Epithelial Tissue

Question 1

what is epithelium?

where is it found?

Answer 1

epithelium is avascular tissue formed from cells found covering the external surfaces of the body, lining the body cavities and constitutes glands.

Question 2

what are the three principle characteristics of epithelium?

Answer 2

1. cells are closely apposed and adhere to each other through specific cell-to-cell adhesion molecues forming cell junctions.
2. each cell exhibit functional and morphological polaritiy. different functions are associated to three specific surface morphological domains:
   - apical (free surface) domain
   - lateral domain
   - basal domain
3. the basalsurface is attached to an underlying basement membrane - a non-cellular protein-polysaccharide-rich layer

Question 3

what is epithelioid tissue?

Answer 3

epithelioid tissue contain epithelium without a free surface. epithelioid tissue is found in specialised situations, such as most endocrine glands.
example: interstitial cells of Leydig in the testis, islets of langerhans in the pancreas

Question 4

what are the two traditional factors that classify epithelium?
and what are the classifications of each?

what is the thrid factor that is relevent in some cases?

Answer 4

the number of cell layers

• simple: one layer
• stratified: more than one layer

the shape of the individual surface cells
- squamous: flattened, width greater than height
- cuboidal: square shape, width = height = depth
- columnar: height exceeds width (low columnar - height
exceeds width only by a short amount)

third factor is the specialisation of the apical suface domain, for instance:

- apical surface somain may contain cilia
- surface layer ay be non-keratinised or keratinised

Question 5

what is pseudostratified epithelium?

where is it mainly found?

Answer 5

pseudostratified epithelium appears stratified but all cells are attached to the basement membrane, so in fact it is simple.

it is found mainly in respiratory system: trachae and bronchial tree
but also found in the ductus deferens as well as the efferent ductules of the epididymis

Question 6

what are the two further specialised classes of epithelium, found in limited areas of the body?

Answer 6

pseudostratified epithelium
transitional epithelium (urothelium)

Question 7

specific names are given to epithelium in certain locations, define these types of epithelium:

• endothelium
• endocardium
• mesothelium

what classification of epithelium are they and give exceptions.

Answer 7

endothelium lines blood and lymphatic vessels

endocardium lines the ventricles and atria of the heart

mesothelium lines the walls and covers the contents of the closed cavities of the body (abdominal, pericardial, pleural)

all three are mostly simple squamoud epithelium although exceptions include:

• lining of postcapillary venules of certain lymphatic tissues, endothelium os cuboidal. these venules are called high endothelial venules (HEV)
• spleen, endothelial cells of venous sinuses are rod-shaped

Question 8

what are HEVs?

Answer 8

HEV are high endothelial venules and are found in certain lymphatic tissues in which the endothelial lining is simple cuboidal and not simple squamous.

Question 9

what are the main five functions that epithelium may serve?

Answer 9

1. secretion
2. protection
3. transportation
4. absorption
5. receptor function

Question 10

what are the three domains of epithelial cells?

define each domain

Answer 10

Free/Apical domain is always directed toward the exterior surface or lumen of the enclosed cavity/tube

Lateral domain communicates with adjacent cells, characterised by specialised attachment areas

Basal domain rests on the basal lamina anchoring the cell ti underlying connective tissue.

Question 11

what are the three structural modifications that can be exhibited on the apical suface of epithelium?

Answer 11

Microvilli - cytoplasmic processes with an actin filament core

stereocilia - long microvilli

cilia - cytoplasmic processes containing bundle of microtubules

Question 12

describe microvilli…

what are the main functions of microvilli?

Answer 12

microvilli are fingerlike projections of the cytoplasm on the apical surface of most epithelia cells.

the number and shape of microvilli of a given cell type corelates with the absorptive capacity of that cell. the more absoptive cells have closely packed, uniform and regular microvilli that greatly increase the surface area.

* striated border - used to descreibe the uniform microvilli of intestinal epitheium
* brush border - used to describe those at the apical surface of kidney tubule cells

main function is the increase the apical surface area of epithelial cells to increase their absoprtive capacity

Question 13

describe the internal structure of a microvillus…

Answer 13

microvilli contain a core of 20-30 actin filaments.

at the distal end/tip the barbed ends are anchored to an actin-binding protein called villin. villus promotes the growth of microvilli on the apical surface of an epithelial cell.

the actin bundle is connected to to apical cytoplasm throgh a horizontal network of actin filaments, the terminal web.

inside the microvillus shaft, the actin filaments are cross-linked at 10nm intervals by actin-binding proteins, fascin, epsin, fimbrin. these proteins provide support and rigidity to the microvillus.

myosin I is also associated tot eh actin core as a molecule that binds the actin filaments to the plasma membrane of the mircovillus shaft.

the terminal web is composed of actin filaments stabilised by spectrin. spectrin also attaches the terminal web to the apical plasma membrane.

myosin II and tropomyosin is found in the terminal web, providing contracile ability. they decrease the diameter of the apex of the cell, causing the microvilli to spread apart and increase the intermicrovillous space.

Question 14

what is villin?

Answer 14

villin is found at the tips of microvilli.
the actin filament core is anchored to villin.
villin has also been shown to stimulate growth of microvilli when found at the apical surface of an epithelial cell

Question 15

what are epsin, fimbrin and fascin?

Answer 15

epsin, fimbrin and fascin are actin-binding proteins found in the actin core of microvilli.
they appear at 10nm intervals, crosslinking actin filaments throughout the shaft of a microvillus. they provide support and rigidity.

Question 16

what is the terminal web?

also spectrin, myosin II and tropomyosin

Answer 16

the terminal web is found at the apical surface of epithelial cells.
made from actin filaments it is located just beneath and perpendicular to microvilli projecting from the apical surface.

spectrin is an actin-binding protein that stabilises the terminal web aswell as attaching it to the apical plasma membrane.

myosin II and tropomyosin are also found in the terminal web, providing the contractile ability of microvilli.

Question 17

what are the average dimensions of uniform microvilli on the apical surface of an epithelial cell?

Answer 17

microvilli have an:
average length of 1-3um
average diameter of 50-100nm

Question 18

in relation to microvilli, where is myosin I found and for what purpose?

Answer 18

myosin I if found in the shaft of microvilli, binding the actin filaments to the surrounding plasma membrane.

Question 19

what are stereocilia?
where are they found?
what are the main functions?
what are the main structural features?

Answer 19

stereocilli are unusually long, immotile microvilli.
length can reach up to 120um, diameter 100-150nm

found on the apical surface of epithelium of the epididmis and sensory cells of the inner ear

mainly facilitate absorption in the epididymis. and fintion as mechanoreceptors in the inner ear.

like microvilli, sterocilia are supported by a bundle of actin filaments cross-linked by fimbrin. the actin filaments plus end orientated towards the tip, the minus end toward the base.

sterocilia have no villin tip.
an actin binding protein called ezrin is used to anchor the actin filaments to the plasma membrane.
alpha-actinin is a cross-bridge-forming molecule that is found in the stem of the sterocilia aswell as the apical cell protrusion.

Question 20

what are the main differences between microvilli and stereocilia (genital ducts)?

Answer 20

stereocilia do not contain villin in their tips, whereas microvilli do (anchoring the actin filaments)

stereocilia contain ezrin, a substitute for myosin I in microvilli. they both bind the actin bundle to the surrounding plasma membrane.

stereocilia contain alpha-actinin, a cross-bridge-forming molecule. microvilli does not.

microvilli is connected to the underlying terminal web, stereocilia is not.

Question 21

what are cilia?

what is the general structure of cilia?

Answer 21

cilia are the most common surface modifactions found on nearly every cell of the body. cilia are hair like extentions of the apical plasma membrane.

cilia contains an axoneme, a microtubule based internal structure, extending from the basla bodies located in the apical region of a ciliated cell.

Question 22

how are cilia classified?

Answer 22

cilia can be classified into three basic catagories:
motile cilia
primary cilia
nodal cilia

Question 23

what are motile cilia?
where and how are they found?
what is the general structure?

Answer 23

motile cilia are found in large numbers on the apical surface of many epithelial cells. most commonly found on epithelia that transport secretions, protein, foreign bodies or cells on their surface:
(oviduct, trachea, bronchial tree, brain ependyma, olfactory epithelium)

typical motile cilia are 5-10um in length, 250nm diameter.

they possess a typical 9+2 axonemal organisation with microtubule-associated motor proteins that are required for the generation of forces needed to induce motility.

Question 24

describe the structure of motile cilia…

Answer 24

motile cilia contain an axoneme
the axoneme represents an organised core of microtubules arranged in a 9+2 pattern. 9 doublets arranged in a circle with two central microtubules in the centre.

the 9 doublets are constructed so that the wall of one microtubule is completed by the wall of the other microtubule.
the full microtubule is composed of 13 tubulin protofilaments, and the ajoining incomplete mircotubule composed of 10 tubulin protofilaments.

each doublet contains a pair of arms composed of dynein, a microtubule-associated motor protein. the dynein occur at 24nm intervals on the completed microtubule along the length of each doublet, extending out to reach the incomplete microtubule on the adjacent doublet.

nexin is a passive elastic component that permenantly links a fully completed microtubule with the incomplete microtubule on the adjacent doublet at intervals of 86nm.

the two central microtubules are surrounded/partially enclosed by a central sheath projection at 14nm intervals.

radial spokes extend from each completed microtubule from each doublet toward the two central microtubules at 29nm intervals

the 9+2 pattern of microtubules run along the shaft of the cilia from the tip to the base. at the base the central two microtubules do not continue but the 9 doublets join the basal body as a continuation of the triplet form.

the basal body is a modified contriole and finctions as a MTOC consisting of nine short microtubule triplets arranged in a ring.

Question 25

in the 9+2 pattern of microtubules in motile cilia, what molecule makes up the microtubules? (and how many?)

Answer 25

tubulin is the molecule.

each doublet is formed from a complete mircotubule of 13 tubulin molecules and an ajoining microtubule of 10 tubulin molecules

Question 26

in the 9+2 pattern of microtubules in motile cilia, what is the radial spoke?

Answer 26

the radial spoke fixes each doublet to the central sheath surrounding the two central microtubules and occurs at 29nm intervals.

Question 27

in the 9+2 pattern of microtubules in motile cilia, what is dynein?
where is it located?

Answer 27

dynein forms arms coming off each fully formed microtubule of each of the 9 doublets, reaching out towards the adjacent doublet.

the dynein arms use ATP hydrolysis to move along the surface of the adjacent microtubule.

dynein arms are found at 24nm intervals

Question 28

in motile cilia what permenantly links the microtubule of one doublet to the microtubule of the adjacent doublet?

Answer 28

nexin, a passive elastic component, found at 86nm intervals

Question 29

what is the lateral domain of epithelial cells characterised by?

Answer 29

cell adhesion molecules, CAMs

the lipid and protein composition of the cell membrane in the lateral surface of the cell is significantly different to the apical surface.

Question 30

what are junctional complexes?

what is an other name for a junctional complex?

Answer 30

junctional complexes join epithelial cells together through specialised sites. it can also be the site of a barrier to the passage molecules to adjacent epithelial cells.

Question 31

what three types of junctional complexes are there?

Answer 31

1: occluding junctions / tight junctions
2: achoring junctions
3: communicating junctions

Question 32

what are occluding junctions?
what is another name for them?
what are 3 functions of occluding junctions?

Answer 32

occluding junctions are impermeable junctions, allowing epithelial cells to function as a barrier.

they can be known as tight junctions

1. form the primary intercellular diffusion barrier between adjacent cells
2. maintains the integrity of the apical and lateral domains by preventing lipid and protein migration between the two domain membranes.
3. recruit various signalling molecules to the cell surfaceand links them to the cell filaments of the cell cytoplasm.

Question 33

what represents the most apical component in the occluding junctions between epithelial cells?

Answer 33

zonula occludens

Question 34

what are zonula occludens?

how are they formed?

Answer 34

zonula occludens represents the most apical component in the occluding junctions between epithelial cells

zonula occludens are formed by narrowing the space between the plasma membranes of adjoining cells. the zonula accludens is not a single continuous seal but a series of focal fusions between the cells formed by transmembrane proteins of adjoining cells that join in the intercellular space.

Question 35

what are the three major groups of transmembrane proteins involved in the creation of zonula occluden strands that form a functional seal?

Answer 35

1. occludins
2. claudins
3. JAMs - junctional adhesion molecules

Question 36

What are occludins, claudins and JAMs involved in?

Answer 36

they are the three major groups of transmembrane proteins involved in the creation of zonula occluden strands that form a functional seal.

Question 37

what is occludin?

Answer 37

occludin is a 60kD protein that participates in the maintainence of the barrier between adjacent cells as well as between the apical and lateral domain of a cell.
occludin is present in most tight junctiuons

Question 38

what are claudins?

Answer 38

claudins are a family of proteins that form the backbone if each zonula occluden strand involved in creating zonula occludens.

claudins are also involved in the creation of aqueous channels.

Question 39

what are JAMs? (occluding junctions)

Answer 39

JAMs are junctional adhesion molecules. they belong to the immunoglobulin superfamily and is not directly involved in the formation of zonula occluden strands but are associated with claudins.

Question 40

what are the two distinct pathways zonula occludens form by creating a controlled diffusion barrier?

Answer 40

1. transcellular pathway - small substances crosses apical membrane into the cell cytoplasm and then crosses the lateral membrane beneath the level of the occluding junction into the intercellular space. transport is usually active, dependent membrane transport proteins and channels that require energy.
2. paracellular pathway - small molecules cross the zonula occludens between two adjacent epithelial cells. the amount of small molecules transported depends upto the tightness of the zonula occludens.

Question 41

what are anchoring junctions?

Answer 41

anchoring junctions provide lateral adhesions between epithelial cells, using proteins that link into the cytoplasm of adjacent cells.

Question 42

what are the lateral types of anchoring junctions?

Answer 42

two types:

1. zonula adherens - interacts with actin filaments
2. macula adherens (desmosomes) - interacts with IF

Question 43

what are CAMs?

Answer 43

CAMs are cell adhesion molecules
they play an important role in cell-to-cell and cell-to-extracellular matrix adhesion.

CAMs are transmembrane proteins used on the lateral and basal surfaces of cells, that interact with those of simular domains of adjacent cells

Question 44

what is the difference between hetrotypic and homotypic binding?

Answer 44

hetrotypic binding occurs between different types of molecules (CAMs)
homotypic binding occurs between the same types of molecules

Question 45

what are the four classified families of CAMs?

Answer 45

1. cadherins
2. integrins
3. selectins
4. immunoglobulin superfamily

Question 46

what are cadherins?

where are they used?

Answer 46

cadherins are transmembrane calcium dependant proteins found mainly within zonula adherens.
they maintain homotypic binding (only binds with other cadherins of simular type)
they interact with actin filaments through other associated proteins.

Question 47

what are integrins?

Answer 47

integrins are represented by two transmembrane glycoprotein subunits (15alpha and 9beta). different combinations of integrin molecules interact with various proteins, and considered to form hetrotypic interactions.

integrins interact with extra cellular matrix molecules, actin filaments and intermediate filaments.

Question 48

what CAMs produce hetrotypic binding?

Answer 48

integrins and selectins

Question 49

what CAMs produce homotypic binding?

Answer 49

cadherins and IgSF

Question 50

what are zonula adherens?

what is the function?

Answer 50

zonula adherens provides lateral adhesion between epithelial cells below the level of zounla occludens.

zonula adherens reinforces the seal created by zonula occludens as their resistance to mechanical stress is limited.

zonula adherens produce a continuous band around the cell like the zonula occludens.

Question 51

what is the zonula adheren composed of?

Answer 51

the zonula adheren is composed of the transmembrane adhesion molecule E-Cadherin.

actin filaments of adjacent cells are linked through:

1. actin filaments
2. alpha actinin
3. vinculin
4. catenin
5. E-cadherin (transmembrane)
6. calcium ion (intercellular)
7. adjacents E-cadherin

Question 52

what are macula adherens? (alternative name…)

what is there function?

Answer 52

the macula adherens or desmosomes are anchoing junctions that provide strong cell-to-cell adhesions

the junctions are localised on the lateral domain of the cell, spot-like and does not provide a continuous band such that zonula occluden and zounula adherens form.

Question 53

what are macula adherens composed of?

Answer 53

macula adherens anchor to the intermediate filaments of the cytoskeleton of the cell.

a dense material, the desmosomal attachment plaque anchors the IF to transmembrane proteins of the cadherin family, desmocollins and desmogleins, that form a zipper effect in the intercellular space with similar proteins of the adjacent cell.

Question 54

what are communication junctions?

give two alternatives names

Answer 54

communication or Gap junctions or nexuses are the only known cellular structures that permit direct passage of signalling moilecules between adjacent cells.

Question 55

what does a gap junction consist of?

Answer 55

a gap junction consists of an accumulation of transmembrane channels in a tightly packed array, the number of channels in a gap junction varies as does the number of gap junction between adjacent cells.

each gap junction are formed of groups of tightly packed channels, each channel formed of two half channels called connexons embedded in the facing membranes.
the connexon of one cell alignes with one on the adjacent cell.

Question 56

what is a connexon?

Answer 56

a connexon is a transmembrane pore and one have of a channel used in gap junctions.
a connexon is made up of 6 connexin subunits.

Question 57

what are the dimensions of a channel in a gap junction?

Answer 57

one channel is made up of 2 connexons consisting of 6 connexin subsunits each (12 in total)
one channel is 10nm long and 2.8nm in diameter
each connexon is aligned with a similar structure on an opposing cell, narrowing the intercellular space to produce a 2nm wide gap.

Question 58

what characterises the basal domain of epithelial cells?

3 things

Answer 58

1. the basement membrane: located between all epithelial tissue and connective tissue stroma.
2. cell-to-extracellular matrix junctions: anchoring the cell to the extracellular matrix through:
   - focal adhesions
   - hemidesmosomes
3. basal cell membrane infoldings: increase the cell surface area and facilitate morphological interactions between adjacent cells and extracellular matrix protiens.

Question 59

what is the basement membrane of epithelial tissue?

Answer 59

the basement membrane underlying all epithelial tissue is called the basal lamina.
the basal lamina is composed of the lamina densa and lamina lucida. recent suggestions indicate the lamina lucida is however an artifact of fixation.

the lamina densa is an electron-densa region located between the epithelial tissue and the adjacent connective tissue. is it 40-60nm thick composed of a network of fine, 3-4nm filaments of proteins called laminins, type IV collagen molecule and associated proteoglycans and glycoproteins.

the lamina lucida is an electron-leucent region 40nm wide.

Question 60

the basal lamina contains molecule that form a sheetlike structure. how many types of different proteins have been found and how can they be classified? (4 groups)

Answer 60

over 50 proteins have been identified within the basal lamina and they can be classifed into four groups:

1. collagens
2. laminins
3. enactin/nidogen
4. proteoglycans

Question 61

how is the type IV collagen suprastructure of the basal lamina formed?

Answer 61

Type IV collagen monomer (single α-chain)
- 7S domain, colagenous helical domain, NC1 domain

Type IV collagen promotor (trimer of α-chain) determines type of basal lamina
- includes a NC1 trimer and intertwining collagenous helical domain to the 7S domain.

Type IV collagen dimer
- NC1 trimer interacts with another NC1 trimer to form a NC1 hexamer to give a dimer

Type IV collagen tetramer
- 4 7S domains of 4 dimers interact at the 7S box producing a tetramer

Type IV collagen suprastructure
- tetramers interacting with each other at 7S domains, forming more 7S boxes.

Question 62

what various functions are attributed to the basal lamina?

Answer 62

1. structural attachment - cells-to-extracellular matrix
2. compartmentalisation - seperates conntective tissue from cells
   - for substances to travel from one compartment to another, e.g. connective tissue to epithelial tissue, it must first cross the basal lamina
3. filtration - regulation of the movement of substances
4. tissue scaffolding
5. regulation and signalling

Question 63

what two anchoring junctions maintain the morphological integrity of the epitheliam-to-connective tissue interface?

Answer 63

1. focal adhesions

2. hemidesmosomes

Question 64

what are focal adhesions at the basal domain of epithelial tissue?

Answer 64

focal adhesions use the actin filaments of the cytoskeleton to anchor the cell to the basal lamina

focal adhesions create a dynamin link between actin filaments and extracellular matrix glycoproteins.

from inside the cell

1. actin filaments
2. alpha-actinin
3. vinculin + paxillin
4. talin
5. transmembrane: integrins
6. extracellular matix: glycoproteins: laminin and fibronectin

Question 65

what are hemidesmosomes?

Answer 65

hemidesmosomes attach the Intermediate filament of the cytoskeleton to the extracellular matrix.

they occur within epithelia that require strong, stable adhesion to the connective tissue beneath. i.e. skin, cornea, vagina, oral cavity, oesophagus.

Question 66

what are the types of glands based on how their products are released?

Answer 66

1. exocrine glands - directly on the surface of the cell or through ducts
2. endocrine glands - into connective tissue to the blood stream
   products known as hormones
3. paracrine glands - release products into the connective tissue but do not enter the bloodstream. they act of cell close by.