All Epithelial Cells in Contact With Subjacent Connective Tissue Have at Their Basal Surfaces a Felt

7/24/2019 All Epithelial Cells in Contact With Subjacent Connective Tissue Have at Their Basal Surfaces a Felt

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All epithelial cells in contact with subjacent connective tissue have at their basal

surfaces a felt-like sheet of extracellular material called the basal laminaBasal laminae have many functions. In addition to simple structural and ltering

functions they are also able to in!uence cell polarity" regulate cell

proliferation and di#erentiation by binding and concentrating growth factors"

in!uence cell metabolism and survival" organi$e the proteins in the adjacentplasma membrane %a#ecting signal transduction&" and serve as pathways for

cell migration. 'he basal lamina seems to contain the information necessary

for many cell-to-cell interactions such as the reinnervation of denervated

muscle cells. 'he presence of the basal lamina around a muscle cell is

necessary for the establishment of new neuromuscular junctions.

Intercellular Adhesion & Other Junctions

(everal membrane-associated structures contribute to adhesion and

communication between cells. 'hey are present in most tissues but areparticularly numerous and prominent in epithelia and will be describedhere. )pithelial cells are extremely cohesive and relatively strongmechanical forces are necessary to separate them. Intercellular adhesionis especially marked in epithelial tissues that are subjected to traction andpressure %eg in the skin&.

'he lateral membranes of epithelial cells exhibit several speciali$edintercellular junctions.*arious junctions serve to function as+

, (eals to prevent the !ow of materials between the cells%occluding junctions&

, (ites of adhesion %adhesiveor anchoring junctions&

, hannels for communication between adjacent cells %gapjunctions&.

In several epithelia such junctions are present in a denite order from theapical to the basal ends of the cells.

Tight junctions,or zonulae occludens%singular zonulaoccludens& are the most apical of the junctions. 'he atin

terminology gives important information about the geometry of thejunction. /0onula/ indicates that the junctions form bandscompletely encircling each cell and /occludens/ refers to themembrane fusions that close o# the space between the cells. Inproperly stained thin sections viewed in the ')1 the adjacentmembranes appear tightly apposed or fused %2igures 343 and 345&.'he seal between the membranes is due primarily to directinteractions between the transmembrane protein claudinon eachcell. After cryofracture %2igure 346& the replicas show these fusionsites as a band of branching strands around each cell. 'he number

of these sealing strands or fusion sites is inversely correlated withthe leakiness of the epithelium. )pithelia with one or very few fusion


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sites %eg proximal renal tubule& are more permeable to water andsolutes than are epithelia with numerous fusion sites %eg the liningof the urinary bladder&. 'hus the principal function of the tightjunction is to form a seal that prevents the !ow of materialsbetween epithelial cells %the paracellular pathway& in either

direction. In this way $onulae occludens in sheets of epithelial cellshelp form two functional compartments+ an apical compartment thatis composed of an organ cavity %such as the lumen of a secretoryunit or the gut& and a basal compartment that begins at thejunctions and encompasses the underlying tissue

Besides forming a seal between compartments on either side of an

epithelium the $onulae occludens of epithelial cells help prevent the

integral membrane proteins of the apical surface from being

transferred to the basolateral surface and vice versa. 'his allows the

two sides of the epithelium to maintain di#erent receptors andfunction di#erently.

'he next type of junction is the adherent junctionor zonula

adherens%2igures 343 and 345&. 'his junction also encircles the

cell usually immediately below the $onula occludens and provides

for the rm adhesion of one cell to its neighbors. Adhesion is

mediated by transmembrane glycoproteins of each cell the

cadherins which lose their adhesive properties in the absence of

a78

. Inside the cell cadherins bind the protein catenin which islinked by means of actin-binding proteins to actin laments all of

which produce electron-dense pla9ues of material on the

cytoplasmic surfaces of adherent junctions. 'he numerous actin

laments form part of the terminal web,a cytoskeletal feature at

the apical pole in many epithelial cells with a role in cytoplasmic

motility and other functions.

Another junction speciali$ed for adhesion is the desmosomeor

macula adherens%. macula,spot&. As the names imply this

junctional type resembles a single /spot-weld/ and does not form a

belt around the cell. 'he desmosome is a disk-shaped structure at

the surface of one cell that is matched with an identical structure at

the surface of an adjacent cell %2igures 343 and 345&. Between cell

membranes at a desmosome are variable amounts of electron-

dense material principally larger members of the cadherin family.

:n the cytoplasmic side of each cell membrane these cadherin-type

proteins inset into a dense attachment plaqueof anchoringproteins %plaophilin, plaoglobin,and desmoplain& which bind


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#umber o$ %ella!ers

%ell 'orm ()amples o$*istribution

+ain 'unction

olumnar ining ofintestinegallbladder.

=rotectionlubricationabsorptionsecretion.

=seudostratied%layers of cellswith nuclei atdi#erent levels"not all cells reach

surface but alladhere to basallamina&

ining of tracheabronchi nasalcavity.

=rotectionsecretion" cilia-mediated transportof particles trappedin mucus out of the

air passages.

(tratied %two ormore layers&

(9uamouskeratini$ed%dry&

)pidermis. =rotection" preventswater loss.

(9uamous

nonkeratini$ed %moist&

1outh

esophaguslarynx vaginaanal canal.

=rotection

secretion" preventswater loss.

uboidal (weat glandsdevelopingovarian follicles.

=rotectionsecretion.

'ransitional Bladder ureters

renal calyces.

=rotection

distensibility.

olumnar onjunctiva. =rotection.

%O##(%TI( TI--(

%ells o$ %onnective Tissue


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A variety of cells with di#erent origins and functions are present in

connective tissue %2igure 547 and 'able 54>&. 'ibroblastsoriginate locally

from undi#erentiated mesenchymal cells and spend all their life in

connective tissue" other cells such as mast cells macrophages and

plasma cellsoriginate from hematopoietic stem cells in bone marrowcirculate in the blood and then move into connective tissue where they

remain and execute their functions. ;hite blood cells %leukocytes& are

transient cells of most connective tissues" they also originate in the bone

marrow and move to the connective tissue where they reside for a few

days then usually die by apoptosis.

'able 54>. 2unctions of connective tissue cells.

%ell T!pe /epresentative 0roduct orActivit!

/epresentative'unction

2ibroblastchondroblastosteoblast

odontoblast

=roduction of bers and groundsubstance

(tructural

=lasma cell =roduction of antibodies Immunologic%defense&

ymphocyte%several types&

=roduction of immunocompetentcells

Immunologic%defense&

)osinophilicleukocyte

=articipation in allergic andvasoactive reactions modulation ofmast cell activities and thein!ammatory process

Immunologic%defense&

?eutrophilicleukocyte

=hagocytosis of foreign substancesbacteria

@efense

1acrophage (ecretion of cytokines and othermolecules phagocytosis of foreignsubstances and bacteria antigen

@efense


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%ell T!pe /epresentative 0roduct orActivit!

/epresentative'unction

processing and presentation to othercells

1ast cell andbasophilicleukocyte

iberation of pharmacologicallyactive molecules %eg histamine&

@efense%participate inallergicreactions&

Adipocyte (torage of neutral fats )nergy reservoir

heat production

'ibroblasts

2ibroblasts synthesi$e collagen elastin glycosaminoglycans

proteoglycans and multiadhesive glycoproteins. 2ibroblasts are the most

common cells in connective tissue %2igure 54& and are responsible for the

synthesis of extracellular matrix components. 'wo stages of activity

active and 9uiescentare often observed in these cells %2igure 54b&.ells with intense synthetic activity are morphologically distinct from the

9uiescent broblasts that are scattered within the matrix they have

already synthesi$ed. (ome histologists reserve the term 1broblastto

denote the active cell and 1broc!teto denote the 9uiescent cell.

'able 547. @istribution and main functions of the cells of the mononuclearphagocyte system.

%ell T!pe ocation +ain 'unction

1onocyte Blood =recursor of macrophages

1acrophage onnective tissue

lymphoid organslungs bone marrow

=roduction of cytokines chemotactic

factors and several other moleculesthat participate in in!ammation


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%ell T!pe ocation +ain 'unction

%defense& antigen processing and

presentation

Cup#er cell iver (ame as macrophages

1icrogliacell

?erve tissue of thecentral nervoussystem

(ame as macrophages

angerhanscell

(kin Antigen processing and presentation

@endriticcell

ymph nodes Antigen processing and presentation

:steoclast Bone %fusion ofseveralmacrophages&

@igestion of bone

1ultinucleargiant cell

onnective tissue%fusion of severalmacrophages&

(egregation and digestion of foreignbodies

'ibers

'he connective tissue bers are formed by proteins that polymeri$e intoelongated structures. 'he three main types of connective tissue bers are

collagen, reticular,and elastic 1bers.ollagen and reticular bers are

both formed by the protein collagen,and elastic bers are composed

mainly of the protein elastin.'hese bers are distributed une9ually

among the types of connective tissue and the predominant ber type is

usually responsible for conferring specic properties on the tissue.


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%ollagen

'he collagens constitute a family of proteins selected during evolution for

the execution of several %mainly structural& functions. @uring the process

of evolution of multicellular organisms a family of structural proteins wasselected by both environmental in!uences and the functional

re9uirements of the animal organism and developed to ac9uire varying

degrees of rigidity elasticity and strength. 'hese proteins are known

collectively as collagen,and the chief examples among its various types

are present in the skin bone cartilage smooth muscle and basal lamina.

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All Epithelial Cells in Contact With Subjacent Connective Tissue Have at Their Basal Surfaces a Felt