-like, width more or less equal to height

Simple Squamous Epithelium  Simple Squamous

Epithelium- single layer of squamous cells: endothelium- lines the inner surface of the circulatory system; mesothelium- lines outer/inner surfaces of membranes enclosing visceral organs, e.g. pleura of the lungs, pericardium of the

Simple Squamous Epithelium Other sites :  alveolar sacs  outer surface of gallbladder, stomach, uterus,

Simple Cuboidal Epithelium  Simple

Cuboidal Epitheliumsingle layer of cuboidal cells lining o Thyroid

follicles o Renal tubules

Simple Columnar Epithelium 

Simple Columnar Epitheliumsingle layer of columnar cells lining o

o o

Alimentary tract Endometrium Endocervix

Simple columnar epithelium

Pseudostratified Columnar Epithelium 

Pseudostratified Columnar Epithelium- single layer of very tall columnar cells, the nuclei of which are oriented at different levels, giving the appearance of stratification; o

Ciliated- lines the

Pseudostratified Columnar Ciliated

Transitional Epithelium  Transitional

Epithelium or Urothelium- true stratified epithelium with variable number of cell layers, depending on the status of the organ being lined; less number of layers when organ is distended; exclusive to the urinary tract: o Ureter

Stratified Squamous Epithelium  Stratified Squamous Epithelium –

several layers of squamous cells, the uppermost layer composed of flattened cells; further subclassified based on the presence/absence of keratin, into: o

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Keratinizing “Dry” Stratified Squamous epithelium- lining body surfaces that open directly to the outside, or are subject to irritation /abrasion Non-keratinizing “Moist” Stratified Squamous epithelium- lining body surfaces that do not communicate directly with the

Stratified Squamous Epithelium, Keratinized

Stratified Cuboidal Epithelium  Stratified

Cuboidal Epitheliumuppermost layer is composed of cuboidal cells; rare in the human body; found in o small parts of the

sweat gland ducts o some parts of salivary glands o in the

Stratified Cuboidal Epithelium

Stratified Columnar Epithelium 

Stratified Columnar Epitheliumuppermost layer is composed of columnar cells; rare in the human body; found only in  

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the conjunctiva some parts of the salivary glands parts of the male urethra

Special Epithelia  Special Epithelia are those that

cannot be included in any of the categories previously discussed  Ovarian/Graafian follicle  Seminiferous tubule  Chorionic villi of the Placenta

Special Epithelia

Glandular Epithelium        

Simple tubular Simple coiled tubular Simple branched tubular Simple acinar Simple tubulo-acinar Compound tubular Compound acinar Compound tubulo-acinar

Glandular Epithelium 

Simple tubular glands - These glands are epithelial-lined tubules, which open on the apical surface. There are three types. 

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Simple straight tubular glands - crypts of Lieberkühn, located within the small intestines, run a straight, unbranched course. Simple coiled tubular glands - within the dermis, eccrine sweat glands Simple branched tubular glands - found in the stomach, esophagus, oral cavity, tongue, uterus

Simple tubular glands: straight, coiled, & branched

Glandular Epithelium 

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Simple alveolar (acinar) glands – several acini attached to a single duct: paraurethral glands in the penile urethra, sebaceous glands in the skin, Meibomian glands (eyelids) Simple tubular-alveolar glands - some of the secretory cells are arranged as acini (alveoli) and others are arranged as tubules: some of the smaller glands of the respiratory tract; minor mucous salivary glands within the oral cavity,

Simple alveolar & tubuloalveolar

Glandular Epithelium 

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Compound tubular glands - have a highly branched duct system. The secretory cells at the ends of the ducts are in the form of tubules. Brunner’s glands of the duodenum Compound alveolar glands - similar to the compound tubular and compound tubulo-alveolar glands but the ducts end in alveoli with dilated sac-like lumina: exocrine pancreas and parotid glands Compound tubulo-alveolar glands – glands that also have a highly branched

Compound glands

Compound tubular (Brunner’s) glands

Compound tubulo-alveolar

Modifications in Epithelial Cells 

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Microvilli – outward cylindrical folds of the apical membrane that markedly increase surface area; primarily specialized for absorption although, since they contain actin filaments, some movement (extension and contraction) may occur: renal PCT (“brush border”) Stereocilia – long, large microvilli : male reproductive tract;

Apical Modifications 

Cilia – multiple short motile hairlike organelles that can move in one plane only; the beating motion is designed for moving fluids along the surface, but some absorption may occur: respiratory tract

Epithelial Cell Modifications 

Junctional complex – Terminal bar: zonulae occludens &adherens  Zonula occludens – encircles the entire cell (permeability barrier) near the apex  Zonula adherens – “intercellular glue” – also encircles entire cell; just below the z occludens

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By light microscopy the junctional complex is seen as a dot and is called the terminal bar. It is resolved into at least 3 components by electron microscopy, the zonula occludens that provides a seal and for physical attachment the zonula adherens and macula adherens. The basement membrane is a layer of extracellular matrix as seen with the light microscope. It consists of at least two regions that are identifiable with

Basal modifications 

Basement membrane – structure on which epithelial cells rest – structure depends on location & function of epithelia: thin in capillaries, thick in the trachea, thick & rugged in the epidermis; laminin, proteoglycans, highly glycosylated type IV collagen 

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Basal lamina – amorphous; made by epithelial cells Reticular lamina – fibrous; made by connective tissue cells

Hemidesmosome – anchors the epithelial

Epithelial Cell Modifications  

Hemidesmosome Basement membrane 

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Basal lamina – type IV collagen, heparan sulfate, fibronectin and laminin. It is usually separated from the epithelium by an electron clear band (lamina rara) thought to be bridged by laminin and fibronectin. These molecules have sites that bind more or less tightly to collagen, heparan and integrins that span the cell membrane. Reticular lamina – a rather thick layer of reticular fibers (type III collagen) in an amorphous matrix. It is produced by the connective tissue and is argyrophilic

Epithelial Regeneration 

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Stem cells – undifferentiated, capable of mitosis – located at the basal portions of epithelium, produce daughter cells that move toward the apex Stratified squamous epithelium of the epidermis – stem cells in the stratum basale/germinativum Simple columnar Intestinal epithelium – stem cells at the base of intestinal

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