LECTURE - Epithelial Tissue Flashcards
Epithelia mediate a wide range of activities
selective diffusion absorption secretion physical protection containment
all epithelia are supported by a ..
basement membrane
- separate epithelia from underlying supporting tissues and are never penetrated by blood vessels
- epithelia depend on diffusion of oxygen and metabolite from supporting tissues
classification of epithelia (3)
- number of cell layers
- shape of component cells
- presence of surface specializations
simple epithelia
- selective diffusion, absorption, secretion
- not found on surfaces subject to stress bc little protection against mechanical forces
simple epithelia
- selective diffusion, absorption, secretion
- not found on surfaces subject to stress bc little protection against mechanical forces
simple epithelia
- selective diffusion, absorption, secretion
- not found on surfaces subject to stress bc little protection against mechanical forces
simple squamous epithelium
- flattened, irregularly shaped cells forming a continuous surface = pavement epithelium
- line surfaces involved in passive transport (diffusion) of gases or fluids
- lining of of pleural (lung), pericardial (heart), and peritoneal (abdominal) cavities
simple cuboidal epithelium
- in sections perpendicular to basement membrane = square
- nucleus usually round; centre of cell
- lines small duct and tubules which may have excretory, secretory, or absorptive functions
simple columnar epithelium
- similar to cuboidal except cells are taller
- nuclei elongated; may be located towards base, center, or apex of cytoplasm
- found mostly on highly absorptive surfaces such as small intestine
simple columnar ciliated epithelium
- presence of cilia makes it different from simple columnar epithelium
- among ciliated cells are scattered non-ciliated cells which usually have a secretory function
pseudostratified columnar ciliated epithelium
- appearance of more than one layer of cells; but because all cells rest on basement membrane, this is true simple epithelium
- nuclei at different levels (illusion of stratification)
- almost exclusive to larger airways of resp. tract = respiratory epithelium
how do we distinguish pseudostratified from true stratified epithelia
- nuclei are mainly confined to basal two-thirds of epithelium
- clia are never present on stratified epithelia
stratified epithelia
- two or more layers of cells
- mainly a protective function; poorly suited for secretion/absorption due to thickness
- classification based on shape + structure of surface cells (basal cells usually cuboidal)
stratified squamous epithelium
- cuboidal basal layer to a flattened surface layer
- basal cell divide continuously to replenish surface cells that are lost
- lines oral cavity, pharynx, esophagus, anal canal, uterine cervix, vagina
keratinized stratified squamous epithelium
- specialized form of stratified squamous epithelium that contributes to epithelial surface of skin epidermis
- maturation = epithelial cells accumulate cross-linked cytoskeletal proteins = keratinization
- keratin allows epithelium to withstand the constant abrasion and dessication to which it is exposed
stratified cuboidal epithelium
- thin, stratified epithelium = only two or three layers of cuboidal or low columnar cells
- larger excretory ducts
transitional epithelium
- urinary tract
- features of both stratified cuboidal and squamous = transitional
relaxed (contracted) state = 4-5 cell layers thick ; stretched = 2-3 cells thick (intermediate and surface layers extremely flattened) - basal cells cuboidal for relaxed state, intermediate is polygonal, and surface cells are large and rounded and may contain two nuclei = binucleate cells; umbrella cells
- urothelium
intercellular surfaces
adjacent cells have cell junctions that permit the epithelia to form a continuous cohesive layer in which all of the cells ‘communicate’ and cooperate
three functional types of cell junctions:
- occluding junctions (tight)
- adhering junctions
- communicating junctions (gap or nexus junctions)
occluding junctions
- tight junctions
- immediately beneath luminal surface of simple columnar epithelium where they seal intercellular spaces and may prevent migration of floating membrane proteins from their appropriate domains on the cell surface
- each tight junction forms continuous circumferential band around the cell
adhering junctions
- tightly bind constituent cells of epithelium together
- act as anchorage sites for cytoskeleton each cell so cytoskeleton of all cells are effectively linked into a single functional unit
two types: - zonula adherens: deep to tight junctions of columnar epithelial cells, forms a continuous band around cell providing structural; reinforcement to occluding junction
- desmosomes (macula adherens): small circular patches or pots that are circumferentially arranged around columnar cells deep to the continuous zonula adherens
adhering junctions
- tightly bind constituent cells of epithelium together
- act as anchorage sites for cytoskeleton each cell so cytoskeleton of all cells are effectively linked into a single functional unit
two types: - zonula adherens: deep to tight junctions of columnar epithelial cells, forms a continuous band around cell providing structural; reinforcement to occluding junction
- desmosomes (macula adherens): small circular patches or pots that are circumferentially arranged around columnar cells deep to the continuous zonula adherens
communicating junctions (gap/nexus)
circular intercellular contact areas containing hundreds of tiny pores which permit the passage of small molecules between adjacent cells
junctional complex
tight junctions, zonula adherens, desmosomes
the luminal surface may incorporate three main types of specialization:
- cilia
- microvilli
- stereocilia
cilia
- relatively long
- motile structures which project in parallel rows from certain epithelial surfaces (resp and female reproductive tract)
- beat with a wave-like synchronous rhythm propelling surface films of mucus or fluid in a consistent direction over the epithelial surface
- axoneme = central core; 20 microtubules arranged as a central pair surrounded by nin peripheral doublets; base = inserts into basal body
- each axoneme doublet = one tubule which is c-shaped in cross-section
- dynein extends towards incomplete tubule of adjacent doublet and towards central microtubule pair
microvilli
- minute finger-linked projections of luminal PM
- can be seen as brush border on surface of cells
- usually for absorption; can increase SA available for absorption by up to 30x
- cytoplasmic core = fine actin filaments which insert into terminal web
- terminal web anchored to zonula adherens at periphery of cell
stereocilia
- extremely long microvilli
- found in small number in parts of male reproductive tract
- filamentous skeleton inside
- may facilitate absorption
basal surface
- hemidesmosomes are present on the inner aspect of basal plasma membrane adjacent to basement membrane and provides a means of anchorage of cell via cytoskeleton to basement membrane and underlying supporting tissues
gland that discharge their secretory product via a duct onto an epithelial surface
exocrine gland
exocrine glands classified according to two major characteristics
MORPHOLOGY
- simple: single, unbranched duct; secretory portions of simple glands have two main forms = tubular or acinar (which may be coiled and/or branched)
- compound: branched duct system and their secretory portions have similar morphological forms as simple glands
MEANS of discharge of secretory products from cells:
- eccrine secretion: involves process of exocytosis; proteins are major secretory product
- apocrine: discharge of free, unbroken membrane-bound vesicles containing secretory product; applies to lipid secretory products in beast and some sweat glands
- holocrine: disintegration of secretory cell to release secretory product; sebaceous glands
secretory products of some exocrine glands are embraced by contractile cells called
myoepithelial cells
ductless glands
endocrine
- secretory products diffuse directly into bloodstream
secretory products of endocrine glands
hormones
- control activity of cell and tissues far removed from the site of secretion
- most consist of groups or cords of secretory cells surrounded by a rich network of blood vessels; each group of cells surrounded by basement membrane reflecting its epithelial origin
- release hormones into intercellular space and then diffuse rapidly into surrounding blood vessels
epithelial cells are closely bound to one another by a variety of membrane specialization called …
cell junctions
- provide physical strength and mediate exchange of ‘information’ and metabolites
