Histology: Epithelium Flashcards
What is epithelium?
the thin tissue forming the outer layer of a body’s surface and lining the alimentary canal (esphogous) and other hollow structures.
What does the epithelium do? (5)
1) keeps things in (tissue& organ boundary)
2) keeps things out (barrier from disease)
3) directs traffic (ducts/vessels)
4) enable transport across gradients
5) secrete stuff (glands)
components of epithelia?
surface, glandular, & special epithelia
components of connective tissue?
- connective tissue proper
- tissues and blood
- cartilage & bone
components of muscular tissues?
-smooth, skeletal, & cardiac
components nervous tissues?
- autonomic, motor, central
how are epithelial cells nourished?
- by diffusion from capillaries in underlying connective tissue
Why are some epithelial cells more or less active than others?
- cells closets to the source of nutrients (capillaries) get most nutrients, more active, divide more to give rise to the upper cell layer
- activity decreases as move away from capillary, leading to differentiation in cell types and death if TOO far away
how are epithelia cells identified/ defined?
-sheet of cells sitting on a basement membrane
The basement membrane?
1) provides solid support & integrity so epithelia cells don’t shift relative to eachother
2) gives cells something to attach to
3) most components made by epithelial cells themself
4) help define polarity of epithelium ( top vs bottom)
Basement membrane vs apical membrane?
- basement membrane= bottom of cell (leads to tissue/ organ)
- apical membrane = top surface of epithelia (has microvilli, borders lumen)
-diff functions & specializations define them
How does epithelial sheet have polarity?
-because has an inside (basolateral) vs outside-lumenal (apical)
What does polarity provide/do?
1) provides barrier to hold things in/keep things out
2) surface to organize differentiated structures
3) ability for transport down tubes
4) boundary for directed movement (selective permeability) to ions, molecules etc
Apical boundary specializations?
- has microvilli that define apical/lumenal space, absorb nutrients from capillaries
- Junctional complexes between neighboring epithelial sites
Basal membrane specializations?
1) transport functions (selectively permeable. membrane & transport proteins)
2) functions as barrier
Two components of the basement membrane? Why is distinction important?
- the basal lamina
- reticular fibers
*distinction between basal lamina and membrane is important for function & structure of lungs & kidneys
What is the basal lateral lamina?
- an ordered combo of type IV collagen, proteoglycans, & glycoproteins
- acts as size & charge filter to regulate access to the epithelium
- thickness & properties vary according to the tissue
What are reticular fibers?
- type III collagen wrapped in glycoprotein coat
- serve to anchor the basal lamina to the underlying general connective tissue
- provide integrity to epithelial sheets
Anchorage of cells to the basal lamina?
- done via hemidesmosomes
interactions between transmembrane protein of the cell & basal lamina provide?
1) Stimuli for polarization & differentiation of epithelial
2) regulation of epithelial proliferation
Simple epithelia?
when only one layer of cells sits on the basement membrane
- what most tissue is composed of
1) simple squamous
2) simple cuboidal
3) simple columnar
4) Pseudostratifed columnar
Simple squamous epithelia?
-cells look like flat, fried eggs
Most common= lining blood vessels (endothelial cells)
Simple cuboidal epithelia?
-cells look like cubes
-nucleus in center
-if not squamous or columnar…are cuboidal
Most common= line ducts since help move secretions; not usually secretory cells
Simple cuboidal epithelia?
-cells look like cubes
-nucleus in center
-if not squamous or columnar…are cuboidal
Most common= line ducts since help move secretions; not usually secretory cells
Pseudostratified columnar?
-looks stratified, but all cells sit on the basement membrane so NOT stratified
-nucleus at diff levels
Most common: airway epithelium
stratified epithelia?
- when have more than 1 layer of cells siting on basement membrane
- epithelium is defined by shape of cells in TOP layer
- occurs when epithelial membrane are actively diving, old cells move up as new cells are formed
- so has active layer (on basement) and inactive layer (apical side)
transitional-stretched epithelia?
-stratified epithelia
-looks like stratified squamous, but cells are bi-nucleated and surface is very pink
-tissue able to handle stretch while maintaining barrier function & integrity
Most common: bladder
transitional un-stretched?
-looks like stratified columnar bit are gaps between the surface cells and basement membrane making basement membrane look wavy…which allows for stretch
Most common: bladder
Simple squamous function?
- lining of blood vessels
- if lost in endothelium of blood vessels
Simple squamous structure-function?
- lining of blood vessels
- if lost in endothelium of blood vessels will result in blood clotting
- is lost in mesothelium will cause adhesions
-passive diffusion
simple cuboidal structure-function?
- line ducts & help move secretions
- increased structural rigidity for tubes compared to just simple squamous
- increased activity of the cell (secretion/absorption)
simple columnar structure-function?
- further increased structural rigidity for tubes
- high cell activity, this form packs as many cells as possible into the epithelium
pseudo stratified columnar structure-function?
- same properties as columnar as well as:
- regeneration from basal cells which are in area that is highly subject to damage, so needs very active cells to replace damaged one
stratified cuboidal functions?
- inactive surface cells, but only two cells thick so is a non-problem
- strong wall for tubes
ex: sweat glands
stratified columnar function?
- stronger walls (than cuboidal)
- are the walls of large tubes & ducts
stratified squamous function?
- best protection against outside world
- but surface cells can’t absorb/secret
- no dead keratinized layer in wet areas but have best protection when it is present
- this layer also helps prevent dehydration
transitional epithelia function?
protection and stretch of the epithelium itself
-ex: urinary bladder
How ID sepithelia?
1) ID basement membrane, allows you to see polarization
2) the connective tissue below basement membrane
3) luminal side (where blood flows) on the apical side
How ID simple cuboidal epithelium?
- nucleus centered
- seen in tubules/ ducts (salivary glands, kidneys)
- not always clear where basmenet membrane is, so look for lumen (apical side) where material is flowing
- ducts situated in complex tissue
How ID transitional epithelium? (airway vs intestinal tract)
1) Basement membrane not as clear depending on looseness of connective tissue
2) apical tissue specialized w/ microvilli in intestines (is absent in airways, have cilia instead))
3) look jumbled when not stretched
Cillia vs. micro villi with transitional epithelium?
- cillia in the airways, have microtubules that help sweep things through the lumen
- microvilli in intestines to help absorb nutrients, have actin filament but microvilli don’t move since all are crosslinked
How ID pseudostratifed vs. stratified columnar epithelium
pseudo: in airway epithelium, has cilli not microvilli
stratified: when need extra layers to maintain integrity of barrier or in salivary glands where 2 cell layers are needed
3 types of junctions in epithelial cells?
1) Adhesive junctions (provide seal between epithelial cells and basement membrane)
2) sealing junction (tight junction, between neighboring epithelial cells)
3) communicating junctions (gap junctions, between epithelial cells)
Types of adhesive junctions (3)
1) hemidesmosomes: cell to basement membrane adhesion
2) desmosomes
3) adherens
Desmosomes?
Mechanical boundary keeping cells stuck together
- cadherins (glue made by transmembrane proteins) hold cells together via spot welds
- keep epithelial cells from shifting relative to eachother; therefore keeps basement membrane barrier tight & intact
Hemidesmosomes ?
-keep cells connected to basal lamina via connective tissue
Hemidesmosomes & Desmosomes together?
- desmosomes keep cells connected to eachother; Hemidesmosomes keep them connected to the basal membrane
- together ensure that cells don’t shift or fall off basal membrane
Adherens junctions?
- epithelial cells in a sheet use these to control permeabiltiy across barrier
- use cadherins as glue between the cells, keep them together and plug gaps
- use actin filaments since spread pull through the cell
Tight Junctions
zonula occludens
- form selective seal to limit/ prevent fluids dissolved in substances passing between the space between epithelial cells (prevents passing from one side of epithelium to the other)
- at apical end
- some cells require extremely tight junctions, other can handle looser connections
tight junction selectivity depends on?
the number of rows of protein complexes& the charge on the protein that make up the complexes.
- proteins are made by both neighboring cells and are transmembrane
- permeability is not static, can be regulated
gap junctions
- communication between cells
- selectively allows molecules to go between epithelial cells via connexons channels (6 connexin combined)
- allows spread of message via a single point & cells to respond to signals they may not have had receptor for
more than 1 junction?
- yes can form complexes
- different junctions can work together to help maintain integrity and boundary of epithelia
- diff tissues have diff combo of junctions
- ex: junction to hold cells to eachother, the basal lamina, and prevent flow of molecules between them
glands and 2 main types?
- develop embryologically from surface of epithelia
1) endocrine (lost polarity): when connection to surface is lost, secrets apically into blood.
2) exocrine(have polarity): when connection to surface is maintained, secrets onto original surface
complex exocrine glands?
- have branched ducts, seen in pancreas/kidneys
- if have branching and get a blockage can have different pathology within the gland
acinar?
- collections of exocrine secreting gland cells (still epithelial w/ poalrity)
- secreting into the gut/ organ they are on
shapes of secretory glands?
- secretory portions of glands have a variety of shapes,
- can occur on single or branched ducts
Globlet cells look? what are they?
- round and clear; near a stem going toward basement membrane
- intestinal endothelial cells that secret mucuous into lumen
- aren’t gland structure, single cells distributed in intestine
types of exocrine secretions?
1) mucuous
2) serous
3) mumcoserous- mixed glands
4) 0ther- neither mucous or serous (ex= sweat glands)
Modes of exocrine secretions? (2 main)
1) merocrine: exocytosis of product w/ H20 & ions, membrane= intact
2) apocrine: small number of cells secrete a part of the cell membrane w/ the product
- cuz lipid isn’t bound to membrane
special modes of secretions?
1) microvesciles
2) exosomes
* both variations of apocrine*
- contain miRNA & mRNA
- can travel in circulation & fuse w/ other cells linking them physiologically & pathologically
holocrine secretions:
glands that secrete whole cells
development of epithelia to organs?
- during embryonic development tissue/organs made from simple epithelia
- glands,ducts, branch and become complex tissue (pancreas)
- epithelia tissues form cords that separate from basement membrane & fold to become solid tissue
blood vessels?
- all vessels lined by endothelial cells on basement membrane of epithelium
- endothelial cells have various purposes (barriers) & permeability (for nutrients in the blood)
apical specialization?
epithelial cells apical surfaces can vary
1) AIRWAY epithelium: cillia
2) intestinal epithelium: microvilli
3) auditory epithelium
cillia
made from tubulin filament (microtubules) that provide motor for cilia beating
- use ATP, are mechanically active
- in airway epithelium
airway epithelium?
uses CILIA to transport luminal contents in mucus &help sweep mucus up the airway passages to help keep airways clear
-helps us swallow secretions from lung & esphogous
intestinal apical epithelium?
have Microvilli (actin) that are cross linked so they don't move -provide extra surface area for absorption/secretions
Microvilli in intestinal epithelium ?
-formed from arrangement of actin filaments that provide structure but DON’T actively beat since are cross linked at tips
auditory apical epithelium?
- have kinocilium (actin based)
- opens K+ channels when associated stereocilia are displaced by movement or sound vibrations
- how we sense sound
