LEC # 9 CELL JUNCTIONS + MODIFICATIONS

8/7/2019 LEC # 9 CELL JUNCTIONS + MODIFICATIONS

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` Epithelial cells are held together by a mutual force of

cohesion but in addition specialized structural

arrangements called

CELL JUNCTIONSAre present


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Two factors are taken in account

There are three functional types


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` This may be in the form of a spot or limited extend called a

maculaor it can pass around the entire cell like belt or

girdle called a zonula


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OCCLUDENS:Outer surfaces of two cell membranesappear in contact or even fused

x Are located immediately beneath the luminal surface, wherethey seal the intercellular spaces

ADHERENS: The intercellular space is being about 20 to25 nm wide

GAP: shows a very narrow intercellular space of about

3nm.


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` This is the diffusion barrier and it is located at the most

apical point of adjoining epithelial cells. It forms a ring

(zonula) or circumferential band around the cells.

` It is a belt-like specialized zone.` Adjacent membranes are fused at multiple levels

separated by short segments with membranes closely

apposed.

` EXAMPLES: intestinal mucosa, urinary bladder mucosa


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` This is a continuous band adhesion. This structure also

surrounds the cell and adjoins to its neighbor.

` The membranes of adjacent cells are separated by a

distance of 20-25 nm.` The cytoplasm immediately subjacent to the membranes

is relatively dense

` It is found among epithelial cells, fibroblasts, smooth

muscle cells etc


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` This structure is a localized spot adhesion, as opposed

to a zonula adherens, and they are located at multiplesites on the upper lateral surface of adjoining cells/

perpendicular to the basement membrane of the

epithelium

` The adjacent membranes are separated by aninterspace of 25 nm.

` Bundles of tonofilaments converge upon and terminate

in dense plaques associated with the cytoplasmic

surface of the opposing membranes.


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Figure 7: Intercellular Junctions. Taken from: Wheaters Functional Histology, a text

and colour atlas, p.88, Figure 5.10.


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` THE COMBINATION OF

ZONULAOCCLUDENS

+

ZONULAADHERENS

&

DESMOSOMES

JUNCTIONAL COMPLEX


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` A variant of the desmosome is found in certain epithelial

cells were abrasion and mechanical shearing forces

would tend to separate the epithelium from the

underlying connective tissue (cornea, skin, esophagus

and vagina). In these locations, only half a desmosomeis present hemidesmosome. They are found on the

basal surface where they provide increased adhesion to

the basal lamina.


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` These are broad patches where adjacent plasmamembranes of adjacent cells are very closely apposedleaving a narrow intervening gap

` 2. The adjacent membranes are separated by an

interspace of 2 to 3 nm.` 3. Integral membrane proteins are tightly packed in

patches to form transmembrane channels or poreswhich permit positively charged ions and other smallmolecules (eg: nutrients & chemicals) between

adjacent cells Each pore consists of a small tubular structure called a

connexon, which traverses the intercellular gaps andcomprises a pair of grommet-like cylinders, one penetratingeach of the opposing cell membranes.

` EXAMPLES: in cardiac muscle, smooth muscle liver kidney thyroid skinpancreas and nervous tissue


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Figure 8: Communicating Junctions. Taken from:, Wheaters Functional

Histology, a text and colour atlas, p.89, Figure 5.14.


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Key: t.j. tight junction (occludens),

a.b. adhesion belt (adherens),

d.s. desmosome (spot),

g.j. gap junction (communicating),

h.d. hemidesmosome


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The apical and basal surfaces of the epithelial cells exhibit

various specializations to carry out diversified functions.

The specializations are as follows :-


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` These are small slender finger like projections/specializations found on free surface of epithelial cellsform a structure called the brush/ striated border

` They are 0.5 to1.0 m in length and about 0.1 mwide.` (can not be seen under light microscope)

` They increase the luminal surface area several fold (asmuch as 30 folds)and help in absorption.

` Each microvillus is stabilized by a core of actin filaments.

` EXAMPLES: (Theyhaveupto 3000microvilli/cell)

Epithelial cells of small intestine Epithelial cells of proximal convulated tubule (PCT) of nephron


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` Differ from microvilli only in their length and flexibility.

(visible with light microscopy)

` Average length 30m

` These structures have a limited distribution, occurring inthe ductus deferens, epididymis, and the inner ear.

` The interior of a stereocilium is filled with closely-

packed, longitudinally-oriented microfilaments.

` They are thought to facilitate absorptive processes in theepididymis But the reason for their unusual form is not known


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` These are minute motile/ hair like processes thatoccur on epithelial cell which are specialized fortransport of fluid or mucus over the epithelial surface

` They vary from 6 to 10 m in length and 0.2 m indiameter

` Cilia are supported by an internal structure ofmicrotubules arranged parallel to their long axis. Themicrotubules always possess the same pattern: acentral pair surrounded by nine peripheral doublets.

` EXAMPLES: ( Asingleepithelialcellmayhaveupto 300 cilia) cilia are found in the lining of the trachea (windpipe), where

they sweep mucus and dirt out of the lungs. In female the beating of cilia in the Fallopian tubes moves

the ovum from the ovary to the uterus


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` It is a tail-like / long, motile projection that

protrudes from the cell body

` The only human cell that possess flagellum is

sperm` Average length is 55 m

` Function is locomotion.


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` Flagella serve for the propulsion of single cells

(e.g. swimming of protozoa and spermatozoa),

and motile cilia for the transport of fluids (e.g.

transport of mucus by stationary ciliated cells inthe trachea).

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LEC # 9 CELL JUNCTIONS + MODIFICATIONS